An Empirical Potential Function of α-Glycine Derived from Infrared Spectroscopic Data of D-, 13C-, 15N-, and 18O-Labeled Species

Abstract

Abstract The infrared spectra (4000-100 cm-1) of the α-form crystal ofglycine (NH3 + - CH2 - COO-) and of thirteen isotopic modifications comprising D, 13C, 15N, and 18O were measured at 80 and 290 K. Excellent resolution was reached at the low temperature. In the low temperature spectra the fine structure of the nearly degenerate NH3 and ND3 antisymmetric deformational modes and the CO2-torsional bands in the vicinity of 200 cm-1 for each of the isotopic molecules, which in the low-frequency region are strongly overlapped by a number of lattice modes, clearly showed up. Based upon the frequency data of the 14 isotopic analogs and the precisely known structure of the molecule, a normal coordinate analysis was carried out. 307 observed frequencies were utilized to derive a new empirical valence force field reduced to a set of 50 force constants by a number of restrictive assumptions. The resulting force field reproduced the 307 frequencies with a root-mean- square deviation of 3.32 cm -1. This force field emphasizes the importance of interaction force constants of the in-plane CO2-rocking and CO2-deformational coordinates with the CH2-twisting coordinate, which can come form a significant deviation of this molecule from an ideal Cs-symmetry. The composition of normal vibrations from generally accepted local-symmetry coordinates is given in terms of the potential energy distribution (PED). The PED results indicate that almost all the normal modes below 1600 cm-1 are extensively intermixed group modes, thus precluding a simple normal vibrational decription. Interestingly the PED description for several vibrations associated with the NH3 - CH2 - C fragment exhibits strong mixing between quasi-A′ symmetric and -A″ antisymmetric coordinates with respect to a pseudo molecular symmetry (CCN) plane in this molecule.