Vibrational spectrum of the hydrogen-bonded complex H2CO...HOCH=NH:: an ab initio study

The vibrational spectra for free and complexed H2CO and HN=CHOH have been studied by ab initio calculations employing the 6-31G basis set. The changes in the vibrational characteristics (vibrational frequencies, infrared intensities and Raman activities) upon hydrogen bonding have been estimated. In agreement with the behaviour always observed with H-bond formation, the ab initio calculations show that the most sensitive to the complexation are the v(O-H) and v(C=O) stretching vibrations for both monomer bonds involved in the hydrogen bonding. These vibrations are shifted to lower frequency, corresponding to bond weakening. Their infrared intensities increase with the complex formation. The vibrational characteristics of the remaining vibrations are changed with smaller magnitudes. (C) 1998 Elsevier Science B.V. All rights reserved.