Competition between hydrogen bonding and dispersion interactions in the crystal structures of the primary amines

Abstract

The crystal structures of the primary amines from ethylamine to decylamine have been determined by X-ray diffraction following in situ crystallisation from the liquids. In the series from propylamine to decylamine structures remain in the same phase on cooling from the melting point to 150 K, and the structures of these compounds were determined by single-crystal methods. By contrast, ethylamine undergoes a slow reconstructive phase transition on cooling to 150 K. The structure of the high-temperature form was determined by single-crystal methods at 180 K, while that of the low-temperature form was determined by powder diffraction at 150 K. The stability of the low-temperature form can be ascribed in part to more energetic hydrogen bond formation. PIXEL calculations indicate that hydrogen bonding and methyl–methyl interactions at the chain ends are optimised in the early members of the series, with particularly inefficient inter-chain interactions observed for propylamine and pentylamine. In the later members of the series dispersion interactions become the principal structure-directing interaction and the energies of the hydrogen bonds and methyl–methyl interactions become weaker to accommodate more efficient inter-chain packing. The weakest methyl–methyl interactions occur in heptyl- and nonyl-amines. Overall, intermolecular interactions in the even membered amines are stronger and the packing more efficient than in the odd members, leading to an alternation in melting points along the series, an effect reminiscent of results obtained for the alkanes, carboxylic acids and several α–ω alkyl derivatives.