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

The FT-infrared absorption spectrum of the hydrogen-bonded acetone-wat er complex has been investigated in solid argon matrices. Acetone and water vapors were co-condensed with an ''cess of argon gas at 12 K, gi ving rise to red- or blue-shifted absorptions near most of those assoc iated with the fundamental transitions of matrix-isolated acetone or w ater. Vibrational shifts are indicative of a 1:1 complex of acetone an d water, in which water is hydrogen-bonded to the carbonyl oxygen of a cetone. Accordingly, red shifts are observed for the C=O stretching mo de of acetone and the O-H stretching modes of water. Corresponding shi fts of similar magnitude were observed when d6-acetone was used. Ab in itio SCF computations of the equilibrium structure and fundamental vib rational frequencies indicate a cyclic hydrogen-bonded structure invol ving interactions between water and acetone at both the carbonyl oxyge n 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 s tages of a water-catalyzed keto-enol transformation. Predicted frequen cy shifts were found to be in excellent agreement with most of the obs erved complex vibrational frequency shifts, when the 6-31G*basis set was employed. The influence of hydrogen bonding on the structure, elec tronic distribution, and vibrational frequencies of acetone and water is discussed.