Ž . Chemical Physics 246 1999 167–208 www.elsevier.nlrlocaterchemphys Vibrational study of intramolecular hydrogen bonding in o-hydroxybenzoyl compounds J. Palomar ) , J.L.G. De Paz, J. Catalan ´ Departamento de Quımica Fısica Aplicada, UniÕersidad Autonoma de Madrid, Cantoblanco, E-28049 Madrid, Spain ´ ´ ´ Received 1 March 1999 Abstract Ž The vibrational study of a set of o-hydroxybenzoyl compounds salicylaldehyde, o-hydroxyacetophenone, methyl . Ž salicylate and salicylamide and their parent compounds phenol, benzaldehyde, acetophenone, methyl benzoate and . Ž . benzamide has been performed by infrared IR measurements and by quantum chemical calculations using B3LYP density )) Ž . functional in conjunction with 6-31G and 6-311 qqG 3df, 2p basis sets. The assignment of the infrared bands results Ž . 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. q 1999 Elsevier Science B.V. All rights reserved. 1. Introduction Ž . Historically, the infrared IR spectroscopy has been the most important spectroscopic method to detect and characterize hydrogen bonds because of the sensitivity of vibrational modes to the presence w x of this type of interaction 1,2 . Ž . A strong intramolecular hydrogen bond IMHB occurs in the molecular structures of the o-hydroxy- benzoyl compounds as a result of their bearing a hydroxyl group and a carbonyl group in adjacent ) Corresponding author. E-mail: pepeo@tendilla.qfa.uam.es Ž . positions that act as a proton donor acid and accep- Ž . tor base , respectively. It is widely accepted that the presence of this strong IMHB is partially responsible in these compounds for their interesting and charac- w x teristic photophysical properties 3–7 . Measurements by IR spectroscopy of the charac- Ž . teristic vibrations of the donor group, n OH , and Ž . acceptor group, n C5O , have been widely used for an approximate evaluation of the strength of the w x hydrogen bond of the type –O–H PPP O5C 8,9 . One recent IR application consisted of the evaluation of the greatly decreased basicity of the carbonyl free lone pair of hydrogen-bonded salicylic acid deriva- w x tives 10 . This spectroscopic technique has also shown the presence of a number of rotamers of these 0301-0104r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0301-0104 99 00159-7