Structure and acidity of carbamic acid, and its thio, dithio and sila dervatives
An ab initio molecular orbital method (at the 6–31G** level of theory) has been used to study the molecular structure of carbamic acid, thiocarbamic O-acid, thiocarbamic S-acid, dithiocarbamic acid and silacarbamic acid, respectively. The total internal energy of the molecules investigated was calculated as a sum of the HF energy, the electron correlation MP2 contribution and zero-point vibrational energy. Fully optimized geometries of the syn and anti conformers of the compounds were determined, and the ionization energies of the above-mentioned acids were computed and the effects of the oxygen-by-sulfur and carbon-by-silicon substitution analysed. The calculations predict the syn forms always to be more stable. The highest gas-phase acidity (1513.9 kJ mol–1) has been calculated for carbamic acid. The replacement of the carbon and/or oxygen atoms of the carbamic acid in the substituted sila and thio derivatives evokes a decrease in the acidity. Changes in the Mulliken gross atomic populations were examined with respect to their dependence on the configuration and thio- and sila-substitution of carbamic acid.