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.