The effect of hydrogen bonding on the nonadiabatic dynamics of a thymine-water cluster

Hans Lischka,Mario Barbatti,Farhan Siddique,Anita Das,Adelia J.A Aquino

doi:10.1016/j.chemphys.2018.07.050

Abstract

Surface hopping photodynamics simulations have been performed on a cluster of thymine interacting with six water molecules (T(H2O)6). The second-order algebraic diagrammatic construction method (ADC(2)) has been used for calculating the required electronic energies and excited state gradients. Comparison with the previously performed photodynamics for the isolated thymine (Molecules 21 (2016) 1603) shows a similar global behavior and the central role of the S1(nπ∗) minimum for further long-term dynamics. The main difference comes from the destabilization of the nπ∗ state by hydrogen bonding, which leads to a significantly enhanced conversion rate from the bright S2(ππ∗) state to S1(nπ∗) for the T(H2O)6 cluster. On the other hand, the decay time to S0 and the trapping in S1 is significantly increased. Due to the localized character of the lone pair orbital involved in the nπ∗ transition at one oxygen atom, specific changes in the structure of the hydrogen bonded network are observed. Since the hydrogen bonding of the water molecules connected to that oxygen atom is specifically weakened, they show dissociations from thymine during the photodynamics, starting within 30 fs after electronic excitation of thymine.

Highlights

The photodynamics of the DNA nucleobase thymine (T) still poses many puzzling questions concerning the origin of the experimentally observed decay processes
In the femtosecond transient absorption study of pyrimidine bases in aqueous solution by Hare et al [23], two decay times have been found for thymine, a fast one at 0.72 and 2.8 ps depending on the probe wavelength and a slow one of 30 ps
The photodynamics of isolated thymine starts in the bright S2(ππ*) state

Summary

Introduction

The photodynamics of the DNA nucleobase thymine (T) still poses many puzzling questions concerning the origin of the experimentally observed decay processes. These simulations represent well the reduced trapping in S2 as compared to the surface-hopping dynamics based on CASSCF [14] but do not seem to explain completely the experimentally observed time constant of about 6 ps since 70% of the population decays to the ground state in the sub-picosecond time scale.

Results

Conclusion