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

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 S-1(n pi*) minimum for further long-term dynamics. The main difference comes from the destabilization of the n pi* state by hydrogen bonding, which leads to a significantly enhanced conversion rate from the bright S-2(pi pi*) state to S-1(n pi*) for the T(H2O)(6) cluster. On the other hand, the decay time to S-0 and the trapping in S-1 is significantly increased. Due to the localized character of the lone pair orbital involved in the n pi* 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, starling within 30 fs after electronic excitation of thymine.