| 1. |
- Erdelyi, Mate, 1975, et al.
(författare)
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Dynamics of the Glycosidic Linkage: Conformational Space of Lactose
- 2011
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Ingår i: Chemistry - A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 17:34, s. 9280-9282
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Tidskriftsartikel (refereegranskat)abstract
- Abstract: The dynamics of the glycosidic bond of lactose was studied by a paramagnetic tagging-based NMR technique, which allowed the collection of an unusually large series of NMR data for a single compound. By the use of distance- and orientation-dependent residual dipolar couplings and pseudocontact shifts, the simultaneous fitting of the probabilities of computed conformations and the orientation of the magnetic susceptibility tensor of a series of lanthanide complexes of lactose show that its glycosidic bond samples syn/syn, anti/syn and syn/anti f/y regions of the conformational space in water. The analysis indicates a higher reliability of pseudocontact shift data as compared to residual dipolar couplings with the presently available weakly orienting paramagnetic tagging technique. The method presented herein allows for an improved understanding of the dynamic behaviour of oligosaccharides.
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| 2. |
- Schmitt, H., et al.
(författare)
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Synthesis, redox properties, and EPR spectroscopy of manganese(III) complexes of the ligand N,N-bis(2-hydroxybenzyl)-N '-2-hdroxybenzylidene-1,2-diaminoethane : Formation of mononuclear, dinuclear, and even higher nuclearity complexes
- 2002
-
Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 8:16, s. 3757-3768
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Tidskriftsartikel (refereegranskat)abstract
- The synthesis and characterization of the title trisphenolate ligand are described. From its reaction with manganese(iii) three complexes were isolated. The crystal structures revealed one pentacoordinate monomer and two similar dimers with different solvents of crystallization. In the dimers the metal ions are hexacoordinate and connected through bridging of two phenolates. A combination of electrochemistry and EPR spectroscopy showed that, in acetonitrile, the isolated batches were all identical and mainly monomeric, indicating that the mononuclear complex is in equilibrium with the dimer and perhaps also with complexes of higher nuclearity, as suggested by the detection of both the trimer and the tetramer by electrospray ionization mass spectrometry (ESI-MS). The successful use of the monomer batch as an epoxidation catalyst indicated that a high-valent manganese-oxo species can be formed, although it is probably short-lived. This is also suggested by EPR studies of the species formed by electrochemical oxidation of the complex. Upon one-electron oxidation, a manganese(iv) species was formed, which was at least partly converted to another species containing a phenoxy radical.
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| 3. |
- Brandt, Peter, et al.
(författare)
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Chromium-salen-mediated alkene epoxidation : a theoretical and experimental study indicates the importance of spin-surface crossing and the presence of a discrete intermediate.
- 2002
-
Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 8:18, s. 4299-307
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Tidskriftsartikel (refereegranskat)abstract
- The mechanism of alkene epoxidation by chromium(v) oxo salen complexes has been studied by DFT and experimental methods. The reaction is compared to the closely related Mn-catalyzed process in an attempt to understand the dramatic difference in selectivity between the two systems. Overall, the studies show that the reactions have many similarities, but also a few critical differences. In agreement with experiment, the chromium system requires a change from low- to high-spin in the catalytic cycle, whereas the manganese system can proceed either with spin inversion or entirely on the high-spin surface. The low-spin addition of metal oxo species to an alkene leads to an intermediate which forms epoxide either with a barrier on the low-spin surface or without a barrier after spin inversion. Supporting evidence for this intermediate was obtained by using vinylcyclopropane traps. The chromium(v) oxo complexes can adopt a stepped shape or form a more concave surface, analogous to previous results on manganese salen complexes.
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| 4. |
- Gawronski, J., et al.
(författare)
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Novel Chiral Pyromellitdiimide (1,2,4,5-Benzenetetracarboxydiimide) Dimers and Trimers: Exploring Their Structure, Electronic Transitions and Exciton Coupling
- 2002
-
Ingår i: Chemistry - A European Journal. - 1521-3765 .- 0947-6539. ; 8, s. 2484-2494 and ibid. 2833
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Tidskriftsartikel (refereegranskat)abstract
- The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2-5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2-disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X-ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low-energy tail (340-360 nm) of the absorption envelope can be assigned to out-of-plane polarized n-pi* transitions (I, II). The higher energy bands are due to contributions from up to six pi-pi* transitions, these being polarized either along the long (IV-VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc-pVDZ and semiempirical INDO/S-CI calculations have been compared with the experimental data. CD Cotton effects in the region 200260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2-5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems.
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| 5. |
- Goeppert, Alain, et al.
(författare)
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Methane activation and oxidation in sulfuric acid
- 2002
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Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 8:14, s. 3277-3283
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Tidskriftsartikel (refereegranskat)abstract
- The H/D exchange observed when methane is contacted with D(2)SO(4) at 270-330 degrees C shows that the alkane behaves as a sigma base and undergoes rapid and reversible protonation at this temperature. DFT studies of the hydrogen exchange between a monomer and a dimer of sulfuric acid and methane show that the transition states involved in the exchange are bifunctional, that is one hydrogen atom is transferred from a hydroxy group in sulfuric acid to methane, while one hydrogen atom is abstracted from methane by a non-hydroxy oxygen atom in sulfuric acid. All the transition states include a CH(5) moiety, which shows similarities to the methanium ion CH(5) (+). The calculated potential activation energy of the hydrogen exchange for the monomer is 174 kJ mol(-1), which is close to the experimental value (176 kJ mol(-1)). Solvation of the monomer and the transition state of the monomer with an extra sulfuric acid molecule, decrease the potential activation energy by 6 kJ mol(-1). The acid-base process is in competition, however, with an oxidative process involving methane and sulfuric acid which leads to CO(2), SO(2), and water, and thus to a decrease of acidity and loss of reactivity of the medium.
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| 6. |
- Jalilehvand, F., et al.
(författare)
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Tl-Pt(CN)(5) in the solid state - A multimethod study of an unusual compound containing inorganic wires
- 2001
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Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 7:10, s. 2167-2177
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Tidskriftsartikel (refereegranskat)abstract
- The crystal and molecular structure of a polycrystalline powder with a metal-metal bond and the composition TlPt(CN)(5) has been determined by combining results from X-ray powder diffraction (XRD), extended X-ray absorption fine structure (EXAFS) and vibrational spectroscopic studies. The XRD data gave the tetragonal space group P4/nmm (No. 129). with a = 7.647(3), c = 8.049(3) Angstrom, Z = 2, and well-determined positions of the heavy metal atoms. The Pt-Tl bond length in the compound is 2.627(2) Angstrom. The platinum atom coordinates four equivalent equatorial cyano ligands, with a fifth axial CN ligand and a thallium atom completing a distorted octahedral coordination geometry. The Tl-Pt(CN)(5) entities are linked together in linear -NC-Pt-Tl-NC-Pt-Tl chains through the axial cyano ligand. These linear wires are the essential structural features and influence the properties of the com-pound. A three-dimensional network is formed by the four equatorial cyano ligands of the platinum atom that form bridges to the thallium atoms of neighbouring antiparallel chains. The platinum atom and the five nitrogen atoms from the bridging cyano groups form a distorted octahedron around the thallium atom. EXAFS data were recorded at the Pt and Tl L-III edges for a more complete description of the local structure around the Pt and Tl atoms. The excessive multiple scattering was evaluated by means of the FEFF program. Raman and infrared absorption spectroscopy reveal strong coupling of the vibrational modes of the TlPt(CN)(5) entities, in particular the metal - metal stretching mode, which is split into four Raman and two IR bands. Factor group theory shows that a structural unit larger than the crystallographic unit cell must be used to assign vibrational bands. Intra- and intermolecular force constants have also been calculated. The compound exhibits red luminescence at 700+/- 3 nm in glycerol and has a corresponding excitation maximum at 240 nm. X-ray photoelectron spectra (XPS) show that the metal atoms have intermediate oxidation states, Pt3.2+ and Tl1.6+, between those in the parent Pt-II and Tl-III species and the decomposition products, Pt-IV and Tl-I. The solid compound TlPt(CN)(5) is stable to 520 degreesC. However in presence of water, a two-electron transfer between the metal atoms results in the cleavage of the metal-metal bond at 80 degreesC, forming a Pt-IV pentacyanohydrate complex and a monovalent thallium ion.
