Theoretical study of the structure, energetics, and the n-π* electronic transition of the acetone plus nH2O (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 functional and ab initio B3LYP, MP2, and CCSD(T) methods. The excitation energies 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 oxygen atom of H2O forms a weak hydrogen bond with a methyl hydrogen. The second H2O occupies a position between the first water and a methyl group, and the third H2O occupies a position between the second H2O and 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 for acetone and water, particularly, red shifts for the OH stretching vibrations (up to 358 cm(-1)) and CO stretching vibrations, as well as a blue shift for the HOH bending vibrations. A small but noticeable red 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 agreement with the experimental blue shift observed in acetone/H2O. The oscillator strength for the n-pi* transition increases from zero to similar to 10(-4) in (CH3)(2)CO .(H2O)(3). The effect of the coordination of water molecules on the spectral intensity is expected to be weaker than the effect due to vibronic coupling.