Single-crystal structures and electron density distributions of ethane, ethylene and acetylene: II. Single-crystal X-ray structure determination of acetylene at 141 K

Crystalline acetylene at 141 K is cubic, a = 6.091 (3) /~, space group Pa3, Z = 4. The intensities of all 164 independent reflexions up to sin 0/2 = 0.80 A -1 were measured accurately on a Nonius four-circle diffractometer. For higher sin 0/2, very few significant reflexion intensities exist due to the strong thermal motion. A valence analysis, with multipole deformation terms and restricted radial functions centred on the atoms, was performed for the 164 independent reflexions by least-squares refinement on L Both ~ and SCF scattering factors were considered. H was constrained to C. The radial functions corresponding to the SCF scattering factors were modified by introduction of an isotropic extinction parameter and an additional isotropic temperature factor for H in the refinement. Rw(I; 0 = 0.0176 and Rw(I;SCF ) = 0.0170, both with 11 independent parameters. Third and fourth cumulant terms describing the non-linearity of the librational motion had no significant effect. Filtered and unfiltered maps including multipole deformations only are based on the 138 reflexions with I _> 0. For the (modified) SCF atomic scattering factors the agreement with theory is better than for the ~ scattering factors. In the present analysis static deformations and thermal smearing could not be separated and no reliable value could be found for the scale factor. The situation would be more favourable for compounds with smaller thermal motion for which both X-ray and neutron data are available