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| 7. |
- Kilså, Kristine, et al.
(författare)
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Enhanced Intersystem Crossing in Donor/Acceptor Systems Based on Zinc/Iron or Free-Base/Iron Porphyrins
- 2001
-
Ingår i: Chemistry, A European Journal. - 1521-3765 .- 0947-6539. ; 7:10, s. 2122-33
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Tidskriftsartikel (refereegranskat)abstract
- The deactivation pathways of the singlet excited state of a series of zinc or free-base donor porphyrins covalently linked by a bridge to a paramagnetic iron(iii) chloride porphyrin acceptor have been studied. These donor-bridge-acceptor systems all share a similar geometry (25 Å donor-acceptor center-to-center distance), but the bridges vary in electronic structure. In previously reported investigations of zinc/iron porphyrin systems, the fluorescence quenching of the donor has predominantly been assigned to electron transfer. However, for the porphyrin systems studied in this paper, we show that the dominant deactivation channels are enhanced intersystem crossing and singlet energy transfer. In both series, the intersystem crossing rate (S1T1) of the donor moiety is almost doubled in the presence of a paramagnetic high-spin metal-porphyrin acceptor. The significant spectral overlap of the donor fluorescence and acceptor absorption in both series allows for efficient singlet energy transfer (Förster mechanism). Furthermore, the bridging chromophores mediate energy transfer and the enhancement is inversely dependent upon the energy gap between the donor and bridge excited states. Although Marcus theory predicts thermodynamically favorable electron transfer to occur in the systems investigated, the quenching rate constants were found to be independent of solvent polarity, and no charge-separated state could be detected, indicating very small electronic coupling for electron transfer.Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2001/f2779_s.pdf or from the author. 1H NMR spectra of FeP, FeP-OB, FeP-BB, FeP-NB, FeP-AB, ZnP-OB-FeP, ZnP-BB-FeP, ZnP-NB-FeP, and ZnP-AB-FeP.
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| 8. |
- Kuznetsov, Alexei N., et al.
(författare)
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Ab initio Calculations on Bismuth Cluster Polycations
- 2001
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Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 7:13, s. 2821-2828
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio calculations on bismuth polycationic species of the types Bi-n((n-2)+), Bi-n((n-4)+), and Bi-n((n-6)+) (n = 3 -12) were performed at the Hartree -Fock and density functional theory levels in order to investigate their general properties and the applicability of Wade's rules on bismuth polycations. Some exceptions to Wade's rules were encountered, and, moreover, several predicted and calculated minima shaw only meta-stable behavior. The bonding in bismuth polycations is characterized by a high degree of electron delocalization and "three-dimensional aromaticity".
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| 9. |
- Lindström, U. M., et al.
(författare)
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Aza- 3,3 -claisen enolate rearrangement in vinylaziridines : Stereoselective synthesis of mono-, di-, and trisubstituted seven-membered lactams
- 2001
-
Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 7:1, s. 94-98
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Tidskriftsartikel (refereegranskat)abstract
- Several 2,3-disubstituted vinylaziridines have been N-acylated and subjected to LiHMDS in THF at - 78 degreesC. Upon warming to room temperature, the resulting amide enolates underwent a highly stereoselective [3,3]-sigmatropic rearrangement to give mono-, di-, and trisubstituted seven-membered lactams in good yields. The scope and limitations of the process have been investigated by using variously substituted vinylaziridines. A kinetically controlled process proceeding through a six-membered boatlike transition state assembly has been invoked to explain the stereochemical outcome of the reaction.
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| 10. |
- Samec, Joseph S. M., et al.
(författare)
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Ruthenium-catalyzed transfer hydrogenation of imines by propan-2-ol in benzene.
- 2002
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Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 8:13, s. 2955-2961
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Tidskriftsartikel (refereegranskat)abstract
- Transfer hydrogenation of a variety of different imines to the corresponding amines by propan-2-ol in benzene catalyzed by [Ru2(CO)4(Ό-H)(C4Ph4COHOCC4Ph4)] (1) has been studied. The reaction is highly efficient with turnover frequencies of over 800 per h, and the product amines were obtained in excellent yields. A remarkable concn. dependence of propan-2-ol was obsd. when the reaction was run in benzene as cosolvent. An optimum was obtained at 24 equiv of propan-2-ol to imine, and further increase of the propan-2-ol led to a dramatic decrease in rate. Also the use of polar cosolvents with 24 equiv of propan-2-ol gave a low rate. It was found that ketimines react faster than aldimines and that electron-donating substituents on the imine increase the rate of the catalytic transfer hydrogenation. Electron-withdrawing substituents decreased the rate. An isomerization was obsd. with imines having an α-hydrogen at the N-alkyl substituent, which is in accordance with a mechanism involving a ruthenium-amine intermediate. It was demonstrated that the ruthenium-amine complex from α-methylbenzylamine, corresponding to the postulated intermediate, can replace 1 as catalyst in the transfer hydrogenation of imines. A primary deuterium isotope effect of kCH/CD = 2.7 ± 0.25 was obsd. when 2-deuterio-propan-2-ol was used in place of propan-2-ol in the transfer hydrogenation of N-phenyl-(1-phenylethylidene)amine. [on SciFinder(R)]
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| 11. |
- Sohnel, T., et al.
(författare)
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The stability of gold iodides in the gas phase and the solid state
- 2001
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Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 7:14, s. 3167-3173
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Tidskriftsartikel (refereegranskat)abstract
- The stability of gold iodides in the oxidation state +I and +III is investigated at the ab initio and density functional level using relativistic and nonrelativistic energy-adjusted pseudopotentials for gold and iodine. The calculations reveal that relativistic effects stabilize the higher oxidation state of gold as expected, that is Au2I6 is thermodynamically stable at the relativistic level, whilst at the nonrelativistic level the complex of two iodine molecules weakly bound to both gold atoms in Au2I2 is energetically preferred. The rather low stability of AuI3 with respect to dissociation into AuI and I-2 Will make it difficult to isolate this species in the solid state as (possibly) Au2I6 or detect it by matrix-isolation techniques. The monomer Au-3, is Jahn-Teller distorted from the ideal trigonal planar (D-3h) form, but adopts a Y-shaped structure (in contrast to AuF3 and AuCl3), and in the nonrelativistic case can be described as I-2 weakly bound to AuI. Relativistic effects turn AuI3 from a static Jahn-Teller system to a dynamic one. For the yet undetected gas-phase species AuI accurate coupled-cluster calculations for the potential energy curve are used to predict vibrational-rotational constants. Solid-state density functional calculations are performed for AuI and Au2I6 in order to predict cohesive energies.
