GAS-PHASE ACIDITIES OF HM(=X)XH (M=C, SI, X=O, S) ACIDS CALCULATED BYAB-INITIO MOLECULAR-ORBITAL METHODS AT THE G2 LEVEL OF THEORY

Ab initio molecular orbital methods at the G2 level of theory have bee n used to study the structure and acidity of formic acid, silanoic aci d and their thio and dithio derivatives. The effects of the oxygen-by- sulphur and carbon-by-silicon substitution on the stability and acidit y of these acids were analysed. The calculations predict the syn forms always to be more stable. Of the acids studied dithiosilanoic acid wa s found to be the strongest and formic acid the weakest acid. Absolute values of gas-phase acidities computed by the G2 and G2(MP2) methods were comparable in accuracy. Thus the G2(MP2) method appears as a chea per alternative for obtaining accurate gas-phase acidities for larger molecules.