ExcitedState Proton Transfer Dynamics of Thioacetamidein S2(ππ*) State: Resonance Raman Spectroscopicand Quantum Mechanical CalculationsStudy.

The photophysics and photochemistryof thioacetamide (CH3CSNH2) after excitationto the S2electronicstate were investigated by using resonance Raman spectroscopy in conjunctionwith the complete active space self-consistent field (CASSCF) methodand density functional theory (DFT) calculations. The A-band resonanceRaman spectra in acetonitrile, methanol, and water were obtained at299.1, 282.4, 266.0, 252.7, and 245.9 nm excitation wavelengths toprobe the structural dynamics of thioacetamide in the S2state. CASSCF calculations were done to determine the transitionenergies and structures of the lower-lying excited states, the conicalintersection points CI(S2/S1) and CI(S1/S0), and intersystem crossing points. The structuraldynamics of thioacetamide in the S2state was revealedto be along eight Franck–Condon active vibrational modes ν15, ν11, ν14, ν10, ν8, ν12, ν18, and ν19, mostly in the CC/CS/CN stretchesand the CNH8,9/CCH5,6,7/CCN/CCS in-planebends as indicated by the corresponding normal mode descriptions.The S2→ S1decay process via the S2/S1conical intersection point as the major channelwere excluded. The thione–thiol photoisomerization reactionmechanism of thioacetamide via the S2,FC→ S′1,minexcited state proton transfer (ESPT) reaction channelwas proposed. [ABSTRACT FROM AUTHOR]