ACIDITY AND BASICITY OF THIOCARBOXAMIDES IN THE GAS-PHASE

The gas-phase acidity and basicity of thioacetamide and the basicity of N,N dimethylthioformamide were measured by Fourier transform ion cyclotron resonance (FT-ICR) mass sectrometry under conditions which minimized the extent of their decomposition. Thiocarboxamides are both much stronger acids and stronger bases than carboxamides. The relative stabilities of individual neutral and ionic species were assessed in terms of isodesmic reactions, using the published or estimated enthalpies of formation. The neutral molecules of carboxamides and thiocarboxamides are stabilized by interaction between the C = X and NH2 functional groups. This interaction is of a similar magnitude in the corresponding protonated forms but it is of greater strength in the deprotonated forms. With regard to the difference between thiocarboxamides and carboxamides, the most significant factor is probably the lone pair-lone pair repulsion operating in the anions.