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The (6–4) photolesion is a key photodamage that occurs when two adjacent pyrimidine bases in a DNA strand bond together. To better understand how the absorption of UVB and UVA radiation by the 2‐pyrimidinone moiety in a (6–4) lesion can damage DNA, it is important to study the electronic deactivation mechanism of its 2‐pyrimidinone chromophore. This study employs theoretical (MS‐CASPT2/cc‐pVDZ level) and experimental (steady state and femtosecond broadband spectroscopic) methods to elucidate the photochemical relaxation mechanisms of 2‐(1H)‐pyrimidinone and 1‐methyl‐2‐(1H)‐pyrimidinone in aqueous solution (pH 7.4). In short, excitation at 320 nm leads to the population of the S11(ππ*) state with excess vibrational energy, which relaxes to the S11(ππ*) minimum in one picosecond or less. A trifurcation event in the S11(ππ*) minimum ensued, leading to radiative and nonradiative decay of the population to the ground state or the population of the long‐lived and reactive T13(ππ*) state in hundreds of picoseconds. Collectively, the theoretical and experimental results support the idea that in DNA and RNA, the T13(ππ*) state of the 2‐pyrimidinone moiety in the (6–4) lesion can further participate in photosensitized chemical reactions increasing DNA and RNA damage. [ABSTRACT FROM AUTHOR]

9H‐ and 7H‐2,6‐Diaminopurine (26DAP) photoinduced events in vacuum were studied at the MS‐CASPT2/cc‐pVDZ level of theory. The S11(ππ* La) state is initially populated evolving barrierless towards its minimum energy structure, from where two photochemical events can take place in both tautomers. The first is the return of the electronic population to the ground state via the C6 conical intersection (CI‐C6). The second involves an internal conversion to the ground through the C2 conical intersection (CI‐C2). According to our geodesic interpolated paths connecting the critical structures, the second route is less favorable in both tautomers, due to the presence of high energy barriers. Our calculations suggest a competition between fluorescence and ultrafast relaxation to the electronic ground state via internal conversion process. Based on our calculated potential energy surfaces and experimental excited state lifetimes from the literature, we can infer that the 7H‐ must have a greater fluorescence yield than the 9H‐tautomer. We also explored the triplet state population mechanisms on the 7H‐26DAP to understand their long‐lived components observed experimentally. [ABSTRACT FROM AUTHOR]

UV–VIS photoinduced events of tzA and tzG embedded into DNA and RNA are described by combining the Extended Multi‐State Second‐Order Perturbation Theory (XMS‐CASPT2) and electrostatic embedding molecular mechanics methods (QM/MM). Our results point out that the S11(ππ* La) state is the bright state in both environments. After the photoexcitation to the S11(ππ* La) state, the electronic population evolves barrierless towards its minimum, from where the excess of energy can be dissipated by fluorescence. As the minimum energy crossing point structure between the ground and first bright states lies in a high‐energy region, the direct internal conversion to the ground state is an unviable mechanism. Other spectroscopic properties (for instance, absorption and Stokes shifts) and comparisons with photochemical properties of canonical nucleobases are also provided. [ABSTRACT FROM AUTHOR]

The guanine derivative, 5‐aza‐7‐deazaguanine (5N7CG) has recently been proposed as one of four unnatural bases, termed Hachimoji (8‐letter) to expand the genetic code. We apply steady‐state and time‐resolved spectroscopy to investigate its electronic relaxation mechanism and probe the effect of atom substitution on the relaxation mechanism in polar protic and polar aprotic solvents. Mapping of the excited state potential energy surfaces is performed, from which the critical points are optimized by using the state‐of‐art extended multi‐state complete active space second‐order perturbation theory. It is demonstrated that excitation to the lowest energy 1ππ* state of 5N7CG results in complex dynamics leading to ca. 10‐ to 30‐fold slower relaxation (depending on solvent) compared with guanine. A significant conformational change occurs at the S1 minimum, resulting in a 10‐fold greater fluorescence quantum yield compared with guanine. The fluorescence quantum yield and S1 decay lifetime increase going from water to acetonitrile to propanol. The solvent‐dependent results are supported by the quantum chemical calculations showing an increase in the energy barrier between the S1 minimum and the S1/S0 conical intersection going from water to propanol. The longer lifetimes might make 5N7CG more photochemically active to adjacent nucleobases than guanine or other nucleobases within DNA. [ABSTRACT FROM AUTHOR]