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| 12. |
- Svensson, Per H., et al.
(författare)
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Metal Iodides in Polyiodide Networks -The Structural Chemistry of Complex Gold Iodides with Excess of Iodine
- 1999
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Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 5:1, s. 305-311
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Tidskriftsartikel (refereegranskat)abstract
- Theoretical calculations that compare the Ij ion and the [AuI4](-) ion show that they are closely related and have potential energy surface (PES) minima corresponding to an L-shaped structure. These calculations also indicate that the I-3(-) and [AuI2](-) ions should be exchangeable. These results were confirmed by the synthesis of the compounds (Et3S)[AuI4]. 2I(2) (1) and (Me3S)(2)[AuI4][I-3] (4), which have been characterised by X-ray diffraction, Raman and far-IR spectroscopy. The structure of 1 is made up of [AuI4](-) units coordinated by infinite zig-zag chains of I, molecules, and can be regarded as [AuI4](-) ions incorporated into a polyiodide network. The structure of 4 is closely related to those of the compounds of the M2Au2X6 family (M = Cs+, Rb+, NH4+, K+; X = Cl-, Br-, I-) which consist of square-planar [AuX4](-) and linear [AuX2](-) ions. However, in the structure of 4, the [AuX2](-) ions are replaced by IS ions.
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| 13. |
- Abbasi, Alireza, et al.
(författare)
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Highly hydrated cations : Deficiency, mobility and coordination of water in crystalline nonahydrated scandium(III), yttrium(III) and lanthanoid(III) trifluoromethanesulfonate
- 2005
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 11:14, s. 4065-4077
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Tidskriftsartikel (refereegranskat)abstract
- Trivalent lanthanide-like metal ions coordinate nine water oxygen atoms, which form a tricapped trigonal prism in a large number of crystalline hydrates. Water deficiency, randomly distributed over the capping positions, was found for the smallest metal ions in the isomorphous nonahydrated trifluoromethanesulfonates, [M(H2O)(n)]CF3SO3)(3), in which M=Sc-III, Lu-III, Yb-III, Tm-III or Er-III. The hydration number n increases (n=8.0(1), 8.4(1), 8.7(1), 8.8(1) and 8.96(5), respectively) with increasing ionic size. Deuterium (H-2) solid-state NMR spectroscopy revealed fast positional exchange between the coordinated capping and prism water molecules; this exchange started at temperatures higher than about 280 K for lutetium(m) and below 268 K for scandium(m). Similar positional exchange for the fully nonahydrated yttrium(m) and lanthanum(m) compounds started at higher temperatures, over about 330 and 360 K, respectively. An exchange mechanism is proposed that can exchange equatorial and capping water molecules within the restrictions of the crystal lattice, even for fully hydrated lanthanoid(III) ions. Phase transitions occurred for all the water-deficient compounds at; 185 K. The hydrated scandium(III) trifluoromethanesulfonate transforms reversibly (Delta H degrees= -0.80(1) kJ mol(-1) on cooling) to a trigonal unit cell that is almost nine times larger, with the scandium ion surrounded by seven fully occupied and two partly occupied oxygen atom positions in a distorted capped trigonal prism. The hydrogen bonding to the trifluoromethanesulfonate anions stabilises the trigonal prism of water ligands, even for the crowded hydration sphere of the smallest metal ions in the series. Implications for the Lewis acid catalytic activity of the hydrated scandium(III) and lanthanoid(III) trifluoromethanesulfonates for organic syntheses performed in aqueous media are discussed.
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| 14. |
- Abdi, Zahra, et al.
(författare)
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In Situ Synthesis of Manganese Oxide as an Oxygen-Evolving Catalyst : A New Strategy
- 2021
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Ingår i: Chemistry - A European Journal. - : John Wiley & Sons. - 0947-6539 .- 1521-3765. ; 27:4, s. 1330-1336
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Tidskriftsartikel (refereegranskat)abstract
- All studies on oxygen-evolution reaction by Mn oxides in the presence of cerium(IV) ammonium nitrate (CAN) have been so far carried out by synthesizing Mn oxides in the first step. And then, followed by the investigation of the Mn oxides in the presence of oxidants for oxygen-evolution reaction (OER). This paper presents a case study of a new and promising strategy for in situ catalyst synthesis by the adding Mn-II to either CAN or KMnO4/CAN solution, resulting in the formation of Mn-based catalysts for OER. The catalysts were characterized by scanning electron microscopy, energy-dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. Both compounds contained nano-sized particles that catalyzed OER in the presence of CAN. The turnover frequencies for both catalysts were 0.02 (mmolO2 /mol(Mn).
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| 15. |
- Abujubara, Helal, et al.
(författare)
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Ionic Liquid-Mediated Approach for the Synthesis of Site-Specific Thioether Conjugates
- 2023
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Ingår i: Chemistry-a European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 29:28
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Tidskriftsartikel (refereegranskat)abstract
- Site-specific conjugation approaches are of great importance in drug discovery, notably for the synthesis of biochemical probes or molecular conjugates for targeted delivery. Herein, we report a mild ionic liquid (IL)-mediated thiolation technique that relies on the use of 1,3-ethyl-methyl imidazolium acetate, [C(2)mim][OAc] as a solvent and precursor to generate activated IL, as well as a solvent for the conjugation reaction. First, a focused library of active ILs was prepared for functionalizing/conjugating cysteine-containing small molecules and unprotected peptides. Interestingly, a bifunctional active IL could also be successfully employed as a linker for the conjugation of peptides lacking Cys. This study sets the ground for further investigation of the use of active ILs for modifying, labeling or conjugating larger and more complex therapeutic modalities such as proteins and antibodies.
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| 16. |
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| 17. |
- Afewerki, Samson, et al.
(författare)
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Direct Regiospecific and Highly Enantioselective Intermolecular α-Allylic Alkylation of Aldehydes by a Combination of Transition-Metal and Chiral Amine Catalysts
- 2012
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 18:10, s. 2972-2977
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Tidskriftsartikel (refereegranskat)abstract
- The first direct intermolecular regiospecific and highly enantioselective a-allylic alkylation of linear aldehydes by a combination of achiral bench-stable Pd0 complexes and simple chiral amines as co-catalysts is disclosed. The co-catalytic asymmetric chemoselective and regiospecific a-allylic alkylation reaction is linked in tandem with in situ reduction to give the corresponding 2-alkyl alcohols with high enantiomeric ratios (up to 98:2 e.r.; e.r.=enantiomeric ratio). It is also an expeditious entry to valuable 2-alkyl substituted hemiacetals, 2-alkyl-butane-1,4-diols, and amines. The concise co-catalytic asymmetric total syntheses of biologically active natural products (e.g., Arundic acid) are disclosed.
