| 1. |
- Chang, Liu, 1986-
(författare)
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Ghost in the shell : Studies on subsurface oxygen in oxide-derived copper nanocube catalysts
- 2017
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the passage of time and the advancement of our industrial civilization, environmental concerns have become more and more recognized since the 1990s. Carbon dioxide reduction reactions are capable of converting carbon dioxide into valuable hydrocarbons and reducing the carbon emission from the combustion of fossil fuels. This is a promising direction for sustainable energy resources given that the scarcity of fossil fuels is becoming more threatening to the survival of mankind. In recent years, oxide-derived metal nanostructures have been synthesized and show unique catalytic features. Recently, Sloan et al. synthesized a novel oxide-derived copper nanocube structure, which showed a high selectivity toward ethylene over methane and low overpotentials. In this work, the presence of subsurface oxygen in the catalyst surface is tested with density functional theory (DFT) calculations, as a complement to experimental x-ray photoelectron spectroscopy. Due to limitations on the scale of modeling with DFT, the results indicate a very low stability of subsurface oxygen, which give rise to a question if subsurface oxygen would be stable with a reasonably large cluster model. Self-consistent charge density functional tight binding (SCC-DFTB) is adopted to investigate a nanocube model. In this model, a manually reduced cuprious oxide nanocube is constructed and investigated. Subsurface oxygen atoms close to facets are found to be more stable inside. A higher degree of disorder is proposed to be the cause of this difference in stabilizing subsurface oxygen atoms between the slab and nanocube models. The presence of subsurface oxygen enhances the adsorption of CO on the Cu(100) surface, increasing the likelihood for adsorbed CO molecules to dimerize, which is the rate determining step for ethylene production on Cu(100) under low-overpotential conditions. With subsurface electronegative atoms such as oxygen or fluorine, it is also found that the d-band scaling relation could be broken.
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| 2. |
- Liljeblad, Jonathan F.D. 1978-
(författare)
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Biomimetic Membranes: : Molecular Structure and Stability Studies by Vibrational Sum Frequency Spectroscopy
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the research presented in this licentiate thesis the surface specific technique Vibrational Sum Frequency Spectroscopy, VSFS, combined with the Langmuir trough has been utilized to investigate Langmuir monolayers and Langmuir-Blodgett (LB) deposited mono- and bilayers of phospholipids. Their molecular structure, stability, and hydration were probed to gain additional understanding of important properties aiming at facilitating the use of such layers as model systems for biological membranes. VSFS was applied to in situ studies of the degradation of Langmuir monolayers of 1,2-diacyl-phosphocholines with identical C-18 chains having various degrees of unsaturation. The time-dependent change of the monolayer area at constant surface pressure as well as the sum frequency intensity of the vinyl-CH stretch at the C=C double bonds were measured to monitor the degradation. It was shown that a rapid degradation of the monolayers of unsaturated phospholipids occurred when exposed to the laboratory air compared to the fully saturated lipid, and that the degradation could be inhibited by purging the ambient air with nitrogen. The degradation was attributed to oxidation mediated by reactive species in the air. The molecular structure and order of Langmuir monolayers of 1,2-distearoyl-phosphocholine (18:0 PC) and their hydrating water were investigated at different surface pressures using VSFS. The spectroscopic data indicated a well ordered monolayer at all surface pressures with a more intense signal at higher pressures attributed to the subsequent increase of the number density and more ordered lipid molecules due to the tighter packing. Water molecules hydrating the headgroups or being in contact with the hydrophobic parts were observed and distinguished by their vibrational frequencies, and found to have different average orientations. Additionally, monolayers of 18:0 PC, its fully deuterated analogue, and 1,2-distearoyl-phosphoserine (18:0 PS) were Langmuir-Blodgett (LB) deposited on CaF2 substrates and VSFS was used to investigate the structure and order of the films as well as the hydrating water. The CH-region, water region, and lower wavenumber region containing phosphate, ester, carboxylic acid, and amine signals were probed to obtain a complete picture of the molecule. The data indicates that all deposited monolayers formed a well ordered and stable film and the average orientation of the aliphatic chains was determined using the antisymmetric methyl stretch.
