| 1. |
- Albinsson, Bo, 1963, et al.
(författare)
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Anti, ortho, and gauche conformers of perfluoro-n-butane: Matrix-isolation IR spectra and calculations
- 1996
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Ingår i: Journal of Physical Chemistry. - 0022-3654. ; 100:9, s. 3418-3429
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Tidskriftsartikel (refereegranskat)abstract
- Nitrogen matrix-isolation IR spectra have been obtained for each of the three conformers of n-C4F10 (gauche, ortho, and anti) by trapping a hot conformer mixture on a cold CsI window and subsequent matrix annealing and spectral differencing. They were assigned by comparison with results of HF/6-31G* calculations, and the nature of the normal modes has been analyzed using the total energy distribution procedure. At the fully optimized MP2/6-31G* (frozen core) level, the CCCC dihedral angles and relative energies (kcal/mol) are 54.2 degrees and 0.68 (gauche), 94.8 degrees and 1.63 (ortho), and 165.5 degrees and 0 (anti). Single-point MP2/6-311G* (frozen core) relative energies at these geometries are 0.85, 2.12, and 0 kcal/mol, respectively. Only a minute amount of the ortho conformer is trapped in nitrogen matrix, and none in other matrices that were tried. A variation of the relative intensities of IR peaks of the gauche and anti conformers as a function of the temperature of the gas before deposition yields an ''average'' Delta H value of about 0.9 kcal/mol, with the anti conformer more stable. The temperature range covered was too small to reveal the expected bilinear nature of the van't Hoff plot.
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| 2. |
- Albinsson, Bo, 1963, et al.
(författare)
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Conformers of n-Si5Me12: A comparison of ab initio and molecular mechanics methods
- 1999
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Ingår i: Journal of Physical Chemistry A. - 1089-5639 .- 1520-5215. ; 103:14, s. 2184-2196
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Tidskriftsartikel (refereegranskat)abstract
- Optimized geometries of the conformers of permethylated linear pentasilane, n-Si5Me12, were calculated by the HF/3-21G*, MM3, MM2, and MM+ methods, which predict eight, nine, six, and six energetically distinct enantiomeric conformer pairs, respectively, at geometries representing various combinations of the anti (similar to 165 degrees), ortho (similar to 90 degrees), and gauche (similar to 55 degrees) SiSiSiSi dihedral angles in the backbone. The results of the MM2 and MM+ methods, based on the same force field, differ insignificantly. The barriers between conformers appear to be exaggerated by the molecular mechanics methods, particularly MM2. Contour maps showing the groundstate energy as a function of the full range of two backbone SiSiSiSi dihedral angles, with all other geometrical variables optimized, computed by each of the methods (only a limited range of angles near the anti,anti geometry in the case of HF/3-21G*) are compared with each other and with analogous results for a model compound, Si4Me10. Conformer interconversion paths are discussed, and two meso transition states for enantiomer interconversion have been located at the HF/3-21G* level of calculation. At the eight HF/3-21G* optimized geometries, single-point energies (HF/6-31G* and MP2/6-31G*) and vibrational frequencies (HF/3-21G*) were computed. The predicted IR and Raman spectra suggest that about half of the expected conformers will be identifiable by vibrational spectroscopy under conditions of matrix isolation. Relative conformer energies calculated by the MM2 and HF methods are similar and favor the anti dihedral angles over gauche and ortho, in agreement with results of solution experiments. Those calculated by the hIM3 and HF methods are similar to each other and favor both anti and gauche dihedral angles nearly equally over ortho, in agreement with indications provided by gas-phase experiments. A rationalization of these solvent effects is proposed. The energies of the conformers of Si4Me10 and Si3Me12 were used to set up a system of additive increments at the MM2, MM3, HF/3-21G*, HF/6-31G*, and MP2/6-31G* levels of calculation, which can be used to predict conformational energies of longer permethylated oligosilanes. An intrinsic energy value is assigned to each of the a, o, and g dihedral angles, and interaction energy values are assigned to each combination of two dihedral angles. The interaction values follow the expected rules in that equal twist sense is favored for adjacent aa, ag, oo, and gg pairs, whereas opposite twist sense is generally favored for adjacent so and go pairs. The MM3-derived set of increments has been tested against results computed for Si6Me14 and found to perform well.
