VIBRATIONAL-SPECTRUM OF THE ACETONE WATER COMPLEX - A MATRIX-ISOLATION FTIR AND THEORETICAL-STUDY

The FT-infrared absorption spectrum of the hydrogen-bonded acetone-water complex has been investigated in solid argon matrices. Acetone and water vapors were co-condensed with an ''cess of argon gas at 12 K, giving rise to red- or blue-shifted absorptions near most of those associated with the fundamental transitions of matrix-isolated acetone or water. Vibrational shifts are indicative of a 1:1 complex of acetone and water, in which water is hydrogen-bonded to the carbonyl oxygen of acetone. Accordingly, red shifts are observed for the C=O stretching mode of acetone and the O-H stretching modes of water. Corresponding shifts of similar magnitude were observed when d6-acetone was used. Ab initio SCF computations of the equilibrium structure and fundamental vibrational frequencies indicate a cyclic hydrogen-bonded structure involving interactions between water and acetone at both the carbonyl oxygen and one methyl hydrogen. Changes in the net charges and bond orders upon formation of the complex are as would be expected for the early stages of a water-catalyzed keto-enol transformation. Predicted frequency shifts were found to be in excellent agreement with most of the observed complex vibrational frequency shifts, when the 6-31G**basis set was employed. The influence of hydrogen bonding on the structure, electronic distribution, and vibrational frequencies of acetone and water is discussed.