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| 3. |
- Yaempongsa, Dhebbajaj, Professor, et al.
(författare)
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Improving the Stability of Trinitramide by Chemical Substitution : N(NF2)(3) has Higher Stability and Excellent Propulsion Performance
- 2021
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Ingår i: Propellants, explosives, pyrotechnics. - : Wiley. - 0721-3115 .- 1521-4087. ; 46:2, s. 245-252
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Tidskriftsartikel (refereegranskat)abstract
- The potential for improving the stability of trinitramide (N(NO2)(3)) by chemical substitution of the NO2 group has been investigated using Kohn-Sham density functional theory [M06-2X/6-31+G(d,p)] and ab initio quantum chemistry [CBS-QB3]. Monosubstituted analogs are generally found to have a decreased N-NO2 bond dissociation enthalpy (BDE) because of increased stabilization of the N(NO2)X radical intermediate resulting from the bond cleavage. This is particularly apparent for N(NO2)(2)NH2, which has a BDE of only 54 kJ/mol. Instead it is shown that the stability of TNA can be significantly improved by substituting all three NO2 for the NF2 group. The resulting molecule, N(NF2)(3), has a N-N BDE of 138 kJ/mol, which is 17 kJ/mol higher than the N-N BDE of N(NO2)(3). In contrast to N(NO2)(3), there are no indications that the stability of N(NF2)(3) is significantly reduced in polar solvents. Condensed phase properties of N(NF2)(3) have been estimated based on surface electrostatic potential calculations, and N(NF2)(3) is estimated to be a liquid in the approximate temperature range of 170-290 K because of its nonpolar character. The performance of N(NF2)(3) in propellant formulations with fuels rich in hydrogen and/or aluminum has been investigated. N(NF2)(3) propellants are shown to outperform propellants based on standard oxidizers by up to 20 % in specific impulse and up to 100 % in density impulse. Compositions of N(NF2)(3) and HMX have significantly higher detonation performance than CL-20.
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| 4. |
- Erdtman, Edvin, 1981-
(författare)
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5-Aminolevulinic acid and derivatives thereof : properties, lipid permeability and enzymatic reactions
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 5-aminolevulinic acid (5-ALA) and derivatives thereof are widely usedprodrugs in treatment of pre-malignant skin diseases of the cancer treatmentmethod photodynamic therapy (PDT). The target molecule in 5-ALAPDTis protoporphyrin IX (PpIX), which is synthesized endogenously from5-ALA via the heme pathway in the cell. This thesis is focused on 5-ALA,which is studied in different perspectives and with a variety of computationalmethods. The structural and energetic properties of 5-ALA, itsmethyl-, ethyl- and hexyl esters, four different 5-ALA enols, and hydrated5-ALA have been investigated using Quantum Mechanical (QM) first principlesdensity functional theory (DFT) calculations. 5-ALA is found to bemore stable than its isomers and the hydrolysations of the esters are morespontaneous for longer 5-ALA ester chains than shorter. The keto-enoltautomerization mechanism of 5-ALA has been studied, and a self-catalysismechanism has been proposed to be the most probable. Molecular Dynamics(MD) simulations of a lipid bilayer have been performed to study themembrane permeability of 5-ALA and its esters. The methyl ester of 5-ALAwas found to have the highest permeability constant (PMe-5-ALA = 52.8 cm/s).The mechanism of the two heme pathway enzymes; Porphobilinogen synthase(PBGS) and Uroporphyrinogen III decarboxylase (UROD), have beenstudied by DFT calculations and QM/MM methodology. The rate-limitingstep is found to have a barrier of 19.4 kcal/mol for PBGS and 13.7kcal/mol for the first decarboxylation step in UROD. Generally, the resultsare in good agreement with experimental results available to date.
