Resonant 2-photon ionization study of the conformation and the binding of water molecules to 2-phenylethanethiol (PhCH2CH2SH).

The structures of 2-phenylethanethiol (PET, PhCH(2)CH(2)SH) and its 1:1 water clusters have been studied using resonant two-photon ionization spectroscopy including band contour analysis and UV-UV holeburning, combined with extensive ab initio calculations on ground and excited states. The most populated conformer, labeled Ggpi, has a gauche arrangement about the SCCC and HSCC bonds that permits a stabilizing SH...dpi type of hydrogen bond. The other observed conformer, Ag, is anti with respect to the SCCC bond. In the dominant 1:1 water cluster, a water molecule binds to the Ggpi conformer via an OH...S hydrogen bond and two significant CH...O interactions. There is also evidence for water binding to conformer Ag with a similar arrangement, and for a second Ggpi cluster where water inserts between the SH and the aromatic ring. The additional interactions to the water molecules result in net D(e) binding energies approximately double those resulting from a single thiol-water hydrogen bond. The (1)(pi,pi(*)) excited state lifetimes in the bare molecules are very short because of internal conversion to a dissociative (1)(n,pi(*)) state related to the thiol. In the dominant Gw(1) cluster, the lifetime is significantly increased from <1 to approximately 4 ns. Hydrogen bonding to the thiol, which raises the energy of the dissociative (1)(n,pi(*)) state, accounts for this behavior.