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| 18. |
- Agarwala, Hemlata
(författare)
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Correspondence of RuIIIRuII and RuIVRuIII Mixed Valent States in a SmallDinuclear Complex
- 2012
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 18, s. 5667-5675
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Tidskriftsartikel (refereegranskat)abstract
- The dirutheniumACHTUNGTRENUNG(III) compound[(m-oxa){RuACHTUNGTRENUNG(acac)2}2] [1, oxa2=oxamidato(2), acac=2,4-pentanedionato]exhibits an S=1 ground statewith antiferromagnetic spin-spin coupling(J=40 cm1). The molecularstructure in the crystal of 1·2C7H8 revealedan intramolecular metal–metaldistance of 5.433 and a notableasymmetry within the bridging ligand.Cyclic voltammetry and spectroelectrochemistry(EPR, UV/Vis/NIR) of thetwo-step reduction and of the two-stepoxidation (irreversible second step)produced monocation and monoanionintermediates (Kc=105.9) with broadNIR absorption bands (e ca.2000m1cm1) and maxima at 1800 (1)and 1500 nm (1+). TD-DFT calculationssupport a RuIIIRuII formulationfor 1 with a doublet ground state. The1+ ion (RuIVRuIII) was calculated withan S=3/2 ground state and the doubletstate higher in energy (DE=694.6 cm1). The Mulliken spin densitycalculations showed little participationof the ligand bridge in the spin accommodationfor all paramagnetic species[(m-oxa){RuACHTUNGTRENUNG(acac)2}2]n, n=+1, 0, 1,and, accordingly, the NIR absorptionswere identified as metal-to-metal (intervalence)charge transfers. Whereasonly one such NIR band was observedfor the RuIIIRuII (4d5/4d6) system 1,the RuIVRuIII (4d4/4d5) form 1+ exhibitedextended absorbance over the UV/Vis/NIR range.
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| 19. |
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| 20. |
- Ahlberg, P., et al.
(författare)
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Solvated CH5+ in liquid superacid
- 2001
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Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 7:12, s. 2501-2510
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Tidskriftsartikel (refereegranskat)
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| 21. |
- Ahlford, Katrin, et al.
(författare)
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Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Amino Acid Derived Rhodium Complexes : On the Origin of Enantioselectivity and Enantioswitchability
- 2009
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 15:42, s. 11197-11209
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Tidskriftsartikel (refereegranskat)abstract
- Amino acid based thioamides, hydroxamic acids, and hydrazides have been evaluated as ligands in the rhodium-catalyzed asymmetric transfer hydrogenation of ketones in 2-propanol. Catalysts containing thioamide ligands derived from L-valine were found to selectively generate the product with an R configuration (95 % ee), whereas the corresponding L-valine-based hydroxamic acids or hydrazides facilitated the formation of the (S)-alcohols (97 and 91 % ee, respectively). The catalytic reduction was examined by performing a structure–activity correlation investigation with differently functionalized or substituted ligands and the results obtained indicate that the major difference between the thioamide and hydroxamic acid based catalysts is the coordination mode of the ligands. Kinetic experiments were performed and the rate constants for the reduction reactions were determined by using rhodium–arene catalysts derived from amino acid thioamide and hydroxamic acid ligands. The data obtained show that the thioamide-based catalyst systems demonstrate a pseudo-first-order dependence on the substrate, whereas pseudo-zero-order dependence was observed for the hydroxamic acid containing catalysts. Furthermore, the kinetic experiments revealed that the rate-limiting steps of the two catalytic systems differ. From the data obtained in the structure–activity correlation investigation and along with the kinetic investigation it was concluded that the enantioswitchable nature of the catalysts studied originates from different ligand coordination, which affects the rate-limiting step of the catalytic reduction reaction.
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| 22. |
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| 23. |
- Ahlquist, Mårten, et al.
(författare)
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An experimental and theoretical study of the mechanism of stannylcupration of alpha, beta-acetylenic ketones and esters
- 2006
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 12:10, s. 2866-2873
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Tidskriftsartikel (refereegranskat)abstract
- The title reaction has been investigated by experimental and computational (DFT) techniques, and subsequently compared to the corresponding carbocupration reaction, with particular emphasis oil the stereoselectivity. For stannylcupration of an ynone substrate, only the anti-addition product is observed, whereas for the corresponding ynoate substrate, the stereoselectivity can be affected by the reaction conditions: in the presence of methanol as proton donor, the initial syn-addition product can be trapped, whereas a syn/anti mixture is obtained in a non-protic solvent. This is in sharp contrast to the carbocupration of the same ynone substrate with a cyanocuprate (RCu(CN)Li), which is highly selective for syn-addition. The product selectivities can be understood from a detailed computational characterization of the reaction paths, and in particular from the relative stabilities of the vinyl cuprate and allenolate intermediates. It is suggested that the stereodetermining step is protonation of vinyl cuprate intermediates.
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| 24. |
- Ahlstrand, David A., et al.
(författare)
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Csp(3)-H Activation without Chelation Assistance in an Iridium Pincer Complex Forming Cyclometallated Products
- 2017
-
Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765. ; 23:8, s. 1748-1751
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Tidskriftsartikel (refereegranskat)abstract
- Cyclometallation of 8-methylquinoline and 2-(dimethylamino)-pyridine in an iridium-based pincer complex is described. The C-H activation of 2-(dimethylamino) pyridine is not chelation assisted, which has not been described before for Csp(3)-H bonds in cyclometallation reactions. The mechanism of the cyclometallation of 2-(dimethylamino) pyridine was studied by DFT calculations and kinetic measurements.
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| 25. |
- Ahmed, Mukhtiar, et al.
(författare)
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Ionic Liquids and Electrolytes with Flexible Aromatic Anions
- 2023
-
Ingår i: Chemistry - A European Journal. - : John Wiley & Sons. - 0947-6539 .- 1521-3765. ; 29:41
-
Tidskriftsartikel (refereegranskat)abstract
- Five new n-tetrabutylphosphonium (P4444)+ cation based ionic liquids (ILs) with oligoether substituted aromatic carboxylate anions have been synthesized. The nature and position of the oligoether chain affect thermal stability (up to 330 ºC), phase behaviour (Tg < -55 ºC) and ion transport. Furthermore, with the aim of application in lithium batteries, electrolytes were created for two of the ILs by 10 mol% doping using the corresponding Li-salts. This affects the ion diffusion negatively, from being higher and equal for cations and anions to lower for all ions and unequal. This is due to the stronger ionic interactions and formation of aggregates, primarily between the Li+ ions and the carboxylate group of the anions. Electrochemically, the electrolytes have electrochemical stability windows up to 3.5 V, giving some promise for battery application.
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| 26. |
- Akkarasamiyo, Sunisa, et al.
