Structure of trans- N-methylacetamide: planar or non-planar symmetry?

Abstract

The structure of trans- N-methylacetamide has been fully optimized, i.e., all frequencies real, at the Hartree-Fock (HF) level with twelve basis sets from 6G–31G ∗ through 6–311 + + G ∗∗ and with electron correlation at the second-order Møller-Plesset perturbation level (MP2) with the 6G–31G ∗, 6G–311G ∗∗, 6–31 + G ∗, 6–311 + G ∗, and 6–311 + + G ∗∗ basis sets. Minimum energy structures are non-planar at HF and MP2 levels without diffuse functions. With diffuse functions included, the minimum energy structure is planar for all basis sets at the HF level and tends toward planarity at more complete levels of MP2 electron correlation. These results, as well as MP3, MP4 (SDQ), and CISD optimizations without frequency calculations, lead us to conclude that, within the expected torsion angle variations in such calculations, the equilibrium structure of the isolated molecule has planar symmetry.