Gas-phase acidities of HM(=X)XH (MC, Si; XO, S) acids calculated by ab initio molecular orbital methods at the G2 level of theory

Ab initio molecular orbital methods at the G2 level of theory have been used to study the structure and acidity of formic acid, silanoic acid and their thio and dithio derivatives. The effects of the oxygen-by-sulphur and carbon-by-silicon substitution on the stability and acidity of these acids were analysed. The calculations predict the syn forms always to be more stable. Of the acids studied dithiosilanoic acid was 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 cheaper alternative for obtaining accurate gas-phase acidities for larger molecules.