Substituent effects on gas-phase acidities of formic acid and its silicon and sulphur derivatives R-M(=X)XH(M=C, Si; X=O, S; R=H, F, Cl, OH, NH2 and CH3)

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the effect of substituent (F, Cl, NH2, OH and CH3) on the gas-phase acidities of formic acid, HCOOH, its silicon and sulphur derivatives R-M(=X)XH(M=C, Si; X=O, S; R=F, Cl, OH, NH2 and CH3). For formic acid and its thio and dithio derivatives the acidity changes upon substitution are irregular and depend on both the type of substituent, position and degree of replacement of oxygen atoms by sulphur atoms. For sila carboxylic acids and their thio and dithio derivatives the calculated acidities regularly increase in the order: R-SiOOH < R-Si(=S)OH << R-Si(=O)SH < R-SiSSH (R=H, F, Cl, OH, NH2 and CH3). The chloro derivatives are the strongest among the acids studied. The highest gas phase acidity (1277.6 kJmol-1) has been calculated for ClC(=S)OH.