Prediction of OH stretching frequencies for intramolecular OH··O linkages

The OH stretching frequencies for a series of compounds with intramolecular OH-O hydrogen bonds are computed with the DVPT2 and GVPT2 anharmonic models of Barone andassociates using B3LYP-D3 density functional theory. The anharmonic models systematically underestimate the observed OH stretching wavenumbers for systems with medium to strong hydrogen-bonding, apparently due to the inability of approaches based on 2nd-order perturbation theory (PT2) to account accurately for the properties of large-amplitude vibrational motions. Nevertheless, the computed wavenumbers correlate linearly with the observed values. But it is noteworthy that the wavenumbers obtained with standard harmonic analyses lead to even better correlations than those computed with the expensive anharmonic models, resulting in linear regression equations of great predictive value (standard deviations close to 20 cm–1). For example, effective OH stretching wavenumbers for 2-acetylmalonamide enol and 2-cyanomalonamide enol are predicted between 2300 and 2400 cm–1. In contrast to the linear correlations observed for intramolecular OH∙∙∙O hydrogen bonds, intramolecular NH-O linkages are characterized by corresponding non-linear relationships.