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

The infrared spectra (4000-100 cm -1 ) of the α-form crystal ofglycine (NH 3 + - CH 2 - COO - ) and of thirteen isotopic modifications comprising D, 13 C, 15 N, and 18 O 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 NH 3 and ND 3 antisymmetric deformational modes and the CO 2 -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 CO 2 -rocking and CO 2 -deformational coordinates with the CH 2 -twisting coordinate, which can come form a significant deviation of this molecule from an ideal C s -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 NH 3 - CH 2 - 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.