Excited State Proton Transfer Dynamics of Thioacetamide in S2(ππ*) State: Resonance Raman Spectroscopic and Quantum Mechanical Calculations Study

The photophysics and photochemistry of thioacetamide (CH3CSNH2) after excitation to the S2 electronic state were investigated by using resonance Raman spectroscopy in conjunction with the complete active space self-consistent field (CASSCF) method and density functional theory (DFT) calculations. The A-band resonance Raman spectra in acetonitrile, methanol, and water were obtained at 299.1, 282.4, 266.0, 252.7, and 245.9 nm excitation wavelengths to probe the structural dynamics of thioacetamide in the S-2 state. CASSCF calculations were done to determine the transition energies and structures of the lower-lying excited states, the conical intersection points CI(S-2/S-1) and CI(S-1/S-0), and intersystem crossing points. The structural dynamics of thioacetamide in the S2 state was revealed to be along eight Franck-Condon active vibrational modes nu(15), nu(11), nu(14), nu(10), nu(8), nu(12), nu(18), and nu(19), mostly in the CC/CS/CN stretches and the CNH8,9/CCH5,6,7/CCN/CCS in-plane bends as indicated by the corresponding normal mode descriptions. The S-2 -> S-1 decay process via the S-2/S-1 conical intersection point as the major channel were excluded. The thione-thiol photoisomerization reaction mechanism of thioacetamide via the S-2,S-FC -> S'(1,min) excited state proton transfer (ESPT) reaction channel was proposed.