Molecular structure of a conformer of glycine by microwave spectroscopy

Abstract

We have observed and assigned the rotational spectra of 18O-glycine and, for the amino and hydroxy groups, all monodeutero and dideutero isotopically substituted species of the conformer of glycine which has the most intense microwave spectrum, gly(2). The derived spectroscopic data made possible the first complete gas-phase structural determination of an aminoacid. The values for the molecular parameters determined in a least-squares fit are well resolved and in harmony with the results for similar molecules and the predictions of high-level ab initio calculations. Calculation of the ab initio potential energy function for the amino torsion at the MP2/6-31G∗∗ level confirms that a large-amplitude vibrational motion is associated with the gly(2) species. It was found that the standard deviations in the results from the least-squares fit of the molecular parameters of both gly(2) were anomalously large. A similar effect was found in a test molecule—formamide. We conclude that when the experimentally derived rotational constants for a molecule are vibrationally averaged over a large-amplitude motion the explicit inclusion of LAM rotation–vibration interaction appears necessary to achieve more precise results.