Thermodynamic stability of carbonyl and silacarbonyl anions R–M−O [M=C, Si, R=H, F, Cl, OH, NH2, CH3, N(H)OH]. A comparative ab initio study

Abstract

Ab initio molecular orbital methods at the CBS–Q level of theory have been used to study the structure and gas-phase stability of carbonyl and silacarbonyl anions R–M⁻O [M=C, Si, R=H, F, Cl, OH, NH₂, CH₃, N(H)OH]. The geometries of carbonyl, silacarbonyl acids and their anions were optimized at the MP2(FC)/6-31G() level of theory. The calculations showed that the thermodynamic stability of the carbonyl and silacarbonyl anions depends on both the type of substituent R and the possibility for competitive existence of O- and N-anions resulting from the monoionization of the multiple ionization sites. Thus formic acid, formamide, acetaldehyde and formohydroxamic acid are O- or N-acids. The result of the effect of the carbon-by-silicon substitution is considerable stabilization of the silaformyl anion. The relative stability between Si- and O-anions of silanoic acid is substantially lowered. Silaformamide, methylsilanone and silaformohydroxamic acid behave as Si-acids. The investigated acids are weak acids with calculated acidity of about 1400–1650 kJ mol⁻¹. For C-acids the acidity increases in the order: H₂CO⩽CH₃CHO<NH₂CHO<HCOOH<HC(O)NHOH<FCHO<ClCHO. Silaacids were found more acidic than their carbon counterparts. Their Si-acidity order is: CH₃SiHO⩽NH₂SiHO⩽H₂SiO<HSiOOH<HSi(O)NHOH<FSiHO<ClSiHO.