Ab initio calculations of low-lying electronic states of vinyl chloride

The equilibrium geometries, vibrational frequencies, excitation energies, and oscillator strengths of vinyl chloride in the ground and five lowest-lying excited singlet states have been calculated using MP2, CIS, CASSCF, and MRCI methods with the 6-31111G** basis set. The geometries and vibrational frequencies of the ground and excited states are utilized to compute Franck–Condon
factors. Calculated vibronic spectra for the transitions from the ground state to these five excited states are in agreement with experiment at 52 500–60 000 cm21, with major contributions from the ˜A(1 1A9)ˆX ˜ (1 1A8) and C˜ (2 1A8)ˆX ˜ (1 1A8) transitions. In this study, two spin-forbidden transitions of ˜b(1 3A9)ˆX ˜ (1 1A8) and ˜c(2 3A9)ˆX ˜ (1 1A8) are calculated to locate in 45 000–54 000 cm21, and could be responsible for the observed one-photon absorption spectrum due to an intensity borrowing caused by the spin–orbit coupling of the Cl atom. Based on calculation, we speculate that upon the excitation of vinyl chloride at 193 nm the ˜b(1 3A9) or ˜c(2 3A9) excited
state, instead of the ~p,p*!, is initially prepared prior to the subsequent photodissociation processes.