Polymorphism, Intermolecular Interactions, and Properties of Primary Amines at High Pressure

A homologous series of the simplest primary amines, ethylamine (EA), propylamine (PA), butylamine (BA), and pentylamine (PEA), has been studied at high pressure by single-crystal X-ray diffraction. The structures of these compounds have been determined, at ambient temperature, from their freezing pressures up to ca. 5 GPa. Ethylamine at high pressure crystallizes in phase II of space group P2(1)/c, already found at ambient pressure and low temperature. For other amines, six new polymorphs have been found: two of propylamine (phases II and III, both of space group P2(1)/c), two of butylamine (phases II and III, both of space group Pbc2(1)), and two of pentylamine (phases II and III, space groups Pbc2(1) and P2(1)cn, respectively). The competition between NH...N and CH...N intermolecular interactions has been studied for the polymorphic forms of all amines. The properties of investigated compounds have been correlated with the number of carbon atoms in their aliphatic chains.