Crystal and molecular structure of CCl4 III: A high pressure polymorph at 10 kbar

Abstract

The crystal and molecular structure of a high pressure form of carbon tetrachloride (CCl4 III) was determined at approximately 10 kbar using a diamond‐anvil berylliumpressure cell and a modified Buerger‐type precession camera. CCl4 III crystallizes in the monoclinic system with a unit cell of the following dimensions: a=9.079± 0.012 Å , b=5.764± 0.003 Å , c=9.201± 0.004 Å , and β=104.29±0.05°. The space group is P21/c with four molecules per unit cell. Observed infrared spectra indicate that the CCl4 molecule exhibits regular tetrahedral symmetry at 10 kbar. By using an approximation to the repulsion energy in a least‐squares refinement procedure considering only nonbonded Cl–Cl interactions, an approximate structure was obtained. Subsequent structure factor calculations using a grid‐point sampling procedure yielded a final structure with a reliability factor of 9.56%. CCl4 III is isostructural with SnBr4, and contains closest nonbonded Cl–Cl distances of 3.49 Å, significantly less than the normal Van der Waals separation of 3.6 Å. Taking four molecules per unit cell, the calculated density is 2.190 g cm−3. A more compact phase than CCl4 III was predicted in the CCl4 system on the basis of packing efficiency and the predicted phase (CCl4 IV) was subsequently verified by visual observation in microscopic studies at pressures in the 35–40 kbar range and temperatures up to 500°C.