Solvent effects on the ultrafast nonradiative deactivation mechanisms of thymine in aqueous solution: Excited-state QM/MM molecular dynamics simulations

Abstract

On-the-fly excited-state quantum mechanics/molecular mechanics molecular dynamics (QM/MM-MD) simulations of thymine in aqueous solution are performed to investigate the role of solvent water molecules on the nonradiative deactivation process. The complete active space second-order perturbation theory (CASPT2) method is employed for a thymine molecule as the QM part in order to provide a reliable description of the excited-state potential energies. It is found that, in addition to the previously reported deactivation pathway involving the twisting of the C-C double bond in the pyrimidine ring, another efficient deactivation pathway leading to conical intersections that accompanies the out-of-plane displacement of the carbonyl group is observed in aqueous solution. Decay through this pathway is not observed in the gas phase simulations, and our analysis indicates that the hydrogen bonds with solvent water molecules play a key role in stabilizing the potential energies of thymine in this additional decay pathway. © 2013 AIP Publishing LLC.

Assessment of the Potential Energy Hypersurfaces in Thymine within Multiconfigurational Theory: CASSCF vs. CASPT2

“…Regarding the new minimum identified in this section, 1 (ππ*) twisted-min , C6-C5 and C5-C7 bond distances are~1.45 and~1.49 Å, respectively. The CASPT2 method has additionally been benchmarked against MS-CASPT2 (see Table 2), a technique used previously to tackle the photochemical behavior of thymine [32,34,41]. The results here obtained are rather homogeneous, yielding similar estimates at both levels of theory.…”

“…Both static [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and dynamic [16,17,[35][36][37][38][39][40][41] theoretical studies have been performed over the last decade, the former usually employing more accurate and correlated methods due to their lower computational cost. Multiconfigurational methods are usually the preferred choice, particularly the complete active space self-consistent field (CASSCF) [42] and its second-order perturbation theory extension (CASPT2) [43].…”

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…A obvious step further is completeactive-space perturbation theory (e.g., CASPT2 [190] ), which offers very good accuracy (oftentimes better than MRCI) and performance (usually much faster than MRCI). Few surface hopping studies based on CASPT2 have been reported already, [191][192][193][194] albeit no ISC dynamics simulation has been published until now, as no current implementation of CASPT2 can provide all necessary quantities. Other promising electronic structure methods are, for example, the approximately size-consistent MRCI variant linear-response-theory multireference average-quadratic coupled-cluster (LRT-MRAQCC) [177] or density matrix normalization group (DMRG) approaches.…”

Decay dynamics of 6‐azauracil from the light absorbing S2(ππ*) state

“…The most important difference lies in the disappearance of the CO stretch mode for thymine and M24PDA, and the appearance of the strong CO stretch mode for uracil, MTCA and 3M3P2O. To our understanding, the disappearance of the CO stretch mode of thymine in water or M24PDA is mostly due to the out‐of‐plane distortion motion of the CCCO motif occurring suddenly in the Franck–Condon region, if keep in mind that the structure of S 2 (ππ*)/S 0 CI for thymine or S 2 (ππ*)/S 1 CI for M24PDA is severely distorted and that the S 2 (ππ*) → S 0 internal conversion via S 2 (ππ*)/S 0 CI is the predominant channel (~90% population) for thymine. We thus speculate that the very high efficiency of the S 2 (ππ*) → S 0 internal conversion for thymine in water is mostly due to the predominant wave‐packets motion towards the S 2 (ππ*)/S 0 in the Franck–Condon region.…”