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In this study, the conformational preferences and photochemistry of acrylic acid (AA, CH2=CHCOOH) monomer isolated in cryogenic argon and krypton matrices were interpreted, based on results of quantum chemical calculations. Natural bond orbital analysis allowed to shed light on the main electronic effects determining the relative stability of the conformers of the molecule in the ground electronic state. The conformational isomerization taking place upon UV-irradiation of the matrix-isolated compound (λ ∼ 243 nm) was explained, based on theoretical complete active space self-consistent field/complete active space with second order perturbation theory (CASSCF/CASPT2) and time-dependent density functional theory (TD-DFT) results, allowing to rationalize the nearly equal populations of the two lowest energy conformers of the molecule observed in the photostationary state. Besides, details of the infrared spectra of the compound were reinterpreted based on the calculated spectra for the two most stable conformers of the molecule. In particular, the assignments for the out-of-plane A″ symmetry vibrations were revised. [ABSTRACT FROM AUTHOR]

We have taken (dA), (dT), and (dA)·(dT) as model systems to study concerted effects of base pairing and stacking on excited-state nature of DNA oligonucleotides using density functional theory (DFT) and time dependent DFT methods. The spectroscopic states are determined to be of a partial → charge-transfer nature in the oligonucleotides. The → charge-transfer transitions produce dark states, which are hidden in the energy region of the steady-state absorption spectra. This is different from the previous assignment that the → charge-transfer transition is responsible for a shoulder at the red side of the first strong absorption band. The → charge-transfer states were predicted to have relatively high energies in the oligonucleotides. The present calculations predict that the → charge-transfer states are not involved in the spectra and excited-state dynamics of the oligonucleotides. In addition, the influence of base pairing and stacking on the nature of the nπ* and ππ* states are discussed in detail. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]