ACIDITY OF HYDROXAMIC ACIDS - AN AB-INITIO AND SEMIEMPIRICAL STUDY

High-level ab initio calculations, using basis sets with several diffuse and polarization functions and including correlation energy through second-order Moller-Plesset (MP2) theory, were performed on the different possible structures of formo- and acetohydroxamate anions to determine their preferred conformations in the gas phase. We found that, in agreement with recent experimental determinations, both acids behave as NH acids rather than OH acids in the gas phase. AM1 semiempirical and ab initio calculations are in general agreement and agree with this particular conclusion. However, AM1 predicts one conformer, unstable at the MP2/6-311++G(2d,2p) level, to have a heat of formation very similar to that of the most stable structure. Comparison of the AM1 results with the experimental free energies of dissociation for several species in the gas phase supports the conclusion that acetohydroxamic acid is a NH acid. AM1 acidities are compared with the acidities of aceto- and benzohydroxamic acids in DMSO by correlating the (AM1) enthalpies of dissociation with the experimental pK(a)s. Acetohydroxamic acid is clearly predicted to be a NH acid in DMSO, but the behavior of benzohydroxamic acid seems less clear. The possibility that OH dissociation might occur is compatible with the data for benzohydroxamic acid. An analysis of the acidities of substituted anilines and benzoic acids in DMSO is presented to independently assess the ability of AM1 to represent NH and OH dissociations in simpler compounds. Finally, pK(a)s of several N-substituted and other hydroxamic acids in aqueous solution are compared with the calculated AM1 enthalpies of dissociation. The results indicate that in aqueous solution hydroxamic acids are OH (rather than NH) acids. An analysis of the calculated structures of hydroxamate anions solvated with four water or MeOH molecules explains the prevalence of OH over NH dissociation in protic solvents.