Pamela A. McGregor

Pressure-induced polymorphism in phenol

The high-pressure crystal structure of phenol (C(6)H(5)OH), including the positions of the H atoms, has been determined using a combination of single-crystal X-ray diffraction techniques and ab initio density-functional calculations. It is found that at a pressure of 0.16 GPa, which is just sufficient to cause crystallization of a sample held at a temperature just above its ambient-pressure melting point (313 K), a previously unobserved monoclinic structure with P2(1) symmetry is formed. The structure is characterized by the formation of hydrogen-bonded molecular chains, and the molecules within each chain adopt a coplanar arrangement so that they are ordered in an alternating 1-1-1 sequence. Although the crystal structure of the ambient-pressure P112(1) phase is also characterized by the formation of molecular chains, the molecules adopt an approximate threefold arrangement. A series of ab initio calculations indicates that the rearrangement of the molecules from helical to coplanar results in an energy difference of only 0.162 eV molecule(-1) (15.6 kJ mole(-1)) at 0.16 GPa. The calculations also indicate that there is a slight increase in the dipole moment of the molecules, but, as the high-pressure phase has longer hydrogen-bond distances, it is found that, on average, the hydrogen bonds in the ambient-pressure phase are stronger.

Hexamer formation in tertiary butyl alcohol (2-methyl-2-propanol, C4H10O)

The crystal structure of phase II of tertiary butyl alcohol (2-methyl-2-propanol, C(4)H(10)O) has been solved using a combination of single-crystal X-ray diffraction techniques and ab initio density functional calculations. This trigonal P3 phase, which is stable at both low temperature and high pressure, and the triclinic P1 phase (phase IV) have very similar enthalpies, the calculations revealing only a 3.859 kJ mol(-1) enthalpy difference at ambient pressure, despite the substantial change of the intermolecular bonding motif from helical catemer to hexamer with an increase in pressure or reduction in temperature. The hexamers in the trigonal phase adopt a chair conformation. There are two unique hexamers: at low temperature these are centred at (0, 0, 1/2) and (2/3, 1/3, 0.961 (13)), and at high pressure the centres are (0, 0, 1/2) and (2/3, 1/3, 0.958 (14)). A slight flattening of the hexamers is observed at high pressure and the calculations confirm that phase II becomes more stable relative to phase IV on pressure increase.

Comparison of the high-pressure and low-temperature structures of sulfuric acid

We have determined the high-pressure crystal-structure of sulfuric acid, including the positions of the hydrogen atoms, using a combination of single-crystal X-ray diffraction techniques and ab initio density functional calculations. Just above the onset of crystallization, at 0.7 GPa, we find that a previously unobserved monoclinic structure, with P21/c symmetry, is formed which is characterised by SO2(OH)2 tetrahedra interconnected by hydrogen bonds. In contrast to the low-temperature C2/c phase, the tetrahedra in the high-pressure crystal structure are no longer arranged in R44(16) hydrogen-bonded layers but, instead, they form chains where the hydrogen bonding adopts a R33(12) arrangement. A series of ab initio calculations indicates that this rearrangement of the molecules results in a relatively small reduction in the enthalpy (13.603 kJ mol−1) for the P21/c structure at 0.7 GPa.