title: Pressure-induced polymorphism in phenol.
abstract: 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.
authors.affiliation: School of Physics, The University of Edinburgh, Mayfield Road, UK. dra@ph.ed.ac.uk
authors.forename: David R
authors.initials: DR
authors.lastname: Allan
authors.forename: Stewart J
authors.initials: SJ
authors.lastname: Clark
authors.forename: Alice
authors.initials: A
authors.lastname: Dawson
authors.forename: Pamela A
authors.initials: PA
authors.lastname: McGregor
authors.forename: Simon
authors.initials: S
authors.lastname: Parsons