Determination of the Crystal Structure of Hexaphenyldisilane from Powder Diffraction Data and Its Thermodynamic Properties

The crystal structure of hexaphenyldisilane, Si2(C6H5)6, was determined from synchrotron powder diffraction data. The compound crystallizes in orthorhombic space group P212121 with the following unit cell dimensions: a = 20.2889(8) Å, b = 16.9602(7) Å, and c = 8.5506(4) Å. Second-harmonic generation measurements as well as density functional theory calculations were used to confirm the structure determination. The combination of experimental and theoretical studies yields a Si–Si distance [d(Si–Si)] of 2.38 Å. The phenyl rings of a molecule are staggered and slightly distorted, so that the molecule is acentric. Thermodynamic measurements showed no phase transition in the temperature range of 2–400 K. The molar heat capacity (Cp) at 298.15 K of 604(6) J mol–1 K–1 was established experimentally and by lattice dynamic calculations. The molar entropy (S°) and the molar enthalpy (ΔH) in the temperature range of 0–298.15 K are 674(7) J mol–1 K–1 and 97(6) kJ mol–1 respectiveley. The Debye temperature (θD) is 207(5) K. The thermal expansion of Si2(C6H5)6 is strongly anisotropic, and negative in two directions as determined via temperature-dependent X-ray powder diffraction experiments. The linear thermal expansion coefficients at 298.15 K are as follows: αa = −4(2) × 10–6 K–1, αb = −4(2) × 10–6 K–1, and αc = 2.21(4) × 10–4 K–1. The volumetric thermal expansion coefficient (αV) at 298.15 K is 2.13(5) × 10–4 K–1.