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| 5. |
- Linder, Mats, 1983-
(författare)
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Computational Studies and Design of Biomolecular Diels-Alder Catalysis
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Diels-Alder reaction is one of the most powerful synthetic tools in organic chemistry, and asymmetric Diels-Alder catalysis allows for rapid construction of chiral carbon scaffolds. For this reason, considerable effort has been invested in developing efficient and stereoselective organo- and biocatalysts. However, Diels-Alder is a virtually unknown reaction in Nature, and to engineer an enzyme into a Diels-Alderase is therefore a challenging task. Despite several successful designs of catalytic antibodies since the 1980’s, their catalytic activities have remained low, and no true artificial ’Diels-Alderase’ enzyme was reported before 2010.In this thesis, we employ state-of-the-art computational tools to study the mechanism of organocatalyzed Diels-Alder in detail, and to redesign existing enzymes into intermolecular Diels-Alder catalysts. Papers I–IV explore the mechanistic variations when employing increasingly activated reactants and the effect of catalysis. In particular, the relation between the traditionally presumed concerted mechanism and a stepwise pathway, forming one bond at a time, is probed. Papers V–X deal with enzyme design and the computational aspects of predicting catalytic activity. Four novel, computationally designed Diels-Alderase candidates are presented in Papers VI–IX. In Paper X, a new parameterization of the Linear Interaction Energy model for predicting protein-ligand affinities is presented.A general finding in this thesis is that it is difficult to attain large transition state stabilization effects solely by hydrogen bond catalysis. In addition, water (the preferred solvent of enzymes) is well-known for catalyzing Diels- Alder by itself. Therefore, an efficient Diels-Alderase must rely on large binding affinities for the two substrates and preferential binding conformations close to the transition state geometry. In Papers VI–VIII, we co-designed the enzyme active site and substrates in order to achieve the best possible complementarity and maximize binding affinity and pre-organization. Even so, catalysis is limited by the maximum possible stabilization offered by hydrogen bonds, and by the inherently large energy barrier associated with the [4+2] cycloaddition.The stepwise Diels-Alder pathway, proceeding via a zwitterionic intermediate, may offer a productive alternative for enzyme catalysis, since an enzyme active site may be more differentiated towards stabilizing the high-energy states than for the standard mechanism. In Papers I and III, it is demonstrated that a hydrogen bond donor catalyst provides more stabilization of transition states having pronounced charge-transfer character, which shifts the preference towards a stepwise mechanism.Another alternative, explored in Paper IX, is to use an α,β -unsaturated ketone as a ’pro-diene’, and let the enzyme generate the diene in situ by general acid/base catalysis. The results show that the potential reduction in the reaction barrier with such a mechanism is much larger than for conventional Diels-Alder. Moreover, an acid/base-mediated pathway is a better mimic of how natural enzymes function, since remarkably few catalyze their reactions solely by non-covalent interactions.
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| 6. |
- Rahm, Martin, 1982-
(författare)
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Green Propellants
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To enable future environmentally friendly access to space by means of solid rocket propulsion a viable replacement to the hazardous ammonium perchlorate oxidizer is needed. Ammonium dinitramide (ADN) is one of few such compounds currently known. Unfortunately compatibility issues with many polymer binder systems and unexplained solid-state behavior have thus far hampered the development of ADN-based propellants. Chapters one, two and three offer a general introduction to the thesis, and into relevant aspects of quantum chemistry and polymer chemistry. Chapter four of this thesis presents extensive quantum chemical and spectroscopic studies that explain much of ADN’s anomalous reactivity, solid-state behavior and thermal stability. Polarization of surface dinitramide anions has been identified as the main reason for the decreased stability of solid ADN, and theoretical models have been developed to explain and predict the solid-state stability of general dinitramide salts. Experimental decomposition characteristics for ADN, such as activation energy and decomposition products, have been explained for different physical conditions. The reactivity of ADN towards many chemical groups is explained by ammonium-mediated conjugate addition reactions. It is predicted that ADN can be stabilized by changing the surface chemistry with additives, for example by using hydrogen bond donors, and by trapping radical intermediates using suitable amine-functionalities. Chapter five presents several conceptual green energetic materials (GEMs), including different pentazolate derivatives, which have been subjected to thorough theoretical studies. One of these, trinitramide (TNA), has been synthesized and characterized by vibrational and nuclear magnetic resonance spectroscopy. Finally, chapter six covers the synthesis of several polymeric materials based on polyoxetanes, which have been tested for compatibility with ADN. Successful formation of polymer matrices based on the ADN-compatible polyglycidyl azide polymer (GAP) has been demonstrated using a novel type of macromolecular curing agent. In light of these results further work towards ADN-propellants is strongly encouraged.
