Topological analysis of electronic charge densities as a tool to study protonation effects on thiocarbonyl compounds

The substituent effects on the intrinsic basicity of a wide variety of thiocarbonyl compounds were investigated by means of a topological analysis of the electronic charge densities and their Laplacians for the neutral and the protonated species, and also by means of their harmonic vibrational frequencies. For the sake of comparison, a similar analysis was carried out for a reduced set of carbonyl compounds with similar substituents. The results show that the behaviour of the thiocarbonyl group with regard to the protonation process is similar to that predicted for the carbonyl group, in the sense that both groups are not much affected by protonation. The charge transferred to the proton results basically in a polarization and, in some cases, in a depopulation of the bonds in which the thiocarbonyl carbon atom participates. Accordingly, the intrinsic basicities of these kinds of compounds depend strongly on the ability of the substituents to be polarized. This conclusion seems to be ratified also by the shiftings undergone by the C=S stretching frequencies on protonation.