title: Electronic and vibrational spectroscopy of 1-methylthymine and its water clusters: the dark state survives hydration.
abstract: Electronic and vibrational gas phase spectra of 1-methylthymine (1MT) and 1-methyluracil (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 ab initio calculations. In contrast to Y. He, C. Wu, W. Kong; J. Phys. Chem. A, 2004, 108, 94 we are 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 solvation 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*(1)-n pi*(1) and (1)pi pi*-S(0) conical intersections and barriers have been identified, but are not significantly altered by hydration.
authors.affiliation: Institut für Physikalische Chemie und Elektrochemie 1, Heinrich-Heine Universität-Düsseldorf, 40225 Düsseldorf, Germany.
authors.forename: Matthias
authors.initials: M
authors.lastname: Busker
authors.forename: Michael
authors.initials: M
authors.lastname: Nispel
authors.forename: Thomas
authors.initials: T
authors.lastname: Häber
authors.forename: Karl
authors.initials: K
authors.lastname: Kleinermanns
authors.forename: Mihajlo
authors.initials: M
authors.lastname: Etinski
authors.forename: Timo
authors.initials: T
authors.lastname: Fleig
mesh_terms: D000465:Algorithms; D004583:Electrons; D008956:Models, Chemical; D044367:Phase Transition; D010777:Photochemistry; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet; D013057:Spectrum Analysis; D013941:Thymine / Q000031:analogs & derivatives* / Q000737:chemistry; D014498:Uracil / Q000031:analogs & derivatives* / Q000737:chemistry; D014732:Vibration; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; C030630:1-methylthymine; D014498:Uracil; C002596:1-methyluracil; D013941:Thymine