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| 7. |
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| 8. |
- Brinck, Tore, Professor, 1965-, et al.
(författare)
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Anomalous π-backbonding in complexes between B(SiR3)3 and N2 : catalytic activation and breaking of scaling relations
- 2023
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 25:31, s. 21006-21019
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Tidskriftsartikel (refereegranskat)abstract
- Chemical transformations of molecular nitrogen (N2), including the nitrogen reduction reaction (NRR), are difficult to catalyze because of the weak Lewis basicity of N2. In this study, it is shown that Lewis acids of the types B(SiR3)3 and B(GeR3)3 bind N2 and CO with anomalously short and strong B-N or B-C bonds. B(SiH3)3·N2 has a B-N bond length of 1.48 Å and a complexation enthalpy of −15.9 kcal mol−1 at the M06-2X/jun-cc-pVTZ level. The selective binding enhancement of N2 and CO is due to π-backbonding from Lewis acid to Lewis base, as demonstrated by orbital analysis and density difference plots. The π-backbonding is found to be a consequence of constructive orbital interactions between the diffuse and highly polarizable B-Si and B-Ge bond regions and the π and π* orbitals of N2. This interaction is strengthened by electron donating substituents on Si or Ge. The π-backbonding interaction is predicted to activate N2 for chemical transformation and reduction, as it decreases the electron density and increases the length of the N-N bond. The binding of N2 and CO by the B(SiR3)3 and B(GeR3)3 types of Lewis acids also has a strong σ-bonding contribution. The relatively high σ-bond strength is connected to the highly positive surface electrostatic potential [VS(r)] above the B atom in the tetragonal binding conformation, but the σ-bonding also has a significant coordinate covalent (dative) contribution. Electron withdrawing substituents increase the potential and the σ-bond strength, but favor the binding of regular Lewis acids, such as NH3 and F−, more strongly than binding of N2 and CO. Molecules of the types B(SiR3)3 and B(GeR3)3 are chemically labile and difficult to synthesize. Heterogenous catalysts with the wanted B(Si-)3 or B(Ge-)3 bonding motif may be prepared by boron doping of nanostructured silicon or germanium compounds. B-doped and hydrogenated silicene is found to have promising properties as catalyst for the electrochemical NRR.
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| 9. |
- Brinck, Tore, Professor, 1965-, et al.
(författare)
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Octanol Water Partition-Coefficients Expressed In Terms Of Solute Molecular-Surface Areas And Electrostatic Potentials
- 1993
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Ingår i: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 58:25, s. 7070-7073
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Tidskriftsartikel (refereegranskat)abstract
- For 70 molecules of various types and sizes, it is shown that their experimental octanol/water partition coefficients can be represented quantitatively in terms of the solute's molecular surface area in conjunction with two statistically-based quantities calculated from its surface electrostatic potential; the latter are the average deviation of the potential and its total variance. An ab initio SCF approach was used to compute STO-3G*-optimized geometries and STO-5G* electrostatic potentials.
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| 10. |
- Brinck, Tore, Professor, 1965-, et al.
