Intermolecular Hydrogen Bond in Acetic Acid Solutions. Raman Spectra and ab initio Calculations

Abstract

C=O vibrational bands of various aggregated formations of pure acetic acid in the Raman spectra have different values of the depolarization ratio. Ab initio calculations using the Gaussian 98 W program in the Hartree-Fock approximation with the basic set of Gaussian functions RHF 6-31G++(d, p) show that this is how it should be. Thus, the formation of aggregates from molecules leads not only to a change in the dipole moment of the molecules, but also to a change in the bond polarizability tensor. Calculations according to this program showed that one of the hydrogen atoms of CH3 group of acetonitrile can participate in the intermolecular hydrogen bond. This can lead to the formation of closed acetonitrile-acetic acid dimers. However, a comparison of the calculated and experimental data shows that in the case of acid-acetonitrile molecules, the H-bond is formed in the direction of the elongation –О-Н…N of acetonitrile molecules. The activity of one of the hydrogen atoms of the CH3 group of acetonitrile leads to the formation of closed dimeric aggregates in pure acetonitrile with a shift of the СN band of acetonitrile towards lower frequencies. The low-frequency asymmetry of СN acetonitrile band in the Raman spectra is associated with the presence of such aggregates in liquid acetonitrile.