(författare)
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Tsuji-Trost Reaction of Non-Derivatized Allylic Alcohols
- 2018
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 24:14, s. 3488-3498
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Tidskriftsartikel (refereegranskat)abstract
- Palladium-catalyzed allylic substitution of non-derivatized enantioenriched allylic alcohols with a variety of uncharged N-, S-, C- and O-centered nucleophiles using a bidentate BiPhePhos ligand is described. A remarkable effect of the counter ion (X) of the XPd[kappa(2)-BiPhePhos][kappa(3)-C3H5] was observed. When ClPd[kappa(2)-BiPhePhos][eta(3)-C3H5] (complexI) was used as catalyst, non-reproducible results were obtained. Study of the complex by X-ray crystallography, (PNMR)-P-31 spectroscopy, and ESI-MS showed that a decomposition occurred where one of the phosphite ligands was oxidized to the corresponding phosphate, generating ClPd[kappa(1)-BiPhePhosphite-phosphate][eta(3)-C3H5] species (complexII). When the chloride was exchanged to the weaker coordinating OTf- counter ion the more stable Pd[kappa(2)-BiPhePhos][eta(3)-C3H5](+)+[OTf] (-) (complexIII) was formed. ComplexIII performed better and gave higher enantiospecificities in the substitution reactions. ComplexIII was evaluated in Tsuji-Trost reactions of stereogenic non-derivatized allylic alcohols. The desired products were obtained in good to excellent yields (71-98%) and enantiospecificities (73-99%) for both inter- and intramolecular substitution reactions with only water generated as a by-product. The methodology was applied to key steps in total synthesis of (S)-cuspareine and (+)-lentiginosine. A reaction mechanism involving a palladium hydride as a key intermediate in the activation of the hydroxyl group is proposed in the overall transformation.
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| 27. |
- Albinsson, Bo, 1963, et al.
(författare)
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Conformers of saturated chains: Matrix isolation, structure, IR and UV spectra of n-Si(4)Me(10)
- 1996
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Ingår i: Chemistry - A European Journal. - 1521-3765 .- 0947-6539. ; 2:5, s. 529-538
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Tidskriftsartikel (refereegranskat)abstract
- Infrared and ultraviolet spectra of the gauche and anti conformers of matrix-isolated permethyl-n-tetrasilane have been obtained separately by taking advantage of thermally induced gauche-to-anti conversion and of wavelength-selective photochemical destruction of either conformer. The resolved UV spectrum of the gauche conformer provides the first piece of experimental evidence in favor of the recently proposed reinterpretation of conformational effects on tetrasilane electronic states. According to this, it is not the energy but the intensity of the lowest singlet excitation that changes dramatically as the SiSiSiSi dihedral angle is varied, as a result of an avoided crossing between sigma sigma* and sigma pi* states. Implications for the general understanding of sigma conjugation in simple terms are discussed. Unconstrained MP2/6-31G* optimization predicts the existence of a third backbone conformer (ortho), with a dihedral angle of about 90 degrees. Its predicted (HF/3-21G*) mid-IR spectrum is indistinguishable from that of the gauche conformer, and the matrix-isolation spectra thus provide no evidence for or against its presence.
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| 28. |
- Ali, Md. Ehesan, et al.
(författare)
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Unraveling the Electronic Structure, Spin States, Optical and Vibrational Spectra of Malaria Pigment
- 2015
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Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 21:23, s. 8544-8553
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Tidskriftsartikel (refereegranskat)abstract
- A detailed knowledge of the electronic structure and magnetic and optical properties of hemozoin, the malaria pigment, is essential for the design of effective antimalarial drugs and malarial diagnosis. By employing state-of-the-art electronic structure calculations, we have performed an in-depth investigation of the malaria pigment. Specifically, molecular bond lengths and spin states of the two Fe-III heme centers and their exchange interaction, the UV/Vis absorption spectrum, and the IR vibrational spectra were calculated and compared with available experimental data. Our density functional theory (DFT)-based calculations predict a singlet ground spin state that stems from an S=5/2 spin state on each of the Fe heme centers with a very weak antiferromagnetic exchange interaction between them. Our theoretical UV/Vis and IR spectra provide explanations for various spectroscopic studies of hemozoin and -hematin (a synthetic analogue of hemozoin). A good comparison of calculated and measured properties demonstrates the convincing unveiling of the electronic structure of the malaria pigment. Based on the predicted vibrational spectra, we propose a unique spectral band from the nuclear resonance vibrational spectroscopy (NRVS) results that could be used as a key fingerprint for malarial detection.
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| 29. |
- Allert, M, et al.
(författare)
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Setting the stage for new catalytic functions in designed proteins - Exploring the imine pathway in the efficient decarboxylation of oxaloacetate by an Arg-Lys site in a four-helix bundle protein scaffold
- 2002
-
Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 8:11, s. 2549-2560
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Tidskriftsartikel (refereegranskat)abstract
- Fourteen 42-residue polypeptides have been designed to identify reactive sites for the catalysis of the decarboxylation of oxaloacetate, a chemical transformation that proceeds through the formation of an imine intermediate. The sequences fold into helix-loop-helix motifs and dimerise to four-helix bundles. The catalytically active lysine residues were incorporated in several surface exposed positions, but also in positions characterised by hydrophobic properties to reduce their pK(a) values. The molecular environments of the Lys residues were systematically varied, to find which residues were able to stabilise and bind the imine intermediate in the decarboxylation reaction. A two-residue Arg-Lys site formed the main component of the reactive site of the helix-loop-helix dimer Decarb-K34_R33, which obeyed saturation kinetics in catalysing the reaction with a k(cat)/K-M of 0.59m(-1) s(-1). The rate constant measured was nearly three orders of magnitude larger than the second-order rate constant of the butylamine-catalysed reaction (0.0011 M-1 s(-1)), and four orders of magnitude larger than the pseudo first-order rate constant of the uncatalysed reaction (1.3 x 10(-5) s(-1)). The sequence of Decarb-K34_R33 contained only a single lysine residue. It was flanked by an arginine in the preceding position in the sequence. A flanking Arg residue provided more efficient catalysis than a flanking Lys or Gln residue. Arginines in flanking positions in the helix, in positions four residues before or after the Lys in the sequence, are not as important in catalysis as the Arg of the Arg-Lys pair. The effect of pK(a) on the catalytic efficiency of the Lys residue in the decarboxylation reaction is well known. The identification of the role of the flanking Arg residue in catalysing decarboxylation, its optimal position, and the importance of conformational stability reported here sets the stage for developing a number of catalytic systems that depend on the formation of imine intermediates, but that lead to different reaction products.
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| 30. |
- Amorati, Riccardo, et al.
(författare)
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Multi-faceted reactivity of alkyltellurophenols towards peroxyl radicals : Catalytic antioxidant versus thiol-depletion effect
- 2013
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 19:23, s. 7510-7522
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Tidskriftsartikel (refereegranskat)abstract
- Hydroxyaryl alkyl tellurides are effective antioxidants both in organic solution and aqueous biphasic systems. They react by an unconventional mechanism with ROO. radicals with rate constants as high as 107M1s1 at 303K, outperforming common phenols. The reactions proceed by oxygen atom transfer to tellurium followed by hydrogen atom transfer to the resulting RO. radical from the phenolic OH. The reaction rates do not reflect the electronic properties of the ring substituents and, because the reactions occur in a solvent cage, quenching is more efficient when the OH and TeR groups have an ortho arrangement. In the presence of thiols, hydroxyaryl alkyl tellurides act as catalytic antioxidants towards both hydroperoxides (mimicking the glutathione peroxidases) and peroxyl radicals. The high efficiency of the quenching of the peroxyl radicals and hydroperoxides could be advantageous under normal cellular conditions, but pro-oxidative (thiol depletion) when thiol concentrations are low.
