Search

The high-level theoretical ab initio quantum chemistry methods are used for studying the nonadiabatic photochemistry of the guanine molecule (2-amino-6-oxopurine) and some of its tautomers. A common framework is obtained for the ultrafast relaxation of the natural nucleobases in which the predominant role of a [pi][pi]*-type state is confirmed.

The mechanism of population of the lowest triplet state of thymine is analyzed on the basis of singlet-triplet crossings (STCs) occurring along the major deactivation path responsible for the rapid internal conversion (IC) process detected in the molecule. Two singlet-triplet crossings along the minimum-energy path for ultrafast decay of the singlet state at 4.8 and 4.0 eV are observed.

The electronic singlet-singlet and singlet-triplet electronic transitions of the isoalloxazine ring of the flavin core are investigated using second-order perturbation theory. The lowest singlet and triplet excited states are found to be both of [pi] character with a singlet-triplet splitting of 0.57eV and evolution of the lowest singlet excited state along the relaxation pathway upon light absorption leads ultimately to the population of the lowest triplet state.

The mechanism of electron transfer (ET) from reduced pheophytin ([Pheo.sub.-]) to the primary stable photosynthetic acceptor, a quinione molecule is examined by using ab inito computations and realistic molecular models. The results show that the ET process involving the (Pheo-Q) and (PheoQ-) oxidation states can be essentially seen as an ultrafast radiationless transition between the two hypersurfaces taking places through conical intersections (CIs).

The low-lying singlet states of the chromophore of rhodopsin, the protonated Schiff base of 11-cis-retinal (PSB11), and of its all-trans photoproduct are studied in isolated conditions by using ab initio multiconfigurational second-order perturbation theory. The computed spectroscopic features include the vertical excitation, the band origin, and the fluorescence maximum of both isomers.

Ab initio CAASSCF and CASPT2 computation is used to construct the composite multistate relaxation path relevant to cycloocta- 1, 3, 5, 7 -tetraene singlet photochemistry. Computational evidence shows that evolution of photoexcited cyclooctatetraene out of the FC region prompts an efficient radiationless decay of the bright S2((super 1)E) state into the dark lowest S1((super 1) A (sub 1)) state, leading to population of a planar D(sub 8h) excited minimum cyclooctatetraene (COT).

Research has been conducted on the Cl (sub)2 Pt(malonate) (super)- complex. The results of the ab initio calculations of this complex performed to estimate the source and the magnitude of the symmetric and antisymmetric C=O stretch splitting are reported.

The electronic spectrum of benzaldehyde is examined by using multiconfigurational second-order perturbation theory through the multistate extension (MS-CASPT2). In conclusion, the S1(n pi*) and T2(pi-pi*) potential hypersurfaces are found to intersect upon relaxation in S1, providing a possible explanation for the efficient singlet-triplet intersystem crossing occurring in benzaldehyde.

A multistate second-order perturbation theory (MS-CASPT2) study of the lowest lying states in the electronic spectra of urocanic acid is in vacuo is presented. Computation of potential energy curves for each of the energy path for the photoisomerization of these reactions is in progress.

The singlet-singlet electronic spectrum of the anti conformer of n-tetrasilane is studied using multiconfigurational wave functions (CASSCF), second-order perturbation theory (CASPT2), and its multi-state extension (MS-CASPT2) in conjunction with large ANO-type basis sets including Rydberg functions. The results are consistent with the available experimental data and are easily understood in terms of a simple Huckel model of sigma-delocalization.

It is possible to use ab initio methods to gain a relatively accurate insight into the electronic spectrum of the transition metal systems Pd,Pt(thpy)2. All the different excited states were classified as either metal-centred, ligand-centred or metal-to-ligand.

An ab initio investigation of the absorption and emission electronic spectra of 1-phenylpyrrole is presented. Both the ground and the lowest excited state have a twisted structure.

A computational study of the trans- and cis-urocanic acid vertical and O-O excitation spectra is presented. This work provides useful information about the behaviour of the deprotonated system.