(författare)
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The Importance of Electrostatics and Polarization for Noncovalent Interactions : Ionic Hydrogen Bonds vs Ionic Halogen Bonds
- 2022
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Ingår i: Journal of Molecular Modeling. - : Springer Nature. - 1610-2940 .- 0948-5023. ; 28:9
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Tidskriftsartikel (refereegranskat)abstract
- A series of 26 hydrogen-bonded complexes between Br- and halogen, oxygen and sulfur hydrogen-bond (HB) donors is investigated at the M06-2X/6-311 +G(2df,2p) level of theory. Analysis using a model in which Br- is replaced by a point charge shows that the interaction energy (Delta E-Int) of the complexes is accurately reproduced by the scaled interaction energy with the point charge (Delta E-Int(PC)). This is demonstrated by Delta E-Int = 0.86 Delta E-Int(PC) with a correlation coefficient, R-2=0.999. The only outlier is (Br-H-Br)(-), which generally is classified as a strong charge-transfer complex with covalent character rather than a HB complex. Delta E-Int(PC) can be divided rigorously into an electrostatic contribution (Delta E-ES(PC)) and a polarization contribution (Delta E-pol(PC)).Within the set of HB complexes investigated, the former varies between -7.2 and -32.7 kcal mol(-1), whereas the latter varies between -1.6 and -11.5 kcal mol(-1). Compared to our previous study of halogen-bonded (XB) complexes between Br and C-Br XB donors, the electrostatic contribution is generally stronger and the polarization contribution is generally weaker in the HB complexes. However, for both types of bonding, the variation in interaction strength can be reproduced accurately without invoking a charge-transfer term. For the Br-center dot center dot center dot HF complex, the importance of charge penetration on the variation of the interaction energy with intermolecular distance is investigated. It is shown that the repulsive character of Delta E-Int at short distances in this complex to a large extent can be attributed to charge penetration.
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| 11. |
- Brinck, Tore, Professor, 1965-, et al.
(författare)
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The use of quantum chemistry for mechanistic analyses of SeAR reactions
- 2015
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Ingår i: Arene Chemistry: Reaction Mechanisms and Methods for Aromatic Compounds. - Hoboken, NJ : wiley. - 9781118754887 - 9781118752012 ; , s. 83-105
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- In this chapter, we review the current understanding of electrophilic aromatic substitution (SEAr) based on the latest experimental and quantum chemical studies. In addition, the most reliable and computationally effective methods for predicting regioselectivity and relative reactivity of SEAr are evaluated and described. The mechanism of nitration is analyzed in detail based on recent quantum chemical studies. In the gas phase, the reaction often has a contribution from a single-electron transfer (SET), and this contribution increases with the activation tendency of the aromatic substrate. The solution reaction lacks a driving force for SET, and the reaction has an early transition state that resembles an O-C coordinated π-complex in structure. In contrast, halogenation with molecular chlorine as the electrophile proceeds via a much later transition state that is more similar to the α-complex. Among the different reactivity descriptors that have been used to analyze regioselectivity and relative reactivity of SEAr, the average local ionization energy seems to have the best predictive power. As is generally the case for descriptor-based approaches, it is best suited for analyzing SEAr with early transition states. The α-complex approach has emerged as an alternative to reactivity descriptors for predicting regioselectivity. It is based on the assumption that the relative α-complex energies are similar to the corresponding transition state energies and thus reflect the positional selectivity for an aromatic substrate when reacting with a particular electrophile. The method provides quantitative predictions for halogenations but is not reliable for nitrations.
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| 12. |
- Bui, Thi Diem Huong, et al.