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| 31. |
- Andaloussi, Mounir, et al.
(författare)
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Microwave-promoted palladium(II)-catalyzed C-P bond formation by using arylboronic acids or aryltrifluoroborates.
- 2009
-
Ingår i: Chemistry - A European Journal. - Weinheim : Wiley-VCH Verlag GmbH. - 0947-6539 .- 1521-3765. ; 15:47, s. 13069-13074
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Tidskriftsartikel (refereegranskat)abstract
- The first Pd-II-catalyzed P arylation has been performed by using palladium acetate, the rigid bidentate ligand dmphen (dmphen=2,9-dimethyl-1,10-phenanthroline), and without the addition of base or acid. Couplings of arylboronic acids or aryl trifluoroborates with H-phosphonate dialkyl esters were conducted in 30 min with controlled microwave (MW) heating under non-inert conditions. Aryl phosphites were also synthesized at room temperature with atmospheric air as the sole reoxidant. The arylated phosphonates were isolated in 44-90% yields. The excellent chemoselectivity of the method was illustrated in the synthesis of a Mycobacterium tuberculosis glutamine synthetase (MTB-GS) inhibitor. Online ESIMS was used to detect cationic palladium species in ongoing reactions directly, and a catalytic cycle has been proposed based on these results.
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| 32. |
- Anderson, LK, et al.
(författare)
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Control of lysine reactivity in four-helix bundle proteins by site-selective pK(a) depression : Expanding the versatility of proteins by postsynthetic functionalisation
- 2002
-
Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 8:16, s. 3687-3697
-
Tidskriftsartikel (refereegranskat)abstract
- Five 42-residue polypeptides have been designed to fold into hairpin helix-loop-helix motifs that dimerise to form four-helix bundles, and to serve as protein scaffolds for the elucidation at the molecular level of the principles that control and fine-tune lysine and ornithine reactivities in a protein context. Site-selective control of Lys and Orn reactivity provides a mechanism for addressing directly individual residues and is a prerequisite for the site-selective functionalisation of folded proteins. Several lysine and one ornithine residues were introduced on the surface and in the hydrophobic core of the folded motif. The reactivity of each residue was determined by measuring the degree of acylation of the trypsin cleaved fragments by HPLC and mass spectrometry. The most reactive residues were Orn34 and Lys19, both of which were located in d positions in the heptad repeat, and therefore in hydrophobic environments. Upon reaction of the helix-loop-helix dimer KA-I with one equivalent of mono-p-nitrophenyl fumarate, Orn34 was acylated approximately three times more efficiently than Lys19, whereas Lys10 (b position), Lys15 (g position), and Lys33 (c position) remained unmodified. In the sequence KA-I-A(15) Lys15 was replaced by an alanine residue and the selectivity of Orn34 over Lys19 increased to approximately a factor of six, probably because Lys15 had the capacity to reduce the pK(a) value of Lys19 and 85 % of site-selectively monoacylated product was obtained. The pH dependence of the acylation reaction was determined and showed that the pK(a) of the reactive residues were 9.3, more than a pK(a) unit below the magnitude of the corresponding residue in a solvent exposed position. Introducing Lys and Orn residues into a or d positions of the heptad repeat therefore serves as a mechanism of depressing their pKa to increase their reactivity site selectively. Extensive NMR and CD spectroscopic analyses showed that the sequences fold according to prediction.
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| 33. |
- Andersson, Johanna, 1983, et al.
(författare)
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AT-Specific DNA Binding of Binuclear Ruthenium Complexes at the Border of Threading Intercalation
- 2010
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 16:36, s. 11037-11046
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Tidskriftsartikel (refereegranskat)abstract
- The binuclear ruthenium complex [mu-bidppz(phen)(4)Ru-2](4+) has been extensively studied since the discover}, of its unusual threading intercalation interaction with DNA, a binding mode with extremely slow binding and dissociation kinetics. The complex has been shown to be selective towards long stretches of alternating AT base pairs, which makes it interesting, for example, as a model compound for anti-malaria drugs due to the high AT content of the genome of the malaria parasite P falciparum. We have investigated the effect of bridging ligand structure on threading intercalation ability and found that length and rigidity as well as the size of the intercalated ring system are all factors that affect the rate and selectivity of the threading intercalation. In particular, we discovered a new DNA-threading compound, [mu-dppzip(phen)(4)Ru-2](4+) which appears to be just at the border of being capable of threading intercalation and displays even greater selectivity for AT-DNA than the parent compound, [mu-bidppz(phen)(4)Ru-2](4+).
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| 34. |
- Andreasson, Joakim, 1973, et al.
(författare)
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An All-Photonic Molecular Keypad Lock
- 2009
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 15:16, s. 3936-3939
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Tidskriftsartikel (refereegranskat)abstract
- An all photonic molecular keypad lock with a reset capacity, that is capable of distinguishing one two-digit code out of eight possible combinations were described. The synthesis and characterization of triad 1 with distilled 2-methyltetrahydrofuran as the solvent for spectroscopic measurement were reported, where the samples were degassed by size freeze-pump-thaw cycles to a final pressure. The functional principle of a keypad lock is that it gives an output signal only when given the correct inputs in the correct order, also the capacity of the triad to perform as a two-input PAND gate with reset capacity are considered. The keypad lock is found to be operated by five optical signals in the UV region including fluorescence emission that signals the actual opening of the lock, which can also be used numerous times without ant photodegradation affecting the performance.
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| 35. |
- Andréasson, Måns, et al.
(författare)
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Using Macrocyclic G-Quadruplex Ligands to Decipher the Interactions Between Small Molecules and G-Quadruplex DNA
- 2022
-
Ingår i: Chemistry - A European Journal. - : John Wiley & Sons. - 0947-6539 .- 1521-3765. ; 28:65
-
Tidskriftsartikel (refereegranskat)abstract
- This study aims to deepen the knowledge of the current state of rational G4-ligand design through the design and synthesis of a novel set of compounds based on indoles, quinolines, and benzofurans and their comparisons with well-known G4-ligands. This resulted in novel synthetic methods and G4-ligands that bind and stabilize G4 DNA with high selectivity. Furthermore, the study corroborates previous studies on the design of G4-ligands and adds deeper explanations to why a) macrocycles offer advantages in terms of G4-binding and -selectivity, b) molecular pre-organization is of key importance in the development of strong novel binders, c) an electron-deficient aromatic core is essential to engage in strong arene-arene interactions with the G4-surface, and d) aliphatic amines can strengthen interactions indirectly through changing the arene electrostatic nature of the compound. Finally, fundamental physicochemical properties of selected G4-binders are evaluated, underscoring the complexity of aligning the properties required for efficient G4 binding and stabilization with feasible pharmacokinetic properties.
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| 36. |
- Andreoni, Alessio, et al.
