Molecular structure and barrier to internal rotation of gaseous glycolic acid

Abstract

Ab initio calculations have indicated intramolecular hydrogen bonding in glycolic acid and the existence of rotamers in the gas phase [ 1,2]. The most stable rotamer predicted by these calculations has a planar skeleton with a five-membered ring formed by a hydrogen bond between the hydroxyl group and the carbonyl oxygen. In the secondary rotamer, a hydrogen bond is formed between the two hydroxyl groups. The calculated energy difference between them is 1.8 [l] or 2.7 [2] kcal mol-’ . Accordingly, both rotamers would be observed in the gas phase if the energy difference is as small as those shown in the calculations. Microwave spectroscopy studies, however, have reported only the most stable rotamer [3,4]. The less abundant species with a small dipole moment might be overlooked in this method, but gas phase electron diffraction allows the observation of all the species existing in a gas sample, which has been proved in the analysis of glycine [5]. Thus, we undertook the analysis of gaseous glycolic acid by gas phase electron diffraction in order to elucidate the composition of the gas and the potential energy barrier between the rotamers. Commercial glycolic acid (hydroxyacetic acid)