Glycolic Acid as a Vibrational Anharmonicity Benchmark.

Three energetically close pairs of vibrational states in glycolic acid are investigated by Raman spectroscopy in a supersonic jet to provide challenging benchmarks for vibrational and electronic structure theory and to solve some open issues in this prototypical hydroxy acid. The alcoholic OH stretching fundamental is located only 8 cm below the acidic OH stretch at 3586 cm, much less shifted than predicted by previous anharmonic calculations and by experimental analogy to a fluorene derivative. This and further near-degeneracies in the CH and C=O stretching region are used to assess the predictive power of an exploratory set of quantum chemical calculations including anharmonic VPT2 corrections.