The effect of pressure on the crystal structure of L-alanine

L-Alanine crystallises as a zwitterion in space group P212121 at ambient pressure. The strongest intermolecular interactions are three N–H⋯O hydrogen bonds. The H-bonds link the molecules into puckered layers in the ac plane, which are then stacked along the b axis. PIXEL calculations indicate that the H-bond mediated intermolecular energies within the layers are 118 and 145 kJ mol−1, while the stacking interactions are considerably weaker (31 kJ mol−1). Neutron powder diffraction data on L-alanine-d7 to 9.87 GPa show that the effects of pressure are most prominent along a, the direction which is least well aligned with the H-bonds. The a axis decreases in length more rapidly than the c axis, and the two axis lengths are equal at ca. 2 GPa. Although the structure is metrically tetragonal at this point, the true symmetry is still orthorhombic. In fact, the structure remains in a compressed form of its starting orthorhombic phase throughout the pressure range studied, in contradiction of previous Raman and energy dispersive power diffraction studies, in which data were interpreted in terms of phase transitions at ca. 2 GPa and 9 GPa. It is likely that the spectroscopic signature of the apparent transition at 2 GPa is the result of a conformational change at the ammonium group. The molecular coordination number in L-alanine is 14, and the packing topology is a distorted form of body-centred cubic at ambient pressure. As pressure increases this topology becomes more regular, until at 9.87 GPa it is very similar to the perfect BCC topology of a structure such as tungsten. It is suggested that this behaviour can be traced to the sphericity of the alanine molecule.