Ab initio insight into ultrafast nonadiabatic decay of hypoxanthine: keto-N7H and keto-N9H tautomers

National Science Foundation of China (NSFC) [21133007, 21103213, 91233106]; Ministry of Science and Technology [2011CB808504, 2012CB214900]; CAS; Director Innovation Foundation of CAS-QIBEBTNonadiabatic dynamics simulations at the SA-CASSCF level were performed for the two most stable keto-N7H and keto-N9H tautomers of hypoxanthine in order to obtain deep insight into the lifetime of the optically bright S-1((1)pi pi*) excited state and the relevant decay mechanisms. Supporting calculations on the ground-state (S-0) equilibrium structures and minima on the crossing seams of both tautomers were carried out at the MR-CIS and CASSCF levels. These studies indicate that there are four slightly different kinds of conical intersections in each tautomer, exhibiting a chiral character, each of which dominates a barrierless reaction pathway. Moreover, both tautomers reveal the ultrafast S-1 -> S-0 decay, in which the S1 state of keto-N9H in the gas phase has a lifetime of 85.5 fs, whereas that of keto-N7H has a longer lifetime of 137.7 fs. An excellent agreement is found between the present results and the experimental value of 130 +/- 20 fs in aqueous solution

A QM/MM MD insight into photodynamics of hypoxanthine: distinct nonadiabatic decay behaviors between keto-N7H and keto-N9H tautomers in aqueous solution

National Science Foundation of China (NSFC) [21133007, 21373164]; Ministry of Science and Technology [2011CB808504, 2012CB214900]Extensive ab initio surface-hopping dynamics simulations have been used to explore the excited-state nonadiabatic decay of two biologically relevant hypoxanthine keto-N7H and keto-N9H tautomers in aqueous solution. QM/MM calculations and QM/MM-based MD simulations predict different hydrogen bonding networks around these nucleobase analogues, which influence their photodynamical properties remarkably. Furthermore, different solvent effects on the conical intersection formation of keto-N7H and keto-N9H were found in excited-state MD simulations, which also change the lifetimes of the excited states. In comparison with the gas-phase situation, the S-1 -> S-0 nonradiative decay of keto-N7H is slightly faster, while this decay process of keto-N9H becomes much slower in water. The presence of pi-electron hydrogen bonds in the solvated keto-N7H is considered to facilitate the S-1 -> S-0 nonradiative decay process

Theoretical Studies on Excited States of Biorelated Systems from Gas Phase to Aqueous Solution

通讯作者地址: Li, JFExcited-state properties of molecules play a pivotal role in understanding their photophysical and photochemical behaviors. With the fast development of computational methodologies, the low-lying states of biorelated systems have been extensively investigated theoretically. Here, we review our recent works on the excited states of selected nucleobases and their related systems in the gas and condensed phases. The simulated electronic spectra of coumarin reproduce the band shape of experimental spectra and provide a basis to reasonably assign the observed bands. The absorption spectra and the excited-state dynamics of nucleic acid bases and their analogs in the gas phase and in aqueous solution have been explored by the combined quantum mechanics and molecular mechanics (QM/MM) calculations and QM/MM-based dynamics simulations with surface hopping. Based on extensive calculations and dynamics simulations, the solvent effects on the excited states and their dynamical behaviors have been discussed.National Science Foundation of China (NSFC) 21133007 21373164 Ministry of Science and Technology 2011CB808504 2012CB21490

Low-lying electronic states and their nonradiative deactivation of thieno[3,4-b]pyrazine: An ab initio study

National Science Foundation of China (NSFC) [21133007]; Ministry of Science and Technology [2011CB808504, 2012CB214900]State-averaged complete active space self-consistent field (SA-CASSCF) calculations have been used to locate the four low-lying electronic states of thieno[3,4-b]pyrazine (TP), and their vertical excitation energies and emission energies have been determined by means of the multistate complete active space with second-order perturbation theory (MS-CASPT2) calculations. The present results indicate that the first weak (1)n pi* excited state has a C-s-symmetry structure, unlike two bright (1)pi pi* excited states in C-2v symmetry. The predicted vertical excitation energies of the three low-lying excited states in the gas phase are 3.41, 3.92, and 4.13 eV at the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] optimized geometry, respectively. On the basis of calculations, a new assignment to the observed spectra of TP was proposed, in which the (1)n pi* state should be responsible for the weak absorption centred at 3.54 eV and the two closely spaced (1)pi pi* states account for the two adjacent absorption bands observed at 3.99 and 4.15 eV. The predicted vertical emission energies lend further support to our assignments. Surface hopping dynamics simulations performed at the SA-CASSCF level suggest that the plausible deactivation mechanism comprises an ultrafast relaxation of the (1)pi pi* excited states to (1)n pi* excited state, followed by a slow conversion to the S-0 ground state via a conical intersection. This internal conversion is accessible, since the MS-CASPT2 predicted energy barrier is similar to 0.55 eV, much lower than the Franck-Condon point populated initially under excitation. The dynamical simulations on the low-lying states for 500 fs reveal that the relatively high (1)pi pi* excited states can be easily trapped in the (1)n pi* excited state, which will increase the lifetime of the excited thieno[3,4-b] pyrazine. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770229

Ab Initio Study on Ultrafast Excited-State Decay of Allopurinol Keto-N9H Tautomer from Gas Phase to Aqueous Solution

The excited-state decay of the biologically relevant allopurinol keto-N9H tautomer populated at the optically bright S₁(¹<i>ππ</i>*) state in the gas phase and in aqueous solution has been explored theoretically. In solution, the hybrid quantum-mechanical/molecular-mechanical simulations were performed, where the QM region (keto-N9H) was treated at the ab initio SA-CASSCF level, while the MM region (water) was described by the TIP3P model. Here we find that there exist four parallel relaxation pathways in the gas phase, but only two of them occur in aqueous solution. In addition, an ultrafast S₁ → S₀ internal conversion is found in vacuum, with an estimated excited-state lifetime of 104.7 fs, much faster than that in water (242.8 fs), showing reasonable agreement with the available experimental finding in aqueous solution (τ < 200 fs). Calculations indicate that the presence of water solvent plays an important role in the excited-state dynamics of DNA base, showing the pronounced environmental effects on its decay pathways and excited-state lifetimes

<i>Ab initio</i> insights on photophysics of 9-methylhypoxanthine

<p>In this work, the low-lying electronic singlet states of 9-methylhypoxanthine (9MHPX) were explored by the complete active space self-consistent-field (CASSCF) and complete active space second-order perturbation theory (CASPT2) calculations, and the conical intersections between the optically bright excited S₁ state and ground S₀ state were optimised by the two-root state-averaged SA-2-CASSCF approach. These studies indicate that four slightly different kinds of S₁/S₀ conical intersections are identified computationally for 9MHPX, corresponding to four main internal conversion pathways, respectively, all of which are found to show the comparable timescales according to dynamics simulations. At the CASPT2 level, four bright <i>ππ</i>* transitions of 9MHPX are calculated to locate at 4.47, 5.35, 5.97 and 6.30 eV, respectively, responsible for the available experimental absorption peaks of 9MHPX in the vapour phase (4.41, 5.19, 6.05 and 6.42 eV). Though one relatively weak <i>ππ</i>* transition computed at 5.69 eV is not observed in the vapour phase, it is in accordance with the circular dichroism measurement of another hypoxanthine derivative deoxyinosine 5'-phosphate near 5.51 eV.</p