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Ab initio studies of structural features not easily amenable to experiment. 25. Conformational analysis of methyl propanoate and comparison with the methyl ester of glycine

✍ Scribed by V. J. Klimkowski; J. N. Scarsdale; Lothar Schäfer

Publisher
John Wiley and Sons
Year
1983
Tongue
English
Weight
419 KB
Volume
4
Category
Article
ISSN
0192-8651

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✦ Synopsis

The molecular geometries of three conformations of methyl propanoate (MEP) (C-C-C=O torsions of O', 120°, and 180') and the potential-energy surfaces of MEP (C-C-C=O torsions) and of the methyl ester of glycine (MEG) (N-C-C=O torsions) have been determined by ab initio gradient calculations at the 4-21G level. MEP has conformational energy minima at 0' and 120' of the C-C-C=O torsion, while the 60-90" range and 180' are energy maxima. For MEG there are two minima (at 0" and 180') and one barrier to N-C-C=O rotation in the 60-90' range. The N-C-C=O barrier height is about twice as high (4 kcal/mol) as the C-C-C=O barrier. The 180" N-C-C=O minimum is characteristically wide and flat allowing for considerable flexibility of the N-C-C=O torsion in the 150-210" range. This flexibility could be of potential importance for polypeptide systems, since the N-C-C=O angles of helical forms are usually found in this region. The molecular structures of the methyl ester group CH30C(=O)CHRR' in several systems are compared and found to be rather constant when R = H and R' = H, CH3, CH3CHz; or when R = NH2 and R' = H, CH3, or CH(CH3)z.

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