An Ab initio study of tautomerism between formhydroxamic acid and formhydroximic acid

Abstract

Ab initio MO calculations including electron correlation with the 4-31c and 4-31c** basis sets were performed in order to study the formhydroxamic acid-formhydroximic acid tautomerism. The geometries, relative energies, and activation energy of the tautomer and transition state were determined. Based on total-energy calculations at the ~~4 / 4 -3 1~* * / /~~~/ 4 -3 1~* * plus the scaled zero-point vibration energy level, the energy of formhydroxamic acid is determined to be lower than that of formhydroximic acid by 40.7 kJ/mol. The activation energy of the formhydroxamic acid-farmhydroximic acid tautomerism via a 1,3-intramolecular hydrogen shift is 151.4 kJ/mol.

Keto–iminol tautomerism in acetohydroxamic and formohydroxamic acids

“…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].…”

Ab Initio Study of Intramolecular Proton Transfer in Formohydroxamic Acid

“…Formohydroxamic acid (HCONHOH) is the simplest formula among the hydroxamic acids. Several theoretical calculations were performed related to its structural analysis. − There are two tautomeric forms, keto, 1E and 1Z , and iminol, 2E and 2Z , shown in Figure . Some experimental studies on the structure of formohydroxamic acid using X-ray and 17 O NMR concluded that the most stable structure was 1Z .…”

“…The disagreement of the experimental conclusions leaves room for theoretical studies. ,,, Most of these calculations drew their conclusion of N-acids or O-acids based on the stability comparison of the anions. Actually the confusion of these experimental results is strongly related to the experimental conditions, especially to the solvents being applied.…”

Gas-Phase Structure and Acidity of Formohydroxamic Acid and Formamide: A Comparative ab Initio Study

“…Sufficiently reliable ab initio calculations have appeared only in recent times. [6][7][8][9][10][11] Although low-quality calculations suggest that hydroxamic acids exist preferentially as the (£)-1 tautomer in the gas phase, this preference is reduced at more sophisticated theoretical levels. At the best level of calculation used up to now, MP2/6-31 lG(d,p)//SCF/6-31 lG(d,p),7 the three conformers (E)-l, (Z)-l, and (Z)-2 of formohydroxamic acid are within 5.5 kJ/mol of each other.…”