Theoretical prediction of the electronic spectrum of thioacetone and comparison with related systems

Abstract

A series of ab initio CF and CI calculations is reported for thioacetone (CH 3) 2 C  S in whch a double-zeta AO basis augmented by bond-type polarization, sulfur 3d and Rydberg-type functions is employed. All but six of the system's valence electrons are left free to be correlated in the multi-reference double-excitation CI treatment undertaken. The calculations characterize the first singlet-singlet transition as n, π*) and agree with the corresponding observed excitation energy value within 0.1 eV, but they strongly suggest that the next two allowed transitions in this system (below 6.0 eV) should be assigned as (n, 4s) and (n, 4pσ) respectively. The latter conclusion is shown to be quite consistent with recent theoretical and experimental studies for H 2CS and the analogous carbonyl systems formaldehyde and acetone. Further comparison shows that the transitions of thioacetone are quite generally shifted to significantly longer wavelengths than the corresponding species in H 2CS, just as has been observed upon methylation of H 2CO. In addition thiocarbonyl transitions are invariably found at lower excitation energies than the corresponding species in their carbonyl analogues.