Experimental and theoretical studies on protonation of thioketones

The site of protonation in two classes of thioketones, (RC6H4)2CS (R = H or OCH3) and But2CS, was investigated by 1H and 13C n.m.r. spectroscopy. For aromatic thioketones, protonation occurs on the thiocarbonyl carbon from the side of the π-plane, whereas for the aliphatic compound, protonation of the sulphur atom occurs from the side of the σ-plane as in the case of carbonyl compounds. Electronic absorption spectra also provide information on sites of protonation in these molecules. Results are discussed in terms of total energies, charge-density distributions, and excitation energies of R2CS and its protonated derivative elucidated by semi-empirical SCF-MO calculations in the CNDO/2 approximation.