Molecular structure and relative stability of trans and cis isomers of formanilide: Gas-phase electron diffraction and quantum chemical studies

Abstract

Molecular structure of formanilide is determined by gas-phase electron diffraction (GED) augmented by quantum chemical calculations (B3LYP/cc-pVTZ and MP2/cc-pVTZ) and literature microwave (MW) data. The combined GED and MW data are well reproduced for the mixture of trans and cis isomers with the relative abundance of 59 ± 5 and 41 ± 5 %, respectively, at T = 410 K. The trans isomer (C s symmetry) is planar, while the cis isomer (C 1 symmetry) has the twisted structure with the amide group rotated by 36.7 ± 2.7 with respect to the phenyl ring. In accord with theoretical calculations, the amide bond -NH-C(O)- is planar in trans formanilide and a somewhat nonplanar in cis isomer. Accurate structural parameters were obtained by a simultaneous fit of the rotational constants reported in the literature and GED intensities obtained in this study. The N-C(O) and N-CPh bond dissociation energies in formanilide are calculated using Gaussian-4 method. It is revealed that the strength of N-C(O) bond in formanilide is 50 kJ/mol less than that in benzamide. On the contrary, the strength of adjacent bond (N-C Ph) increases by 35 kJ/mol compared to aniline. It is rather unexpectedly that the bond strength weakening does not result in the bond elongating, and vice versa. © 2012 Springer Science+Business Media, LLC.