Microwave investigation of lactonitrile. Potential functions to the hydroxyl and methyl group torsions

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Abstract
The rotational spectrum of lactonitrile, CH3CH(OH)CN, has been reinvestigated. The analysis has been extended to the CH3CH(OD)CN species and to several excited states of the hydroxyl and methyl internal rotations. The molecule exhibits two conformers with the hydroxyl hydrogen gauche with respect to the cyano group. The conformer A with the hydroxyl hydrogen adjacent to the methinic hydrogen is 118±30 cm−1 (75±30 cm−1 for the OD species) more stable than the conformer B with the hydroxyl hydrogen adjacent to the methyl group. The barrier height to the interconversion is about 400 cm−1. Rotational A–E splittings due to methyl internal rotation have been observed in the first excited state of both conformers, yielding effective V3 barriers of 3.7 kcal/mol for conformer A and 4.3 kcal/mol for conformer B. By using a one-dimensional flexible model treatment the potential function and the structural relaxation associated with the OH torsion have been determined from the level spacings and the variations of the rotational constants, upon excitation. The potential profile and the structural relaxation indicate a repulsion of the hydroxyl hydrogen atom by the methyl group and an attraction by the cyano group. A preliminary two-dimensional model including both torsions is applied to the parent species. A comparison of the respective results with observations suggests some potential energy coupling of the two torsions and/or structural relaxation associated also with methyl internal rotation.