Structure, reactivity and vibrational spectra of formohydroxamic and silaformohydroxamic acids:: a comparative ab initio study

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the structure, gas-phase acidities and vibrational spectra of formohydroxamic acid HC(=O)NHOH, and its sila derivative HSi(=O)NHOH. The geometries of various tautomers and rotamers or formohydroxamic and silaformohydroxamic acids, their anions and protonated forms were optimized at the MP2(FC)/6-31G((dagger)) level of theory. The calculations showed that the molecules of neutral acids should exist in several forms very close in energy. The molecules of neutral formohydroxamic and silaformohydroxamic acids exist in more stable keto form. The formohydroxamic tautomer is by about 20 kJ mol(-1) more stable than the hydroximic structure. The result of the effect of the carbon-by-silicon substitution is considerable destabilization of the hydroximic tautomer. In the structure of the neutral form and anion of the parent hydroxamic acid the intramolecular hydrogen bond stabilizes the structure and makes the most stable conformations more planar. In both acids the HON- anion is more stable than the O-anion, hence formohydroxamic and silaformohydroxamic acids in the gas phase are N-acids. Formohydroxamic acid is a weak acid with calculated acidity of about 1427 kJ mol(-1). The acidity increases in the order: HSiONHOH < HCONHOH. The proton affinity of formohydroxamic acid was computed to be -821.0 kJ mol(-1). Silaformohydroxamic acid is a stronger base with computed proton affinity Delta H-298 = -883.0 kJ mol(-1). The fundamental vibrational frequencies have been computed using the MP2(Full)/6-31G(d) method. Sila substitution produces an appreciable shift of the corresponding harmonic vibrational frequencies of formohydroxamic acid. (C) 2000 Elsevier Science B.V. All rights reserved.