Molecular structure of 2-(hydroxyimino)propanohydroxamic acid in solid state and DMSO solution.

The 1H and 13C NMR spectra of 2-(hydroxyimino)propanohydroxamic acid (hpha) were measured in DMSO-d6 solution. The set of several monomeric structures along with the cluster of H-bonded hpha with three DMSO molecules were proposed to fit the experimental data. The calculated chemical shifts [B3LYP/6-311++G(d,p)] strongly suggested the formation of the cluster in which all the labile protons were H-bonded to the solvent molecules. The comparison between experimental and calculated Raman spectra of hpha in DMSO also suggested that in these conditions the investigated compound forms the proposed cluster rather than dimers. According to our calculations [B3LYP/6-31+G(d)] this cluster was energetically stabilized (84-106 kJ mol-1) compared to postulated dimeric structures. On the other hand, formation of dimers was proposed to be present for hpha in solid state. The comparison of the vibrational data (IR, RS) with the computed harmonic frequencies of three most probable dimers [B3LYP/6-31+G(d)] suggested that the dimer in which molecules adopted the zEe-keto form and were linked by two symmetric, almost linear H-bonds between the carbonyl oxygen atoms and the hydroxamic O-H protons was the predominant species of hpha in the solid state. Thus, the structures of hpha in solid state and DMSO solution appeared to be different.