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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
Author(s) -
Klimkowski V. J.,
Scarsdale J. N.,
Schäfer Lothar
Publication year - 1983
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540040407
Subject(s) - chemistry , ab initio , conformational isomerism , methyl group , potential energy , ab initio quantum chemistry methods , crystallography , computational chemistry , stereochemistry , molecule , group (periodic table) , organic chemistry , physics , atomic physics
Abstract 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 .