GAS-PHASE STRUCTURE AND ACIDITY OF FORMOHYDROXAMIC ACID AND FORMAMIDE - A COMPARATIVE AB-INITIO STUDY

The structures and gas-phase acidity of formohydroxamic acid and formamide were calculated at the SCF and MPn levels using several basis sets, up to 6-311++G(2d,2p). The global minimum found on the potential energy surface of formohydroxamic acid presents a nonplanar conformation around the nitrogen atom and a quasi-linear hydrogen bond between both oxygen atoms. Delocalization in this structure is similar to that found in formamide. This minimum is more stable than the previously reported ones, especially that featuring also a nonplanar conformation around the nitrogen atom but a torsional HONC angle of about 90 degrees. This latter structure (a true, albeit local, minimum) corresponds with the structure found in all but one of the crystallographic studies on synperiplanar hydroxamic acids present in the Cambridge Structural Database. The fully planar hydrogen-bonded structure of formohydroxamic acid is a transition state. However, the height of this transition state over the symmetrically located minima is very low at the best theoretical level used here (1.7 kJ/mol). Moreover, if enthalpy and entropy contributions are accounted for, then the transition state is more stable than the minima at room temperature. We conclude then that formohydroxamic acid in the gas phase can be most accurately represented by a hydrogen-bonded planar structure. The OH group in formohydroxamic acid stabilizes the enol form with respect to the keto one, in comparison with the analog situation in formamide. While formimide free energy at 298 K is about 56 kJ/mol higher than that for formamide, the same difference is reduced to around 2 kJ/mol for formohydroxamic and formohydroximic acids. Absolute Delta G degrees(HA) and the relative values of the acidity of formohydroxamic acid with respect to that of formamide agree well with those determined experimentally for acetohydroxamic acid and acetamide in the gas phase. The best ab initio result obtained indicates that formohydroxamic acid requires about 76 kJ/mol less energy to dissociate than formamide. The semiempirical AM1 method is essentially in agreement with this conclusion.