The effect of microhydration on ionization energies of thymine.

A combined theoretical and experimental study of the effect of microhydration on ionization energies (IEs) of thymine is presented. The experimental IEs are derived from photoionization efficiency curves recorded using tunable synchrotron VUV radiation. The onsets of the PIE curves are 8.85 +/- 0.05, 8.60 +/- 0.05, 8.55 +/- 0.05, and 8.40 +/- 0.05 eV for thymine, thymine mono-, di-, and trihydrates, respectively. The computed (EOM-IP-CCSD/cc-pVTZ) AIEs are 8.90, 8.51, 8.52, and 8.35 eV for thymine and the lowest isomers of thymine mono-, di-, and tri-hydrates. Due to large structural relaxation, the Franck-Condon factors for the 0 <-- 0 transitions are very small shifting the apparent PIE onsets to higher energies. Microsolvation strongly affects IEs of thymine--the addition of each water molecule reduces the first vertical IE by 0.10-0.15 eV. The adiabatic IE decreases even more (up to 0.4 eV). The magnitude of the effect varies for different ionized states and for different isomers. For the ionized states that are localized on thymine the dominant contribution to the IE reduction is the electrostatic interaction between the delocalized positive charge on thymine and the dipole moment of the water molecule.