B. H. Torrie

Structure of the α-phase of solid methanol

The crystal structure of α-methanol at 15K has been determined from neutron powder diffraction measurements. The structure is orthorhombic, space group P212121. The molecular geometry is found to be very similar to that in the gas phase, but the methyl group no longer has ideal 3-fold symmetry. The crystal is formed by infinite hydrogen-bonded chains of molecules with adjacent chains ‘pointing’ in opposite directions. The O-H … O hydrogen bonds are almost linear. No phase intermediate between the low temperature α-phase and the high temperature β-phase was found, but a new, metastable phase was discovered.

Raman and infrared studies of the ferroelectric transition in ammonium sulphate

Abstract The Raman and i.r. spectra of ammonium sulphate and its fully deuterated analogue have been measured at temperatures above and below the ferroelectric transition temperature. No marked changes in the spectra occur at T c , although several small peaks appear below T c and increase in intensity as the temperature is reduced. There are no anomalous changes in either frequencies or peak widths, and the normal mode frequencies of the NH 4 + ions are those expected for a tetrahedral rather than a distorted configuration. The results do not support either of the simple models of the phase transition which have been proposed.

The temperature dependence of the crystal structures of the solid halogens, bromine and chlorine

The neutron powder profile refinement technique has been used to determine the crystal structure of solid Br2 at 5, 80, 170 and 250 K and that of solid Cl2 at 22, 55, 100 and 160 K. The results confirm that the structures of the two halogens are isomorphous, with space group Cmca at all temperatures. For both halogens the lattice parameters a and b increase monotonically with temperature, but the lattice parameter c decreases at the highest temperatures. The data were analysed assuming both isotropic and anisotropic thermal parameters. The intramolecular bond length and its orientation relative to the b-axis show little change with temperature. For Br2 at 5 K the bond length is 2·301(2) A while a = 6·5672(3), b = 4·4678(2), c = 8·6938(4) A. For Cl2 at 22 K the bond length is 1·994(2) A while a = 6·1453(2), b = 4·3954(1), c = 8·1537(2) A, where the errors quoted are those produced by the profile fitting program.

Structural analyses of various Cu2+, Zn2+-superoxide dismutases by differential scanning calorimetry and Raman spectroscopy

The thermal denaturation profile of the Cu2+, Zn2+ metalloenzyme, bovine superoxide dismutase, consists of two primary components, the major component denatures irreversibly at Tm = 104 degrees C with a total enthalpy (delta Hcal) of 7.30 cal/g. Reduction of Cu(II) to Cu(I) with potassium ferrocyanide lowers Tm to 96 degrees C and delta Hcal to 6.96 cal/g. The apo-form of bovine superoxide dismutase (both Cu and Zn removed) denatures at 60 degrees C with an enthalpy only one-half that of the holo-form. The reduced thermal stability, which indicates a greater ability to change conformation, may explain the previously observed much greater membrane binding of the apo-enzyme. Reconstitution with Zn2+, Cu2+, or Zn2+ and Cu2+ raises Tm to 80, 89, or 102 degrees C, respectively, with corresponding increases in the enthalpy. Thus, the metal ions considerably stabilize the enzyme and must somewhat affect conformation. The effect of Cu2+ alone is greater than that of Zn2+, although both are needed for full stability. Raman spectroscopy indicates little difference in secondary structure between the apo- and holo-forms, implying that the increased stability due to metal binding is not caused by an extreme structural reorganization. The value of Tm of canine and yeast superoxide dismutase is also lowered by reduction of Cu(II). The reduced form of the yeast enzyme denatures irreversibly, as do all forms of the bovine and canine enzymes, but the oxidized form is unique in that it denatures reversibly. Thus, the copper ion must be oxidized for renaturation and appears to act as a nucleation site.

Phase transition in solid acetonitrile

The structure of the low temperature phase of acetonitrile was determined from neutron powder diffraction data. The lattice parameters were measured as a function of temperature and the structure of the high temperature phase, as previously determined, was confirmed. At 12 K, the structure is orthorhombic, Cmc21, a = 5·9895(3) A, b = 5·2079(2) A, c = 7·7317(4) A. A transformation to a monoclinic phase, P21/c, a = 4·1182(7) A, b = 8·2865(10) A, c = 7·9946(11) A, β = 100·4(6)°, takes places between 220 and 229 K. Equilibrium structures for the two phases were calculated using simple model potentials and the calculated structures are very similar to those observed. The nature of the phase transition was investigated by means of these model potentials.

Raman and far infrared spectra of the solid phases of carbon tetrachloride

Abstract The lattice vibrations of carbon tetrachloride have been studied by Raman and infrared spectroscopic techniques over the temperature range 18 K to the melting point at 250 K. Comparisons are made with previous results on CCl 4 and other tetrahedral molecules, and the observed peaks for the monoclinic phase II are assigned as translational modes.

Raman spectra of crystalline HF and DF

Abstract Raman spectra of polycrystalline samples of HF and DF have been recorded at 80 and 18 K. The observed features are compared to previous infrared and Raman results on the hydrogen halides and assignments of all of the fundamental modes are proposed. A Raman mode at a very low frequency is indicative of surprisingly weak forces between layers of molecules.

Structures of solid deuterium bromide and deuterium iodide

The structures of the three phases of DBr and DI were determined using neutron powder profile techniques. The highest temperature phases are cubic, Fm3m, with the deuteriums in twelvefold disordered positions about the halogens. The intermediate temperature phases are orthorhombic, Cmca, with the deuteriums in twofold disordered positions about the halogens in the mirror planes. In the lowest temperature phase of DBr, molecular ordering results in zigzag chains of molecules in the mirror planes of an orthorhombic structure with space group Cmc21. Molecular ordering appears in a different way in DI giving distorted diamonds (almost squares), rather than chains, and the rotational sense within the diamonds differs from one plane to the next. DI molecules are parallel or perpendicular to each other but the molecules point 6·27° away from the lines joining the iodines. the structure is triclinic, with space group P1.

Crystal structures of methylene bromide and methylene iodide

The crystal structures of the three solid phases of methylene iodide and the stable phase of deuterated methylene bromide have been determined using the neutron powder profile structure refinement technique. For CH2I2, both phase I at 50K and phase III at 16K, and for CD2Br2, the stable phase at 15K, have eight molecules per unit cell on general sites of the monoclinic space group C2/c. The structure of CH2I2, phase II, has been determined at 16K, 30K, 50K and 255K. This phase is face centred orthorhombic, space group Fmm2, with four molecules per unit cell on sites with C 2v symmetry. Molecular bond lengths and angles are given for all structures. The lattice energy of each structure has been calculated using an empirical potential. Phase II is found to be the most stable phase for CH2I2, but the energy differences between all three phases are small.

Structure of solid formamide at 7 K

The structure of formamide was redetermined at 7 K using neutron powder profile analysis. The space group is P21/n with a=3·5432(2), b=8·9512(5), c=6·9741(4) A and β=101·051°. The results are in agreement with earlier X-ray results but a more accurate determination of the hydrogen positions has been made and a lower temperature has reduced anharmonic effects. A simple model using standard exp-6 atom-atom potentials, experimentally determined atomic charges and an exponential attractive potential for the O-D hydrogen bonds gives a good fit to the structural parameters.