Molecular geometry versus crystal packing in N-methyl-N-(2-nitrophenyl) cinnamanilide: an experimental charge density study

Abstract

A charge density investigation of N-methyl-N-(2-nitrophenyl)cinnamanilide has been carried out using X-ray crystallography. The molecular geometry in the solid state is compared with that of a “free” molecule generated using the semi-empirical AM1 calculation. Significant differences are observed in torsion angles, though the bond lengths and angles are quite similar in the two cases. In the solid state, the cinnamide portion of the molecule is quite planar and the nitrobenzene ring is twisted out of this plane. In the “free” molecule, however the two phenyl rings are nearly planar and the bonds joining them are buckled. The molecule in the lattice is bound by C–H···O contacts, the prominent being a pair of centrosymmetrically related bifurcated hydrogen bonds from amidic oxygen. Experimental charge density reveals certain interesting aspects of both intra- and intermolecular bonding. It is observed that there is extensive charge redistribution in the amide group, with the C···O bond carrying relatively high density. Lone-pairs of the amidic oxygen appear to be polarized along the bifurcated hydrogen bonds. The C–H⋯O bonds from the amidic oxygen are stronger than those from nitro-oxygens. Hydrogen bonding from the amidic oxygen thus dictates molecular structure and packing.