Howard C. Knachel

Crystal structures, phase transitions and energy calculations of poly(p-phenylene) oligomers

The room temperature crystal structures, unit cell dimensions at 110K and phase transitions of three poly(p-phenylene) oligomers are reported. The structures of p-quinquephenyl (PQP), C30H22,p-sexiphenyl (PSP), C36H26, and p-septiphenyl (PSeptiP), C42H30, each belonging to space group P21/c, are similar to those of shorter oligomers. The unit cell dimensions are a = 22.056 A, b = 5.581 A, c = 8.070 A and β = 97.91° for PQP, a = 26.241 A, b = 5.568 A, c = 8.091 A and β = 98.17 ° for PSP, and a = 30.577 A, b = 5.547 A, c = 8.034 A and β = 100.52 ° for PSeptiP. The a axis increases with molecular length. The molecules are linear and planar in all three structures. The herringbone nature of the packing is similar for PQP and PSeptiP, while a considerably greater tilt occurs in PSP. At 110K, the unit cell parameters b and c are approximately doubled while a remains nearly the same as in the room temperature cell. A time-dependent solid state transition is observed for PQP, PSP and PSeptiP when crystals are cooled to 110K. At elevated temperatures, thermal measurements indicate the oligomers to be thermotropic liquid crystals. The crystal-smectic transition temperatures are reported for PQP, PSP, PSeptiP and p-octiphenyl (POP), C48H34. The results of a molecular mechanics study on the conformation and packing of PSP are also presented. The competition between intramolecular forces (such as ortho hydrogen repulsions) and intermolecular crystal packing forces was examined in particular. Molecular mechanics calculations predict non-planar conformations in isolated polyphenyls, implying that conjugation between phenyl rings is insufficient to overcome ortho hydrogen repulsions. In a crystalline environment, however, intermolecular forces tend to force a planar conformation. Calculations on arrays of PSP molecules show that changing the phenyl-phenyl torsion angles from the coplanar value increases the total energy of the structure. The most favourable intermolecular interactions between oligomers are achieved for conformations having the phenyl rings coplanar.

Symmetrical Ring Cleavage of the Gold(I) Phosphorus Ylide Dimer [Au(CH2)2PPh2]2 by Hydrogen Halides in Toluene

Abstract The reaction of the cyclic, 8-membered gold(I) ylide dimer, bis [μ-(dimethylenediphenylphosphoranyl-C,C)]-digold(I), [Au(CH2)2PPh2]2, with two moles of HX (X = Cl, Br) in toluene yields the symmetric cleavage product halo(methylmethylenediphenylphosphoranyl)-gold(I), XAuCH2PPh2- (CH3), as the only isolable gold containing product. Further addition of HX affords a dimethyldiphenylphosphonium dihaloaurate(I) salt.

Structural Transformations in Crystalline Oligomers of Polyparaphenylene

Abstract : We report the room and low temperature crystal structures of the unsubstituted p-quinquephenyl (PQP) and p-sexiphenyl (PSP), and substituted 2,4- diphenyl-p-quinquephenyl (DPQP), 2,6-diphenyl-p-septiphenyl (DPSP), and 1,2,4- triphenylbenzene (TPB) polyparaphenylene oligomers. The unsubstituted oligomers exhibit a solid state transition when cooled from room temperature to 110K, as indicated by a change in crystallographic space group. No transition is observed for the substituted oligomers other than the usual thermal contraction of the unit cell. The transition observed for the unsubstituted oligomers is interpreted in terms of a conformational change from an averaged planar structure to a static non-planar one. Comparisons of room temperature and low temperature crystal data are presented. Chemical reactions.

Validating a model to predict the chemistry of the fuel used in radioisotope power systems

U.S. radioisotope power systems convert the decay heat from 238PuO2 fuel into electricity using various forms of thermoelectric conversion technology. When placed in an environment that has low oxygen potential and very high temperatures, the fuel can undergo reduction to a sub-stoichiometric form (PuO2-x). In order to better understand this system, a model was developed that combines the known relationship between stoichiometry (i.e., x), temperature, and oxygen potential with the chemical thermodynamics of graphite, carbon monoxide, and carbon dioxide. Because of the challenges surrounding PuO2 experimentation, the model has been benchmarked against a common PuO2 surrogate: CeO2. Samples of CeO2 were sealed into a reaction tube and allowed to react with graphite using a reaction temperature of 1273 K. When these reaction conditions were used, the model (x = 0.268) and experiment (x = 0.272 + 0.008) found very good agreement. Successful benchmarking of this model suggests that it will be useful in predicting the complex chemical interaction that can occur between the materials inside a radioisotope power system. Kinetic data obtained from these experiments suggests that reaction rates are very slow and that it could take hundreds of hours to reach equilibrium. A change in the rate limiting reaction mechanism is also observed when x > 0.25, but does not appear to impact the thermodynamic equilibrium. In addition to helping inform a 238Pu based system, this model may also be useful for predicting the chemistry found inside an 241Am based system, which is currently under development for the European Space Agency.

Characterization by NMR of reactants and products of hydrofluoroether isomers, CF3(CF2)3OCH3 and (CF3)2C(F)CF2OCH3, reacting with isopropyl alcohol

The 3M Company product Novec™ 71IPA DL, a mixture of methoxyperfluorobutane, methoxyperfluoroisobutane and 4.5 wt.% isopropyl alcohol, has been found to be very stable at ambient temperature, producing fluoride at the rate of ~1 ppm/year. Our earlier kinetic and theoretical studies have identified the reaction mechanism. This paper identifies the 1H and 19F NMR chemical shifts, multiplicities, and coupling constants of reactants and the major products that result from aging the mixture in sealed Pyrex NMR tubes for periods up to 1.8 years at temperatures from 26 °C to 102 °C. Chemical shifts and coupling constants of fluorine and hydrogen atoms on the hydrofluoroethers and isopropyl alcohol are traced through the reactions to their values in the products – esters, isopropylmethyl ether, and HF. These spectral positions, multiplicities, and coupling constants are presented in table format and as figures to clarify the transformations observed as the samples age. Copyright

Single-crystal and high-resolution X-ray diffraction studies and Raman spectrum of a phosphorus(V) chloride complex containing a single bromide ion, P3Cl14Br

The solid-state structure of the compound of empirical formula P3Cl14Br, has been investigated by single-crystal and high-resolution powder diffraction techniques. The results of the structural study are correlated with the Raman spectra of P3Cl14Br and metastable PCl5. Refinement in tetragonal space group l4, with a= 8.739(2), c= 12.320(1)A, Z= 2 and R= 0.058 yielded a structure composed of two PCl4+ tetrahedra of Cs symmetry, one PCl6– octahedron of C4v symmetry, and one Br–. Rietveld refinement of powder data was carried out in l4 and l4/m. The Raman spectrum of the same crystal used for single crystal X-ray analysis was found to be strikingly similar to that of metastable PCl5 with vibrational degeneracies lifted. Metastable PCl5 and P3Cl14Br are structurally very similar.