(författare)
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Fluorescence Correlation Spectroscopy of Labeled Azurin Reveals Photoinduced Electron Transfer between Label and Cu Center
- 2018
-
Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765. ; 24:3, s. 646-654
-
Tidskriftsartikel (refereegranskat)abstract
- Fluorescent labeling of biomacromolecules enjoys increasing popularity for structural, mechanistic, and microscopic investigations. Its success hinges on the ability of the dye to alternate between bright and dark states. Forster resonance energy transfer (FRET) is an important source of fluorescence modulation. Photo-induced electron transfer (PET) may occur as well, but is often considered only when donor and acceptor are in van der Waals contact. In this study, PET is shown between a label and redox centers in oxidoreductases, which may occur over large distances. In the small blue copper protein azurin, labeled with ATTO655, PET is observed when the label is at 18.5 angstrom, but not when it is at 29.1 angstrom from the Cu. For Cu-II, PET from label to Cu occurs at a rate of (4.8 +/- 0.3) x 10(4) s(-1) and back at (0.7 +/- 0.1) x 10(3) s(-1). With Cu-I the numbers are (3.3 +/- 0.7) x 10(6) s(-1) and (1.0 +/- 0.1) x 10(4) s(-1). Reorganization energies and electronic coupling elements are in the range of 0.8-1.2 eV and 0.02-0.5 cm(-1), respectively. These data are compatible with electron transfer (ET) along a through-bond pathway although transient complex formation followed by ET cannot be ruled out. The outcome of this study is a useful guideline for experimental designs in which oxidoreductases are labelled with fluorescent dyes, with particular attention to single molecule investigations. The labelling position for FRET can be optimized to avoid reactions like PET by evaluating the structure and thermodynamics of protein and label.
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| 37. |
- Ankner, Tobias, et al.
(författare)
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Palladium- and Nickel-Catalyzed Alkenylation of Enolates
- 2013
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 19:6, s. 1858-1871
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Forskningsöversikt (refereegranskat)abstract
- Transition-metal-catalyzed alkenylation of enolates provides a direct method to synthesize broadly useful ,-unsaturated carbonyl compounds from the corresponding carbonyl compound and alkenyl halides. Despite being reported in the early seventies, this reaction class saw little development for many years. In the past decade, however, efforts to develop this reaction further have increased considerably, and many research groups have reported efficient coupling protocols, including enantioselective versions. These reactions most commonly employ palladium catalysts, but there are also some important reports using nickel. There are many examples of this powerful transformation being used in the synthesis of complex natural products.
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| 38. |
- Appukkuttan, Prasad, et al.
(författare)
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Microwave-assisted transition-metal-catalyzed synthesis of N-shifted and ring-expanded buflavine analogues
- 2007
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 13:22, s. 6452-6460
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Two novel and efficient strategies for the synthesis of hitherto unknown N-shifted and ring-expanded buflavine analogues are presented. Construction of the medium-sized ring system of the title molecules, a difficult task due to the high activation energy needed for the ring-closure with the additional rigidity imposed by the biaryl skeleton, was achieved by using Suzuki-Miyaura biaryl coupling and a ring-closing metathesis reaction as the key steps. The combination of a second-generation Grubbs catalyst and microwave irradiation proved to be highly useful in generating the otherwise difficult to obtain medium-sized ring system of the buflavine analogues.
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| 39. |
- Aramburo, Luis R., et al.
(författare)
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The Porosity, Acidity, and Reactivity of Dealuminated Zeolite ZSM-5 at the Single Particle Level : The Influence of the Zeolite Architecture
- 2011
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 17:49, s. 13773-13781
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Tidskriftsartikel (refereegranskat)abstract
- A combination of atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), focused-ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron-based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM-5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol-to-olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM-5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM-5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 550 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Bronsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.
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| 40. |
- Arkhypchuk, Anna I., et al.
(författare)
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Oxaphospholes and Bisphospholes from Phosphinophosphonates and alpha,beta-Unsaturated Ketones
- 2013
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 19:41, s. 13692-13704
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Tidskriftsartikel (refereegranskat)abstract
- The reaction of a {W(CO)(5)}-stabilized phosphinophosphonate 1, (CO)(5)WPH(Ph)P(O)(OEt)(2), with ethynyl- (2a-f) and diethynylketones (7-11, 18, and 19) in the presence of lithium diisopropylamide (LDA) is examined. Lithiated 1 undergoes nucleophilic attack in the Michael position of the acetylenic ketones, as long as this position is not sterically encumbered by bulky (iPr)(3)Si substituents. Reaction of all other monoacetylenic ketones with lithiated 1 results in the formation of 2,5-dihydro-1,2-oxaphospholes 3 and 4. When diacetylenic ketones are employed in the reaction, two very different product types can be isolated. If at least one (Me)(3)Si or (Et)(3)Si acetylene terminus is present, as in 7, 8, and 19, an anionic oxaphosphole intermediate can react further with a second equivalent of ketone to give cumulene-decorated oxaphospholes 14, 15, 24, and 25. Diacetylenic ketones 10 and 11, with two aromatic acetylene substituents, react with lithitated 1 to form exclusively ethenyl-bridged bisphospholes 16 and 17. Mechanisms that rationalize the formation of all heterocycles are presented and are supported by DFT calculations. Computational studies suggest that thermodynamic, as well as kinetic, considerations dictate the observed reactivity. The calculated reaction pathways reveal a number of almost isoenergetic intermediates that follow after ring opening of the initially formed oxadiphosphetane. Bisphosphole formation through a carbene intermediate G is greatly favored in the presence of phenyl substituents, whereas the formation of cumulene-decorated oxaphospholes is more exothermic for the trimethylsilyl-containing substrates. The pathway to the latter compounds contains a 1,3-shift of the group that stems from the acetylene terminus of the ketone substrates. For silyl substituents, the 1,3-shift proceeds along a smooth potential energy surface through a transition state that is characterized by a pentacoordinated silicon center. In contrast, a high-lying transition state TS(E-F)(R=Ph) of 37kcalmol(-1) is found when the substituent is a phenyl group, thus explaining the experimental observation that aryl-terminated diethynylketones 10 and 11 exclusively form bisphospholes 16 and 17.
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| 41. |
- Arkhypchuk, Anna I., et al.
(författare)
-
Redox Switching in Ethenyl- Bridged Bisphospholes
- 2014
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 20:49, s. 16083-16087
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Tidskriftsartikel (refereegranskat)abstract
- A 2e(-)/2H(+) redox platform has been implemented in the ethenyl-bridged bisphosphol-3-ol 1 to afford the first phospholes that feature chemically reversible oxidations. Oxidation of the title compounds to the corresponding bisphosphol-3-one 2 leads to a change in conjugation topology and a concomitant hypsochromic shift of the lowest-energy absorption maximum by 100nm. Electrochemical oxidation proceeds without any detectable intermediates, whereas the deprotonated form of 1 can be observed in an aprotic medium during the reduction of 2. This dianionic intermediate 3 is characterized by end absorptions that are bathochromically shifted by circa 200nm compared to those of 2.
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| 42. |
- Arpa Gonzalez, Enrique Manuel, et al.