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| 3. |
- 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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| 4. |
- Albinsson, Bo, 1963
(författare)
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Dual fluorescence from N-6,N-6-dimethyladenosine
- 1997
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Ingår i: Journal of the American Chemical Society. - 1520-5126 .- 0002-7863. ; 119:27, s. 6369-6375
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Tidskriftsartikel (refereegranskat)abstract
- The adenosine derivative N-6,N-6-dimethyladenosine (DMA) shows dual fluorescence in solvents of different polarity. In addition to the ''normal'' fluorescence at 330 nm, another band is observed at 500 nm. The long wavelength emission dominates in aprotic solvents but is dynamically quenched by protic solvents. Steady-state and lifetime measurements show that the emissions originate from two excited state species; the short wavelength emission is from the directly populated excited state which irreversibly isomerizes into the species responsible for the long wavelength emission. It is conceivable to assign the long wavelength emitting species to a twisted intramolecular charge transfer state (TICT). The fluorescence quantum yield of the short wavelength emission is approximately 4 x 10(-4) at room temperature and increases by three orders of magnitude when the temperature is lowered to 80 K in accordance with the behavior of normal nucleic acid bases. In contrast, the long wavelength fluorescence quantum yield is almost temperature independent. The different photophysical processes for DMA are summarized into a kinetic scheme where the temperature quenching of the short wavelength fluorescence is exclusively through isomerization into the long wavelength emitting species. Direct internal conversion to the ground state, commonly believed to be the dominant process for nonradiative deactivation of the DNA bases, makes a negligible contribution for DMA.
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| 5. |
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| 6. |
- Albinsson, Bo, 1963, et al.
(författare)
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ELECTRONIC-TRANSITION MOMENT DIRECTIONS AND IDENTIFICATION OF LOW-ENERGY N-PI-ASTERISK STATES IN WEAKLY PERTURBED PURINE CHROMOPHORES
- 1993
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 115:1, s. 223-231
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of UV linear dichroism on purine and three methyl derivatives partially oriented in poly(vinyl alcohol) matrix gave direct evidence for the assignment of the first singlet npi* state. Intensity distributions and moment directions for the first three pi --> pi* transitions were also determined. The pi --> pi* transitions in purine were found to be polarized at (angles, relative to the pseudo-symmetry long axis, counted positive in the N7 direction): -31-degrees +/- 5-degrees (II at 265 nm), +38-degrees +/- 5-degrees (III at 244 nm), and +36-degrees +/- 10-degrees (IV at 214 nm). The transition energies and moment directions were not markedly perturbed by methyl substitution at the sixth, seventh, or ninth position. Therefore, these methyl substituents could be used as orientational perturbers to resolve a sign ambiguity problem regarding transition moment directions. The orientation were determined by infrared dichroic measurements using both in-plane and out-of-plane polarized vibrational transitions. In addition, the phosphorescence spectra were studied, including phosphorescence anisotropy, phosphorescence lifetimes, and quantum yields, for the purines in an organic glass at 80 K. Based on these measurements, the lowest triplet state is concluded to have effectively pipi* character, and its emission allowedness appears to originate from spin-orbit interactions primarily with singlet sigmapi* states but also with singlet pipi* states via vibronic mixing. The phosphorescence emission spectra of purine and 6-methylpurine are complex, compared to 7-methylpurine and 9-methylpurine, with emission wavelength-dependent lifetimes and excitation spectra. This is ascribed to a prototropic tautomeric equilibrium between the 7H and 9H forms of purine and 6-methylpurine, a ground-state heterogeneity that we believe has caused confusion in earlier studies and, e.g., led to an assignment of the phosphorescence origin of purine.
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| 7. |
- Albinsson, Bo, 1963
(författare)
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Excitation and Excited State Properties of Heterocyclic Chromophores. Indole and Purine Derivatives of Biological Significance
- 1992
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Proteins and nucleic acids absorb light in the near-UV due to the presence of aromatic heterocyclic chromophores. In this work linear dichroism (LD) in combination with fluorescence polarization anisotropy (FPA) and magnetic circular dichroism (MCD) is used to investigate electronic excitations and excited states of indole derivatives, purine derivatives, the wye base, 2-phenylindole and 4',6-diamidino-2-phenylindole (DAPI). The UV spectrum of indole, which is the chromophoric core of tryptophan, is resolved into contributions from the lowest nearly degenerate 1A1 --> 1La and 1A1 --> 1Lb transitions, and the moment directions for the four lowest transitions in indole and eight selected methyl and methoxy derivatives are determined. Substitution does not generally perturb the transition moment directions, with exception for the 1A1 --> 1Lb and 1A1 --> 1Bb transitions whose moment directions are significantly altered by a methoxy group in 4- or 6-position. The transition moment directions and the resolved UV-spectra were also determined for purine, a model compound for the purine nucleic acid bases, and the tRNA base wye (Y-base). Purine has a clearly resolved n-->p* transition lowest in energy, followed by two nearly degenerate p-->p* transitions with almost perpendicular moment directions. The phosphorescence spectra of purine and three methyl derivatives were investigated and the emitting triplet was found to have effectively pp* character. In addition, the purines with mobile imidazolic protons (purine and 6-methylpurine) are shown to exist, in an organic glass at 80 K, in two tautomeric forms (7H and 9H) as judged from phosphorescence emission and excitation spectra. A DNA-binding probe 4',6-diamidino-2-phenylindole (DAPI) and its parent chromophore 2-phenylindole were finally investigated using the above mentioned experimental techniques as well as quantum mechanical calculations. Five distinct electronic transitions are found above 200 nm and their moment directions are determined. The theoretical results indicate a weak relation of the two lowest transitions in 2-phenylindole to the 1A1 --> 1Lb and 1A1 --> 1La transitions of indole.