(författare)
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p-block doped semi-metallic xenes as highly selective and efficient transition-metal free single atom catalysts for electrochemical CO reduction
- 2023
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 12:4, s. 2110-2120
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Tidskriftsartikel (refereegranskat)abstract
- The development of robust and inexpensive catalysts for the electrochemical CO reduction reaction (CORR) is key for sustainable production of valuable chemicals, yet it remains a long-standing challenge. Herein, we conduct a systematic theoretical investigation on p-block doping of semi-metallic xene monolayers to afford transition metal free catalysts for the CORR. Silicene (Si) and germanene (Ge) are suitable platforms for capturing the dopant (B/Al) to ensure high stability. Our single atom catalysts (SACs) are promising candidates for CORR due to their favorable initial CO adsorption and the selectivity of CO reduction over H2 evolution. B@Si, Al@Si, Al@Ge and B@Ge exhibit superior CORR catalytic activity with a limiting potential UL of 0.04, −0.39, −0.40, and −0.40 V, respectively. Notably, B@Si is identified as the best CORR electrocatalyst with an overpotential of less than 0.1 V. B@Si, Al@Si, Al@Ge exhibit high CORR selectivity towards CH3OH production, whereas B@Ge is predicted to form mainly CH4. The fundamental principles behind the outstanding CORR catalytic enhancement are disclosed by analyzing the structural and electronic configurations of two key intermediates, CO* and CHO*. CO* binds the dopant with moderate strength through a combination of σ-donation and π-backdonation unique for a transition metal free catalyst, whereas CHO* adsorbs strongly to the surface by the simultaneous binding to two neighboring atomic sites; consequently, the binding of the two intermediates breaks the scaling relation that limits the CORR activity of conventional catalysts. The optimal adsorption behaviors are attributed to the surface charge modulation induced by the substitutional doping. Hence, these findings may facilitate rational design of xene-based SACs for CORR and advance other catalytic applications.
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| 13. |
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| 14. |
- Halldin Stenlid, Joakim, 1987-
(författare)
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Computational Studies of Chemical Interactions: Molecules, Surfaces and Copper Corrosion
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The chemical bond – a corner stone in science and a prerequisite for life – is the focus of this thesis. Fundamental and applied aspects of chemical bonding are covered including the development of new computational methods for the characterization and rationalization of chemical interactions. The thesis also covers the study of corrosion of copper-based materials. The latter is motivated by the proposed use of copper as encapsulating material for spent nuclear fuel in Sweden.In close collaboration with experimental groups, state-of-the-art computational methods were employed for the study of chemistry at the atomic scale. First, oxidation of nanoparticulate copper was examined in anoxic aqueous media in order to better understand the copper-water thermodynamics in relation to the corrosion of copper material under oxygen free conditions. With a similar ambition, the water-cuprite interface was investigated with regards to its chemical composition and reactivity. This was compared to the behavior of methanol and hydrogen sulfide at the cuprite surface.An overall ambition during the development of computational methods for the analysis of chemical bonding was to bridge the gap between molecular and materials chemistry. Theory and results are thus presented and applied in both a molecular and a solid-state framework. A new property, the local electron attachment energy, for the characterization of a compound’s local electrophilicity was introduced. Together with the surface electrostatic potential, the new property predicts and rationalizes regioselectivity and trends of molecular reactions, and interactions on metal and oxide nanoparticles and extended surfaces.Detailed atomistic understanding of chemical processes is a prerequisite for the efficient development of chemistry. We therefore envisage that the results of this thesis will find widespread use in areas such as heterogeneous catalysis, drug discovery, and nanotechnology.
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| 15. |
- Jorner, Kjell, et al.
