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

Abstract

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.

Solid-State Polymerization of Acetylene under Pressure:Ab InitioSimulation

“…Experimentally the equilibrium phase is the Cmca structure below 133 K and at all measured pressures [15]. Above 133 K a new cubic Pa3 phase is stabilized [16]. At room temperature acetylene undergoes pressure-induced phase transitions [5,6]; the liquid converts into the Pa3 phase at 0.7 GPa and then a transition to the Cmca phase takes place at 0.9 GPa.…”

Cocrystallization with Acetylene. The 1 : 1 Complex with Benzene: Crystal Growth, X‐Ray Diffraction and Molecular Simulations

“…Refined Parameters for the 1 : 1 Acetylene/Benzene Crystal Structure (2) 0.089 (8) 0.048(4) Distances/ä a ) C(1)ÀC (1)' 1.097 (10) 1.152(4) C(1)ÀH (1) 1.01 (9) 0.97(4) C(2)ÀC (2) (2) 0.94(3) 0.960 (15) a ) Symmetry transformations used to generate equivalent atoms: distances among results derived from different methods and refinement procedures would then be improper [11]. Thus, the triple-bond length in the complex is expected to be in the range of 1.17 ± 1.20 ä, as compared to neat acetylene (X-ray at 141 K, 1.176 ä [12]). The triple-bond length in 3-ethynylcyclopropene is 1.184 ä at 103 K [11], longer than the mean distance of 1.174 ä for 367 structures with terminal acetylene groups [13] [14], probably because, in the Cambridge database, most of the structures were determined at room temperature.…”

In situ crystallization of the linear alkynes C n H2 n–2 (n=7, 8, 9, 10)

“…Like the linear alkanes [11] and alkane derivatives [12][13][14][15], the linear alkynes exhibit an alternation in their melting points, where the odds are systematically higher than the evens (Figure 8). 1 This is correlated with an alternation in the crystal density for the four structures described 1 Alternation does not necessarily mean that the melting points alternately increase and decrease in absolute terms.…”

“…The chemical focus in this case is on some linear alkynes: 1-heptyne to 1-decyne, C n H 2n-2 (n = 7, 8,9,10). Apart from ethyne (acetylene) [13][14][15], the Cambridge Structural Database [16] does not contain any 1-alkyne crystal structures, so this is the first report of any structural data for this hydrocarbon series. sample at the top end of the capillary (Figure 1), then aligned horizontally on the diffractometer (χ = 90°) with the N 2 cryostream placed perpendicular to the capillary axis ( Figure 2).…”