Research was conducted to examine the electronically excited states of the two isomers 1,3 and 1,4-cycloohexadiene using multiconfigurational second-order perturbation theory with extended ANO-type basis sets. New optical and resonant-enhanced multiphoton ionization spectra of 1,3-CHD were calculated and analyzed. Computed and experimental vertical transition energy results were in excellent agreement.

The theoretical study of P-aminobenzonitrile (ABD) and (p-dimethylamino)benzonitrile (DMABN) molecules at the CASPT2 and CASSCF level reveals the dual fluorescence process occurring in these systems. The molecules exhibit two fluorescence bands in the polar solvents. The fluorescence occurs primarily due to the small energy gap between the interacting states.

Theoretical and experimental analyses of the lower energy region of the norbornadiene spectrum reveal the associated symmetries, characteristics and vertical energies. Theoretical calculations assess the experimental ability to treat problems involving indirect conjugation and pi, sigma interactions associated with a strained molecule. A comparison of the energy levels of related diene series reveals the accuracy of calculations performed using other methods.

A newly suggested quantum chemical approach for ab initio computations of dynamic electron correlation influences in molecular systems aimed to theoretically examine the lowest valence and Rydberg excited states of cyclopentadiene, furan and pyrrole. These valence excited singlet states and the 1a2 leads to 3s, 3p and 3d Rydberg states for cyclopentadiene and furan were included. The valence excited states 1A (sub 1) (super +) (7.89, 7.46, 7.74 eV) and 1B2 (5.27, 5.92, 6.04 eV) induced the two primary characteristics in the spectra. The results are consistent with experimental findings but vary dramatically from earlier theoretical predictions.

Calculations for the inner-sphere reorganization energies for some small-nolecule inorganic redox couples are presented. Most of the calculations were done using the coupled pair functional approach except for O3- which was done using the complete active space SCF method with dynamic correlation using second-order perturbation theory. A comparison of values showed that the ab initio calculations were accurate to within +/- 2 kcal mol-1 or better than classical values. This showed that earlier conclusions obtained using the latter method are still valid.

Hybrid QM/MM computations are used for mapping the photoisomerization path of the retinal chromophore in Rhodopsin and exploring the reasons behind the photoactivity efficiency and spectral control in the visual pigments. The roles of E113 counterion, the E181 residue and the other amino acids of the protein packet are analyzed, indicating that counterion quenching by the protein environment has played a key role in setting up the chromophore's optical properties and its photochemical efficiency.

The energies and structures of different arrangements of the stacked adenine homodimer are studied in the isolation and in an aqueous environment. Adenine dimers have formed excimer singlet states with different degrees of stacking and interaction and a model is described for a 2-fold decay dynamics of adenine oligomers.

Cytosine photodimer formation is mediated along the triplet and singlet manifold by a singlet-triplet crossing and by a conical intersection. The results have helped in understanding the potential photogenotoxicity of endogenous and exogenous compounds via triplet-triplet sensitization, with a lower bound for cytosine oligonucleotides and have supported the primary role of triplet excited states in the photochemistry of DNA.

The role of the protein in controlling the absorption spectra of photoactive yellow protein (PYP), the archetype xanthopsin photoreceptor, is examined by using quantum mechanics/molecular mechanics (QM/MM) methods based on ab initio multireference perturbation theory. The studies have shown that the spectrum of the PYP can be reproduced within 0.25 eV by using QM/MM calculations.

A high-level ab initio study on the ionization properties of the single charged 2'-deoxythymidine 5'-monophosphate anion is reported. The computed vertical electron detachment energies (VDEs) are consistent with the gas-phase photoelectron spectrum of the molecule.

The intersystem crossing (ISC) process and triplet-state reactivity based on theoretical analysis of the spin - orbit strength and the potential energy surfaces for the relevant singlet and triplet states of cytosine are discussed. The ISC mechanism documented provides a possibility to access the triplet state, which has a key role in the photochemistry of the nucleic acid bases.