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

The molecular geometries of three conformations of methyl propanoate (MEP) (CCCO torsions of 0°, 120°, and 180°) and the potential‐energy surfaces of MEP (CCCO torsions) and of the methyl ester of glycine (MEG) (NCCO 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 CCCO 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 NCCO rotation in the 60–90° range. The NCCO barrier height is about twice as high (4 kcal/mol) as the CCCO barrier. The 180° NCCO minimum is characteristically wide and flat allowing for considerable flexibility of the NCCO torsion in the 150–210° range. This flexibility could be of potential importance for polypeptide systems, since the NCCO angles of helical forms are usually found in this region. The molecular structures of the methyl ester group CH 3 OC(O)CH RR ′ in several systems are compared and found to be rather constant when R H and R ′ H, CH 3 , CH 3 CH 2 ; or when R NH 2 and R ′ H, CH 3 , or CH(CH 3 ) 2 .