Barriers to rotation adjacent to double bonds. 3. The carbon-oxygen barrier in formic acid, methyl formate, acetic acid, and methyl acetate. The origin of ester and amide resonance

TLDR: In this article, the structures of the rotamers about the C-O bonds of formic acid, methyl formate, acetic acid, and methyl acetate were calculated by using the 6-31G* basis set and complete geometrical relaxation.

Abstract: The structures of the rotamers about the C-O bonds of formic acid, methyl formate, acetic acid, and methyl acetate were calculated by using the 6-31G* basis set and complete geometrical relaxation Large basis sets (6-311+G**) and correction for electron correlation were needed in order to obtain calculated barriers that were in good agreement with the available experimental data The factors that control the geometry at a carbonyl group are considered, and it is shown that an analysis in terms of bond path angles leads to a direct connection with electronegativity The nature of the interaction between an amino group and a carbonyl, as in an amide, is examined and shown not to involve charge transfer from the nitrogen to the carbonyl oxygen, but rather it involves charge transfer between carbon and nitrogen The origin of the rotational barrier in esters and of the difference in energy between the E and Z conformers is discussed