Vibrational study of intramolecular hydrogen bonding in o-hydroxybenzoyl compounds Original Research Article

The vibrational study of a set of o-hydroxybenzoyl compounds (salicylaldehyde, o-hydroxyacetophenone, methyl salicylate and salicylamide) and their parent compounds (phenol, benzaldehyde, acetophenone, methyl benzoate and benzamide) has been performed by infrared (IR) measurements and by quantum chemical calculations using B3LYP density functional in conjunction with 6-31G∗∗ and 6-311++G(3df, 2p) basis sets. The assignment of the infrared bands results unequivocal for most cases, including those signals in which the motion of the intramolecular hydrogen bond (IMHB) is involved. The analysis of the IR spectra for the deuterated derivatives was proved to be a very useful tool for this purpose. A model based on isotopically substituted benzenes was used for classifying the phenyl ring movements of the molecules studied. The results obtained show the suitability and limitation of the harmonic treatment using B3LYP method to describe the characteristic IMHB vibrations. Vibrational data are analyzed in order to obtain information about the nature of the hydrogen bonding interaction in this family of compounds. Finally, the available empirical relationships to approximate the IMHB energy from experimental spectroscopic data are revised in view of the new theoretical results.