The Case of Glycine Continued - Some Contradictory Scf Results

by Ramek, M.; Cheng, V. K. W.; Frey, R. F.; Newton, S. Q.; Schafer, L.

The energies of five conformations of glycine were determined by ab initio HF/SCF gradient geometry optimization using 13 different basis sets. The conformations considered are the stretched form (I), a heavy-atom framework planar (II) and non-planar (III) cyclic form, and a planar (IV) and non-planar (V) bifurcated structure. Form I is the global energy minimum in all calculations, but contradictory results are obtained for the stabilities of the other forms. Of the cyclic forms, II is a local energy minimum at the 4-31G, 6-31G, 6-31 + G, and (10s,5p/4s) levels, but a saddle point in calculations with both smaller and larger basis sets (including 6-31G**, 6-31 + G**, and 6-31 + +G**). Similarly contradictory results are obtained for IV. There is considerable scatter in calculated energy differences and in the optimized values of the non-planar N-C-C = O torsions. Form I and the planar cyclic form II were previously found in the microwave spectrum of glycine. The significance of the new calculations for interpretations of the microwave data is discussed. It is concluded that assumption of slight non-planarity of the cyclic form, with a minimum at III and a low lying transition state at II is not necessarily in conflict with the available microwave data. In addition it is possible to conclude that a third conformer, IV or V, hitherto not observed, may exist in glycine vapors in amounts which are similar to those of the observed conformer II.

Journal of Molecular Structure-Theochem
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