OH stretching frequencies in systems with intramolecular hydrogen bonds: Harmonic and anharmonic analyses

OH stretching wavenumbers were investigated for 30 species with intramolecularly hydrogen-bonded hydroxyl groups, covering the range from 3600 to ca. 1900 cm−1. Theoretical wavenumbers were predicted with B3LYP/6-31G(d) density functional theory using the standard harmonic approximation, as well as the second-order perturbation theoretical (PT2) anharmonic approximations available with the Gaussian software package. The wavenumbers computed with the anharmonic procedures were found to be essentially linearly related to those obtained within the harmonic analysis. The theoretical wavenumbers were compared with experimental values taken from the literature, supplemented with values estimated from infrared (IR) absorption spectra recorded for the purpose of this study. An approximately linear relationship was established between the observed wavenumbers νOH and the results of the harmonic analysis. This is significant in view of the fact that the full anharmonic PT2 analysis requires orders-of-magnitude more computing time than the harmonic analysis. νOH also correlates with OH chemical shifts.