Determination of the crystal structure of hexaphenyldisilane from powder diffraction data and its thermodynamic properties

The crystal structure of hexaphenyldisilane, Si₂(C ₆H₅)₆, was determined from synchrotron powder diffraction data. The compound crystallizes in orthorhombic space group P2 ₁2₁2₁ 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 (C_p) 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 (θ_D) is 207(5) K. The thermal expansion of Si₂(C₆H ₅)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: α_a = -4(2) × 10^-6 K^-1, α_b = -4(2) × 10^-6 K^-1, and α_c = 2.21(4) × 10^-4 K^-1. The volumetric thermal expansion coefficient (α_V) at 298.15 K is 2.13(5) × 10^-4 K^-1.