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 lowest lying electronic states of CoN are investigated at the ab initio multireference configuration interaction (MRCI) and multistate multiconfigurational second-order perturbation theory (MS-CASPT2) levels with extended atomic basis sets and inclusion of scalar relativistic effects. The potential energy curves, excitation energies, spectroscopic constants and bonding character for all states are presented.

The electronic spectra of the N(9)H and N(7)H tautomers of purine were investigated. The complete active space SCF method and the multiconfigurational second-order perturbation theory were used to calculate the vertical integration energies, oscillator strengths, dipole moments and transition moment directions in gas phase. The results showed that the electronic absorption spectra of purine can ben satisfactorily explained when the N(9)H tautomer is considered predominant in gas phase and nonpolar solvents while the N(7)H pure is assumed to predominate in polar solvents.

Absorption spectra of a series of cyclic conjugated ketones and thioketones have been computed at the multiconfigurational second-order multistate perturbation level of theory, the CASSCF/ MS-CASPT2 method. Excitation energies, transition dipole moments, oscillator strengths, and static dipole moments are reported and discussed for excited states with energies lower than ≈ 7-8 eV. The main bands of the spectra have been assigned and characterized in most cases for the first time. The spectroscopy of the different systems is compared in detail. Thioketones in particular have low-energy and intense ,π,π[sup *] transitions which suggest corresponding enhanced nonlinear molecular optical properties. Additionally, some of the methods used to estimate these properties from spectroscopic data have been considered in order to analyze the main contributions to the nonlinear optical properties. [ABSTRACT FROM AUTHOR]

The multistage multiconfigurational second-order perturbation theory is employed to study the low-lying singlet and triplet electronic states of RhB. The ground electronic state RhB molecules are shown to be highly symmetrical.

The tzC and tzU emissive properties were investigated with the MS‐CASPT2 method and CASSCF/CASPT2 protocol, with inclusion of solvent effects with the aid of the PCM and Sequential QM/MM, the Free Energy Gradient methods. Theoretical vertical emission energies and Stokes shifts, using the ASEC‐FEG method, are in a good agreement with the experimental data, within the expected accuracy of CASPT2 method. Their emissive properties can be understood in analogy to the photophysical behaviour of the canonical uracil and cytosine pyrimidine nucleobases, for which the twisting of the C5C6 bond is the key for understanding their ultrashort radiationless decay times. For both tzC and tzU, the computed photophysical deactivation pathway, shows that the bright state 1(ππ*) goes directly to a minimum structure on its potential energy hypersurface (1(ππ*)min) , from where no conical intersection with the ground state can be reached, due to the steric hindrance of the C5C6 position, which forms the bridge between the rings in both molecules. Solvent effects place the conical intersection in higher energy region, reinforcing their emissive nature. [ABSTRACT FROM AUTHOR]

The 13 lowest-lying electronic states of a new diatomic molecule, BeC, are described theoretically for the first time using the multireference single and double excitations configuration interaction approach. Potential energy curves are presented for all the states as well as a description of the electronic structure characterizing each of them. Dissociation and vertical excitation energies, and dipole moment functions complete the electronic structure description. Vibrational and rotational levels spacings, on the other hand, allowed the calculation of a whole set of spectroscopic constants characterizing each state. In its ground state, a 3Σ-, this molecule is more strongly bound (De=2.39 eV) than the recently described species BeN (De=1.34 eV) but still less strongly bound than BeO (De=4.69 eV) and BeF (De=5.82 eV). The first excited triplet state (3Π) shows an avoided crossing at short internuclear distances which should certainly reflect in perturbative effects in its spectrum. A distinctive feature is also the existence of a very low-lying strongly bound quintet state. [ABSTRACT FROM AUTHOR]