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| 8. |
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| 9. |
- Albinsson, Bo, 1963, et al.
(författare)
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EXCITED-STATE PROPERTIES OF THE INDOLE CHROMOPHORE - ELECTRONIC-TRANSITION MOMENT DIRECTIONS FROM LINEAR DICHROISM MEASUREMENTS - EFFECT OF METHYL AND METHOXY SUBSTITUENTS
- 1992
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 96:15, s. 6204-6212
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Tidskriftsartikel (refereegranskat)abstract
- From measurements of UV and IR linear dichroism on molecules partially oriented in stretched polyethylene host the transition moment directions for the first four pi-pi* transitions of indole and some indole derivatives were determined. Relative to the pseudosymmetry long axis of indole, the transitions were normally found to be polarized at (angles counted away from the ring nitrogen): +42 +/- 50-degrees (1A1 --> 1L(by) at 287 nm), -46 +/- 5-degrees (1A1 --> 1L(a) at 265 nm), 0 +/- 15-degrees (1A1 --> 1B(by) at 220 nm), and for the 1A1 --> 1B(a) transition occurring around 200 nm, at least at +/- 30-degrees away from this axis. In addition, indication for a weak, essentially short axis polarized transition was found at 235 nm, possibly due to the 1A1 --> 1C transition. An ambiguity problem regarding the sign of the angles was resolved by exploiting the change of orientation properties upon introduction of substituents. Orientation parameters (including diagonalizing angle) were determined by consideration of a large number of in-plane as well as out-of-plane polarized vibrational transitions. The question regarding effects on the excited states by the presence of methyl and methoxy substituents, at varied positions in the indole chromophore, was addressed in terms of the perturbations they caused on the transition moments. Whereas none of the four transitions was found to be very sensitive in this respect to methyl or methoxy groups introduced in 2-, 3-, 5-, or 7-position of indole, the directions of the weak 1A1 --> 1L(by) but also the strong 1A1 --> 1B(by) transition were found to become significantly altered by a methoxy group in 4- as well as 6-position. The conclusions are consistent with recent fluorescence anisotropy data and semiempirical calculations.
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| 10. |
- Albinsson, Bo, 1963, et al.
(författare)
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Matrix-isolation IR and UV spectra of Si3H8 and Si4H10: Isomers and conformers of oligosilanes
- 1996
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Ingår i: Journal of Physical Chemistry. - 0022-3654. ; 100:21, s. 8681-8691
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Tidskriftsartikel (refereegranskat)abstract
- Matrix-isolation IR and UV spectra of Si3H8, i-Si4H10, and the two conformers of n-Si4H10 have been recorded. A quantitative separation of the IR spectrum of n-Si4H10 into contributions from the anti and gauche forms was accomplished by a combination of matrix annealing and selective monochromatic photodestruction experiments. A qualitative separation of their UV spectra was achieved as well. The IR spectra of Si3H8, i-Si4H10, and the two conformers of n-Si4H10 have been assigned by comparison with results of ab initio calculations, which reproduce the frequencies and even the relative intensities quite well. The calculations predict dihedral angles omega of 180 degrees and 57 degrees for the anti and the gauche conformer of n-Si4H10, respectively, and confirm earlier predictions of nearly equal stability for an isolated molecule. In the matrix, the anti conformer is more stable, The conformational effects on the UV spectrum of n-Si4H10 are not those anticipated from simple models of the Sandorfy or ladder C type, in that it is primarily not the energy but the intensity of the low-energy excited singlet states that depends strongly on the SiSiSiSi dihedral angle omega. This result is interpreted in terms of data from 6-in-8 CASSCF 6-31G* calculations, which predict an avoided crossing between a strongly allowed sigma sigma* B state and a very weakly allowed sigma pi* B state as omega changes, with the former lower in energy at 180 degrees and the latter lower at 0 degrees. Consequences for attempts to understand the effects of conformation on optical spectra of polysilanes are noted.
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