Electronic and vibrational spectroscopy of 1-methylthymine and its water clusters:: The dark state survives hydration

Electronic and vibrational gas phase spectra of 1-methylthymine (1MT) and 1-methylurocil (1MU) and their clusters with water are presented Mass selective IR/UV double resonance spectra confirm the formation of pyrimidine-water clusters and are compared to calculated vibrational spectra obtained from ob initio calculations. In contrast to Y He, C Wu, W. Kong; J. Phys. Chem. A, 2004, 108, 94 we ore able to detect 1MT/1MU and their water clusters via resonant two-photon delayed ionization under careful control of the applied water-vapor pressure. The long-living dark electronic state of 1MT and 1MU detected by delayed ionization, survives hydration and the photostability of 1MT/1MU cannot be attributed solely to hydration. Oxygen coexpansions and crossed-beam experiments indicate that the triplet state population is probably small compared to the (1)n pi* and/or hot electronic ground state population. Ab initio theory shows that salvation of 1MT by water does not lead to a substantial modification of the electronic relaxation and quenching of the (1)n pi* state. Relaxation pathways via (1)pi pi-n pi*(1) and (1)pi pi*-S-0 conical intersections and barriers have been identified, but are not significantly altered by hydration.