Dimerization of the keto tautomer of acetohydroxamic acid - infrared matrix isolation and theoretical study

Dimerization of the keto tautomer of acetohydroxamic acid has been studied using FTIR matrix isolation spectroscopy and DFT(B3LYP)/63-1+G(d,p) calculations. Analysis of CH3CONHOH/Ar matrix spectra indicates formation of two dimers in which two intramolecular CO center dot center dot H-ON bonds within two interacting acetohydroxamic acid molecules are retained. A chain dimer I is stabilized by the intermolecular CO center dot center dot center dot H-N hydrogen bond, whereas the cyclic dimer II is stabilized by two intermolecular N-H center dot center dot center dot O(H)N bonds. Twelve vibrations were identified for dimer I and six vibrations for dimer II; the observed frequency shifts show a good agreement with the calculated ones for the structures I and II. Both dimers have comparable binding energies (Delta E-ZPE(CP) (I, II) = -7.02, -6.34 kcal mol(-1)) being less stable than calculated structures III and IV (Delta E-ZPE(CP) (III, IV) = -9.50, -8.87 kcal mol(-1)) in which one or two intramolecular hydrogen bonds are disrupted. In the most stable 10-membered cyclic dimer III, two intermolecular CO center dot center dot center dot H-ON hydrogen bonds are formed at expense of intramolecular hydrogen 41 bonds of the same type. The formation of the less stable (AHA)(2) dimers in the studied matrixes indicates that the formation of (AHA)(2) is kinetically and not thermodynamically controlled. (c) 2004 Elsevier B.V. All rights reserved.