Ab Initio Study of the Stepwise versus Concerted Fragmentation Pathways in Microhydrated Thymine Radical Cations.

Thymine radiation-induced fragmentation is characterised by ring opening and the loss of HNCO/NCO. These pathways have been investigated using DFT calculations in the presence of zero, one and two water molecules. In addition to the already characterised stepwise fragmentation mechanism, we propose a novel concerted pathway reported here for the first time. We show that both the stepwise and concerted mechanisms are competitive with activation energies of 2.05 eV and 2.00 eV, respectively, in the gas phase. The effect of microhydration on these mechanisms are examined based on the most stable conformations found by an exploration of the potential energy surface performed by using DFT-based ab initio molecular dynamics. Microhydration is also accompanied by an increase in the activation energies, with respect to gas phase, amounting to 0.47 eV-an increase that is associated to a stabilising effect of water in agreement with recent experimental studies. However, we also point out that this effect is greatly dependent on the specific water arrangement around thymine and could be limited to only 0.13 eV for some configurations.