Influence of microhydration on the ionization energy thresholds of thymine: comparisons of theoretical calculations with experimental values

In the present study the ionization energy thresholds (IETs) of thymine, and thymine keto-enol tautomers, have been calculated (with the B3LYP, and P3 levels of theory using the standard 6-31++G(d,p) basis set) with 1-3 water molecules placed in the first hydration shell. Calculations show there is a distinct effect of microhydration on the IET of thymine. The first water molecule is seen to decrease the IET by about 0.1 eV, whereas the second and third water molecules cause a further decrease of less than 0.1 eV each. The changes in IET calculated here for thymine with 1-3 waters of hydration are smaller than the experimental values determined by Kim et al. (J. Phys. Chem. 1996, 100, 7933). Therefore calculations have been performed on the microhydrated keto-enol tautomers of thymine. The calculated results on the keto-enol tautomers are seen to be in better agreement with the experimental results. However, the keto-enol thymine tautomers are considerably higher in energy than the canonical form of thymine, and there is presently no good evidence that these thymine tautomers are actually present in a supersonic jet-cooled experiment.