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Stewart J Clark

Publications and source records attributed to Stewart J Clark.

7 recordsLinked to original sources
PubMed2006

Condensed phase ionic polarizabilities from plane wave density functional theory calculations.

A method is presented to allow the calculation of the dipole polarizabilities of ions and molecules in a condensed-phase coordination environment. These values will be useful for understanding the optical properties of materials and for developing simulation potentials which incorporate polarization effects. The reported values are derived from plane wave density functional theory calculations, though the method itself will apply to first-principles calculations on periodic systems more generally. After reporting results of test calculations on atoms to validate the procedure, values for the polarizabilities of the oxide ion and various cations in a range of materials are reported and compared with experimental information as well as previous theoretical results.

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PubMed2006

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.

Journal Article↗
PubMed2006

High-pressure polymorphism in L-cysteine: the crystal structures of L-cysteine-III and L-cysteine-IV.

The crystal structure of the orthorhombic phase of L-cysteine (hereafter L-cysteine-I) consists of chains of molecules linked via NH...O hydrogen bonds. The chains are linked into a layer by other NH...O hydrogen bonds, forming R4(4)(16) ring motifs. The layers are linked by further NH...O and disordered SH...S/SH...O interactions. The main effects of compression to 1.8 GPa are to contract voids in the middle of the R4(4)(16) rings and to reduce S...S distances from 3.8457 (10) to 3.450 (4) angstroms. The latter is at the lower limit for S...S distances and we suggest that strain about the S atom is responsible for the formation of a new phase of L-cysteine, L-cysteine-III, above 1.8 GPa. The phase transition is accompanied by a change in the NCCS torsion angle from ca 60 to ca -60 degrees and small positional displacements, but with no major changes in the orientations of the molecules. The structure of L-cysteine-III contains similar R-type ring motifs to L-cysteine-I, but there are no S...S contacts within 3.6 angstroms. L-Cysteine-III was found to be stable to at least 4.2 GPa. On decompression to 1.7 GPa, another single-crystal to single-crystal phase transition formed another previously uncharacterized phase, L-cysteine-IV. This phase is not observed on increasing pressure. The structure consists of two crystallographically independent cysteine molecules in the same conformations as those found in L-cysteine-I and L-cysteine-III. The structure separates into zones with are alternately phase I-like and phase III-like. L-Cysteine-IV can therefore be thought of as an unusual example of an intermediate phase. Further decompression to ambient pressure generates L-cysteine-I.

Anisotropy↗
PubMed2005

[BDTA]2[Cu(mnt)2]: an almost perfect one-dimensional magnetic material.

[BDTA]2[Cu(mnt)2] (BDTA = benzo-1,3,2-dithiazolyl, mnt = maleonitriledithiolate) was crystallized in the space group P with an inversion center on Cu giving a stacked structure with each metal complex anion sandwiched by two cations. Short intermolecular S...S contacts give rise to a one-dimensional chain lateral to the stacking axis. Variable-temperature magnetic susceptibility and EPR measurements indicate that the salt behaves as an ideal one-dimensional Heisenberg antiferromagnetic material from 2 K < or = T < or = 300 K, with a coupling constant of J/k(B) = 16-17 K; the very low temperature magnetic properties are in quantitative agreement with the predictions of quantum field theory. DFT calculations are consistent with the formation of a one-dimensional magnetic chain with interstack interactions mediated by the BDTA counterions.

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PubMed2004

Calculation of flexoelectric coefficients for a nematic liquid crystal by atomistic simulation.

Equilibrium molecular dynamics calculations have been performed for the liquid crystal molecule n-4-(trans-4-n-pentylcyclohexyl)benzonitrile (PCH5) using a fully atomistic model. Simulation data have been obtained for a series of temperatures in the nematic phase. The simulation data have been used to calculate the flexoelectric coefficients e(s) and e(b) using the linear response formalism of Osipov and Nemtsov [M. A. Osipov and V. B. Nemtsov, Sov. Phys. Crstallogr. 31, 125 (1986)]. The temperature and order parameter dependence of e(s) and e(b) are examined, as are separate contributions from different intermolecular interactions. Values of e(s) and e(b) calculated from simulation are consistent with those found from experiment.

Journal Article↗
PubMed2002

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.

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