THEORETICAL-STUDY OF THE STRUCTURE, ENERGETICS, AND THE N-PI-ASTERISKELECTRONIC-TRANSITION OF THE ACETONE PLUS NH(2)O (N = 1-3) COMPLEXES

The structure, energetics, and vibrational spectra of the (CH3)(2)CO . (H2O)(n) (n = 1-3) complexes have been studied using density functiona l and ab initio B3LYP, MP2, and CCSD(T) methods. The excitation energi es and the oscillator strength for the n-pi electronic transition in acetone and acetone-water complexes have been calculated using the CIS , CASSCF, and CASPT2 approaches. The results show that the first water molecule is coordinated to the carbonyl group of acetone, while the o xygen atom of H2O forms a weak hydrogen bond with a methyl hydrogen. T he second H2O occupies a position between the first water and a methyl group, and the third H2O occupies a position between the second H2O a nd the methyl hydrogen of acetone. The energies of the coordination of the first, second, and third water molecules to the complexes are 3.7 , 5.7, and 6.7 kcal/mol, respectively. The formation of the (CH3)(2)CO .(H2O)(n) complexes results in the shift of vibrational frequencies f or acetone and water, particularly, red shifts for the OH stretching v ibrations (up to 358 cm(-1)) and CO stretching vibrations, as well as a blue shift for the HOH bending vibrations. A small but noticeable re d shift. (similar to 30 cm(-1)) of the C-H stretch can be observed in the (CH3)(2)CO .(H2O)(2) complex 2a. The excitation energy of the n-pi electronic transition is blue shifted by 0.25-0.30 eV, which is in a greement with the experimental blue shift observed in acetone/H2O. The oscillator strength for the n-pi transition increases from zero to s imilar to 10(-4) in (CH3)(2)CO .(H2O)(3). The effect of the coordinati on of water molecules on the spectral intensity is expected to be weak er than the effect due to vibronic coupling.