The role of multiple parallel and antiparallel local dipoles for molecular structure and intermolecular interactions of oxalamides

Abstract

The paper illustrates the role of dipole–dipole interactions in stabilizing the molecular structure and intermolecular interactions in oxalic acid diamides. A CSD search and quantum chemical calculations reveal that within the oxalamide molecule N–H and β(CO) bonds are oriented mutually parallel, so that the local dipoles formed along these bonds are antiparallel. Moreover, the all-trans conformation of secondary oxalamides gives rise to the cooperative intramolecular NH/CO and CH/CO dipolar interactions. Contrary to the situation in hydrogen bonded amide R²₂(8) dimers, in oxalamide R²₂(10) dimers the CO and N–H bonds from two different molecules are parallel, moreover, the angular distribution of proton donors around carbonyl acceptor is much more linear. These findings indicate the dominant role of dipole–dipole interactions in this bimolecular cyclic system. As parallel (and antiparallel) dipole–dipole interactions impose restraints on the position of four atoms such an approach provides an additional predictive value over the identification of hydrogen bonding that imposes restraints on the position of three atoms.