Neutron powder diffraction study of the low-temperature phase of solid acetylene-d2

Acetylene-d2 (CzDz) has two solid phases. The crystal structure of the low-temperature phase has been determined at 4.2 K by neutron powder diffraction. The structure is orthorhombic, with a = 6.193 (3), b = 6.005 (3), c= 5551 (3) A, space group Acam, Z= 4. The linear molecules lie in layers parallel to (001) in slightly off-diagonal directions. The final agreement index Rz = 0.071. Bond lengths are C-C = 1-18 (1), C-D = 1.06 (2),~. The apparent shortening of the C-C bond compared with the value calculated from highresolution Raman spectra in the gas phase is attributed to the effect of molecular libration.