(författare)
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Machine learning meets mechanistic modelling for accurate prediction of experimental activation energies
- 2021
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Ingår i: Chemical Science. - : ROYAL SOC CHEMISTRY. - 2041-6520 .- 2041-6539. ; 12:3, s. 1163-1175
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Tidskriftsartikel (refereegranskat)abstract
- Accurate prediction of chemical reactions in solution is challenging for current state-of-the-art approaches based on transition state modelling with density functional theory. Models based on machine learning have emerged as a promising alternative to address these problems, but these models currently lack the precision to give crucial information on the magnitude of barrier heights, influence of solvents and catalysts and extent of regio- and chemoselectivity. Here, we construct hybrid models which combine the traditional transition state modelling and machine learning to accurately predict reaction barriers. We train a Gaussian Process Regression model to reproduce high-quality experimental kinetic data for the nucleophilic aromatic substitution reaction and use it to predict barriers with a mean absolute error of 0.77 kcal mol(-1) for an external test set. The model was further validated on regio- and chemoselectivity prediction on patent reaction data and achieved a competitive top-1 accuracy of 86%, despite not being trained explicitly for this task. Importantly, the model gives error bars for its predictions that can be used for risk assessment by the end user. Hybrid models emerge as the preferred alternative for accurate reaction prediction in the very common low-data situation where only 100-150 rate constants are available for a reaction class. With recent advances in deep learning for quickly predicting barriers and transition state geometries from density functional theory, we envision that hybrid models will soon become a standard alternative to complement current machine learning approaches based on ground-state physical organic descriptors or structural information such as molecular graphs or fingerprints.
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| 16. |
- Liljenberg, Magnus, 1962-
(författare)
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Quantum Chemical Studies of Aromatic Substitution Reactions
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, density functional theory (DFT) is used to investigate the mechanisms and reactivities of electrophilic and nucleophilic aromatic substitution reactions (SEAr and SNAr respectively). For SEAr, the σ-complex intermediate is preceded by one (halogenation) or two (nitration) π-complex intermediates. Whereas the rate-determining transition state (TS) for nitration resembles the second π-complex, the corresponding chlorination TS is much closer to the σ-complex. The last step, the expulsion of the proton, is modeled with an explicit solvent molecule in combination with PCM and confirmed to be a nearly barrierless process for nitration/chlorination and involves a substantial energy barrier for iodination. It is also shown for nitration that the gas phase structures and energetics are very different from those in polar solvent. The potential energy surface for SNAr reactions differs greatly depending on leaving group; the σ-complex intermediate exist for F-/HF, but for Cl-/HCl or Br-/HBr the calculations indicate a concerted mechanism. These mechanistic results form a basis for the investigations of predictive reactivity models for aromatic substitution reactions. For SEAr reactions, the free energy of the rate-determining TS reproduces both local (regioselectivity) and global reactivity (substrate selectivity) with good to excellent accuracy. For SNAr reactions good accuracies are obtained for Cl-/HCl or Br-/HBr as leaving group, using TS structures representing a one-step concerted mechanism. The σ-complex intermediate can be used as a reactivity indicator for the TS energy, and for SEAr the accuracy of this method varies in a way that can be rationalized with the Hammond postulate. It is more accurate the later the rate-determining TS, that is the more deactivated the reaction. For SNAr reactions with F-/HF as leaving group, the same method gives excellent accuracy for both local and global reactivity irrespective of the degree of activation.
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| 17. |
- POLITZER, P, et al.
(författare)
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RADIAL BEHAVIOR OF THE AVERAGE LOCAL IONIZATION ENERGIES OF ATOMS
- 1991
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Ingår i: Journal of Chemical Physics. - : AMER INST PHYSICS. - 0021-9606 .- 1089-7690. ; 95:9, s. 6699-6704
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Tidskriftsartikel (refereegranskat)abstract
- The radial behavior of the average local ionization energy IBAR(r) has been investigated for the atoms He-Kr, using ab initio Hartree-Fock atomic wave functions. IBAR(r) is found to decrease in a stepwise manner with the inflection points serving effectively to define boundaries between electronic shells. There is a good inverse correlation between polarizability and the ionization energy in the outermost region of the atom, suggesting that IBAR(r) may be a meaningful measure of local polarizabilities in atoms and molecules.
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| 18. |
- POLITZER, P, et al.