(författare)
-
A Proof-of-Principle Design for Through-Space Transmission of Unidirectional Rotary Motion by Molecular Photogears
- 2023
-
Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765.
-
Tidskriftsartikel (refereegranskat)abstract
- The construction of molecular photogears that can achieve through-space transmission of the unidirectional double-bond rotary motion of light-driven molecular motors onto a remote single-bond axis is a formidable challenge in the field of artificial molecular machines. Here, we present a proof-of-principle design of such photogears that is based on the possibility of using stereogenic substituents to control both the relative stabilities of two helical forms of the photogear and the double-bond photoisomerization reaction that connects them. The potential of the design was verified by quantum-chemical modeling through which photogearing was found to be a favorable process compared to free-standing single-bond rotation ("slippage"). Overall, our study unveils a surprisingly simple approach to realizing unidirectional photogearing. A stereochemical approach to transmitting the directional double-bond rotary motion of light-driven molecular motors through space onto a remote single-bond axis is put forth and successfully tested by means of quantum-chemical modeling. A key result in the assessment of the approach is that the desired photogearing process is favorable compared to the undesired, free-standing single-bond rotation process ("slippage") with which it competes.**image
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| 43. |
- Arpa Gonzalez, Enrique Manuel, et al.
(författare)
-
Unveiling Photodegradation and Photosensitization Mechanisms of Unconjugated Pterins
- 2023
-
Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765. ; 29:29
-
Tidskriftsartikel (refereegranskat)abstract
- Unconjugated pterins are ubiquitous molecules that participate in countless enzymatic processes and are potentially involved in the photosensitization of singlet oxygen, amino acids, and nucleotides. Following electronic excitation with UV-A light, some of these pterins degrade, producing hydrogen peroxide as the main side product. This process, which is known to take place in vivo, contributes to oxidative stress and melanocyte destruction in vitiligo. In this work, we present for the first time mechanistic insight into the formation of transient triplet species that simultaneously trigger Type I and Type II photosensitizing processes and the initiation of degradation processes. Our calculations reveal that photodegradation of 6-biopterin, which accumulates in the skin of vitiligo patients, leads to 6-formylpterin through a retro-aldol reaction, and subsequently to 6-carboxypterin through a water-mediated aldehyde oxidation. Additionally, we show that the changes in the photosensitizing potential of these systems with pH come from the modulation of their excited-state redox potentials.
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| 44. |
- Arpa Gonzalez, Enrique Manuel, et al.
(författare)
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Unveiling Photodegradation and Photosensitization Mechanisms of Unconjugated Pterins
- 2023
-
Ingår i: Chemistry - A European Journal. - : WILEY-V C H VERLAG GMBH. - 0947-6539 .- 1521-3765.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Invited for the cover of this issue are Enrique M. Arpa (Linkoping University) and Ines Corral (Universidad Autonoma de Madrid). The image depicts two examples where pterin chemistry is relevant, the wing coloration of some butterflies and the cytotoxic action in vitiligo. Read the full text of the article at 10.1002/chem.202300519.
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| 45. |
- Artacho Ruiz, Josep, et al.
(författare)
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The synthesis and characterization of all diastereomers of a linear symmetrically fused tris-Troger's base analogue: New chiral cleft compounds
- 2006
-
Ingår i: Chemistry: A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 12:10, s. 2692-2701
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Tidskriftsartikel (refereegranskat)abstract
- The synthesis and characterization of all diastereomers of a linear symmetrically fused tris-Troger's base analogue are described. The diastereomers are unambiguously assigned as syn-anti 1a, anti-anti 1b, and syn-syn 1c isomers, by using X-ray diffraction analysis and NMR spectroscopy. For the first time, the anti-anti and the syn-syn diastereomers of a linear symmetrically fused tris-Troger's base analogue have been synthesized. Molecules 1a and 1c are new cleft compounds and analysis of compound 1a in the solid state shows inclusion of one molecule of CH2Cl2 in the larger aromatic cleft, whereas in isomer 1c disordered solvent molecules are trapped in the extended aromatic cleft. Furthermore, in the solid state, isomer 1c forms infinite open channels along one of the crystallographic axes and perpendicular to this axis there are infinitely extending "wedged-ravines". Importantly, each of the diastereomers 1a-c is resistant to inversion at the stereogenic nitrogen atoms under strongly and weakly acidic conditions in the range from room temperature (RT) to 95 degrees C. This observed configurational stability at the stereogenic nitrogens of 1a-c is unique for analogues of Troger's base in general to date. Finally, the ratio of cleft compounds 1a and 1c significantly increased relative to cavity compound 1b when ammonium chloride was used as an additive in the Troger's base condensation to 1a-c suggesting a templating effect of the ammonium ion.
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| 46. |
- Artacho Ruiz, Josep, et al.
(författare)
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Twisted Amide Analogues of Tröger's Base.
- 2012
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Ingår i: Chemistry: A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 18:4, s. 1038-1042
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Tidskriftsartikel (refereegranskat)abstract
- Let's do the twist: The first twisted bis-amide is obtained by the benzylic oxidation of Tröger's base (TB). Kinetic studies of its acidic hydrolysis reveal that the hydrolysis is to a large extent funneled through doubly protonated species.
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| 47. |
- Auer, F, et al.
(författare)
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Switchable assembly of stable, ordered molecular layers
- 1999
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Ingår i: Chemistry - A European Journal. - 0947-6539 .- 1521-3765. ; 5, s. 1150-1159
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Tidskriftsartikel (refereegranskat)abstract
- Bisamidines can be assembled on self-assembled monolayers of mercaptoalkanoic acids on gold to form stable and ordered but pH-switchable layers (see diagram). At basic pH the layers are stable and charge selective towards charged surfactants and plasma proteins. The system can potentially be used to reversibly introduce new surface properties for given applications that use one single substrate.
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| 48. |
- Ayub, Rabia, et al.
(författare)
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Cyclopropyl Group : An Excited-State Aromaticity Indicator?
- 2017
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Ingår i: Chemistry - A European Journal. - : Wiley-VCH Verlag. - 0947-6539 .- 1521-3765. ; 23:55, s. 13684-13695
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Tidskriftsartikel (refereegranskat)abstract
- The cyclopropyl (cPr) group, which is a well-known probe for detecting radical character at atoms to which it is connected, is tested as an indicator for aromaticity in the first ππ* triplet and singlet excited states (T1 and S1). Baird's rule says that the π-electron counts for aromaticity and antiaromaticity in the T1 and S1 states are opposite to Hückel's rule in the ground state (S0). Our hypothesis is that the cPr group, as a result of Baird's rule, will remain closed when attached to an excited-state aromatic ring, enabling it to be used as an indicator to distinguish excited-state aromatic rings from excited-state antiaromatic and nonaromatic rings. Quantum chemical calculations and photoreactivity experiments support our hypothesis; calculated aromaticity indices reveal that openings of cPr substituents on [4n]annulenes ruin the excited-state aromaticity in energetically unfavorable processes. Yet, polycyclic compounds influenced by excited-state aromaticity (e.g., biphenylene), as well as 4nπ-electron heterocycles with two or more heteroatoms represent limitations.
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| 49. |
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| 50. |
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