Ab initio prediction of the vibrational spectra of hydrogen-bonded complexes PhOH•••(H2O)2

The changes of the vibrational characteristics (vibrational frequencies, infrared intensities and Raman activities) for the hydrogen-bonded complex PhOH ...(H2O)(2) have been studied by ab initio calculations at different levels of ab initio MO theory. For the complexes of phenol with two water molecules, three stable structures were determined: two open structures and one ring structure. The best agreement was established between the calculated and experimentally observed frequencies in the OH stretching region for the ring structure. In agreement with experiment, the ab initio calculations show that the most sensitive to the complexation is the OH stretching vibration from the phenol side. The ab initio calculations at the 6-31G/MP2 level predict more reliable values for the red shift (369 cm(-1)) of this vibration. The IR intensity and Raman activity of this vibration increase upon hydrogen-bonding. The hydrogen-bonded OH stretching vibrations from the water side (nu(OH)(SYM)) are more sensitive to the complexation than the free OH stretching vibrations (nu(OH)(asym)). The frequencies of (nu(OH)(sym)) are shifted to a low frequency, indicative of bond weakening. The IR intensities of these vibrations increase in the complex more than 100 times. (C) 2000 Elsevier Science B.V. All rights reserved.