Condensed Gas Calorimetry. III. Heat Capacity, Heat of Fusion, Heat of Vaporization, Vapor Pressures and Entropy of Diborane between 13 °K. and the Boiling Point (180.32 °K.)1

Solubility Parameters of Permanent Gases

The solubility parameters,δH(Tb), of nonreactive permanent gases at their boiling pointsTb(<290 K) are calculated from individually discussed values of their molar enthalpies of vaporization and densities obtained from the literature. These values are tabulated and where available the coefficients of the temperature dependence expressionδH(T)are also tabulated. The trends noted in theδH(Tb)values are dealt with and the values are compared with those reported in the literature and derived from the solubilities of the gases in various solvents. TheδH(Tb)values are shown to correlate linearly with the depths of the potential wells (attractive interaction energies,ε/kB) for binary collisions of the gaseous molecules and with the surface tensions,σ(Tb), of the liquefied gases.

BH3 under pressure: leaving the molecular diborane motif

Molecular and crystalline structures of (BH(3))(n) have been theoretically studied in the pressure regime from 1 atm to 100 GPa. At lower pressures, crystals of the familiar molecular dimer are the structure of choice. At 1 atm, in addition to the well-characterized β diborane structure, we suggest a new polymorph of B(2)H(6), fitting the diffraction lines observed in the very first X-ray diffraction investigation of solid diborane, that of Mark and Pohland in 1925. We also find a number of metastable structures for oligomers of BH(3), including cyclic trimers, tetramers, and hexamers. While the higher oligomers as well as one-dimensional infinite chains (bent at the bridging hydrogens) are less stable than the dimer at ambient pressure, they are stabilized, for reasons of molecular compactness, by application of external pressure. Using periodic DFT calculations, we predict that near 4 GPa a molecular crystal constructed from discrete trimers replaces the β diborane structure as the most stable phase and remains as such until 36 GPa. At higher pressures, a crystal of polymeric, one-dimensional chains is preferred, until at least 100 GPa.