Molecular orbital study of the structures of hydroxamic acids

Ring‐substituted benzohydroxamic acids: 1H, 13C and 15N NMR spectra and NHOH proton exchange

“…Our calculations demonstrate that the benzohydroxamic acid and its ring-substituted derivatives are most stable in the Z-conformer of the hydroxamic form, which is in agreement with previous (ab initio for benzohydroxamic acid 33,34 and o-hydroxybenzohydroxamic acid 35 and semiempirical 36 ) results. The energy difference between the Z and E conformers in the gas phase is 17.2 kJ mol 1 (or 7.9 kJ mol 1 ;…”

Keto–iminol tautomerism in acetohydroxamic and formohydroxamic acids

“…At the higher levels of theory, when more extended basis set and correlation energy were used, the 1Z-keto isomer with the OH bond involved in an intramolecular hydrogen bond, was predicted to be the most stable one. However, the 1Z, 1E-keto rotamers and the iminol tautomer with the trans and cis orientations of the OH bonds around the N-O and C-O bonds, respectively, were calculated to lie within small energy interval [23,25,26,28,31]. Due to the small energy difference the relative stability of the 1E and 2Z tautomers has been found to be dependent both on the method and on the basis set applied in the calculations.…”

“…Low-quality calculations suggested that FHA exists preferentially as the 1E-keto isomer with OH bond in anti-orientation in the gas phase [21,25,28,32]. At the higher levels of theory, when more extended basis set and correlation energy were used, the 1Z-keto isomer with the OH bond involved in an intramolecular hydrogen bond, was predicted to be the most stable one.…”

“…A number of theoretical calculations were performed related to the structural analysis of FHA [21][22][23][24][25][26][27][28][29][30][31][32][33] and AHA [34][35][36][37][38][39]. Low-quality calculations suggested that FHA exists preferentially as the 1E-keto isomer with OH bond in anti-orientation in the gas phase [21,25,28,32].…”

A critical examination of H‐bonding interactions calculated using the AM1 molecular orbital method

“…Even at the HF/6-311G(d,p) level, the 2-keto and Zenol forms are calculated to be 1.7 and 4.3 kcal/mol higher than the E-keto form. AM^ predicts the both keto forms and the Z-enol of methylhydroxamic acid to be within 0.6 kcal/mol of each other, with a slight preference for the Z-enol, whereas it predicts the Z-keto and Z-enol forms of benzohydroxamic acid to be within 0.1 kcal/mol of each other, with the E-keto form 1.1 kcal/mol higher [64]. NMR evidence suggests that mono alkylhydroxamic acids exist in both keto and one enol form in solution [65], in agreement with the calculations.…”