(författare)
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RELATIONSHIPS BETWEEN SOLUTE MOLECULAR-PROPERTIES AND SOLUBILITY IN SUPERCRITICAL CO2
- 1993
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - : AMER CHEMICAL SOC. - 0022-3654 .- 1541-5740. ; 97:3, s. 729-732
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Tidskriftsartikel (refereegranskat)abstract
- For 21 organic compounds, we have investigated the dependence of solubility in supercritical C02 upon several properties of the solute molecules, which were calculated at the ab initio SCF STO-5G*//STO-3G* level. Good double-variable linear regressions were found that relate solubility to (a) the total variance of the electrostatic potential on the molecular surface and (b) the molecular volume. Solubility varies inversely with the variance of the surface potential, consistent with solute-solute interactions dominating over solute-solvent interactions in determining solubility in low-density supercritical solutions.
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| 19. |
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| 20. |
- Sjöberg, P, et al.
(författare)
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AVERAGE LOCAL IONIZATION ENERGIES ON THE MOLECULAR-SURFACES OF AROMATIC SYSTEMS AS GUIDES TO CHEMICAL-REACTIVITY
- 1990
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Ingår i: Canadian journal of chemistry (Print). - : Canadian Science Publishing. - 0008-4042 .- 1480-3291. ; 68:8, s. 1440-1443
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Tidskriftsartikel (refereegranskat)abstract
- The average ionization energy, [Formula: see text], is introduced and is demonstrated to be useful as a guide to chemical reactivity in aromatic systems. [Formula: see text] is rigorously defined within the framework of self-consistent-field molecular orbital (SCF-MO) theory and can be interpreted as the average energy needed to ionize an electron at any point in the space of a molecule. An abinitio SCF-MO approach has been used to calculate [Formula: see text] at the 6-31G* level, using STO-3G optimized geometries. [Formula: see text] has been computed on molecular surfaces defined by the contour of constant electronic density equal to 0.002 electrons/bohr3, for a series of aromatic systems. This surface [Formula: see text] provides site specific predictions for preferred positions of electrophilic aromatic substitution. Relative reactivity toward electrophiles increases as the magnitudes of the smallest [Formula: see text] values [Formula: see text] for these systems decrease. An excellent relationship, with a correlation coefficient of 0.99, has been found between the Hammett constants and [Formula: see text]; this allowed us to predict the values of these constants for the substituents NHF and NF2, for which they were previously not known. Keywords: average local ionizations energy, chemical reactivity, electrophilic aromatic substitution, molecular surfaces, Hammett constants.
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| 21. |
- Tissot, Heloise, et al.
(författare)
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Acetic acid conversion to ketene on Cu2O(100) : Reaction mechanism deduced from experimental observations and theoretical computations
- 2021
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 402, s. 154-165
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Tidskriftsartikel (refereegranskat)abstract
- Ketene, a versatile reagent in production of fine and specialty chemicals, is produced from acetic acid. We investigate the synthesis of ketene from acetic acid over the (3,0;1,1) surface of Cu2O(1 0 0) through analysis of the adsorption and desorption characteristics of formic and acetic acids. The results allow us to establish a reaction mechanism for ketene formation. Observations from x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy, and temperature programmed desorption (TPD), supported by a comparison with formic acid results, suggest that acetic acid reacts with Cu2O through deprotonation to form acetate species coordinated to copper sites and hydroxylation of nearby surface oxygen sites. For formic acid the decomposition of adsorbed formate species results in desorption of CO2 and CO while, for acetic acid, high yields of ketene are observed at temperature >500 K. Modeling by density functional theory (DFT) confirms the strong interaction of acetic acid with the (3,0;1,1) surface and the spontaneous dissociation into adsorbed acetate and hydrogen atom species, the latter forming an OH-group. In an identified reaction intermediate ketene binds via all C and O atoms to Cu surface sites, in agreement with interpretations from XPS. In the vicinity of the adsorbate the surface experiences a local reorganization into a c(2 × 2) reconstruction. The total computed energy barrier for ketene formation is 1.81 eV in good agreement with the 1.74 eV obtained from TPD analysis. Our experimental observations and mechanistic DFT studies suggests that Cu2O can operate as an efficient catalyst for the green generation of ketene from acetic acid.
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