Crystal structure determination of C3Cl +3AlCl –4 and ab initio studies of the structure and vibrational spectrum of the trichlorocyclopropenium cation

Abstract

The crystal structure of C₃Cl⁺₃AlCl^–₄ has been determined using X-ray intensity data measured on a diffractometer. The crystals are triclinic, a= 6.617(1)Å, b= 8.859(2)Å, c= 10.299(2)Å, α= 90.24(1)°, β= 89.90(1)°, γ= 111.86(2)°, z= 2, space group P1. The final R= 0.045 for 2264 observed reflections. In the trichlorocyclopropenium cation the C—C bonds (mean bond length 1.356 Å) and C—Cl bonds (mean bond length 1.631 Å) are exceptionally short, the latter being the shortest C—Cl bond yet reported. Ab initio calculations for [C₃Cl₃]⁺ at the SCF level with a 6-311G** basis give the bond lengths C—C = 1.358 and C—Cl = 1.643 Å, and yield vibrational wavenumbers and infrared and Raman intensities in good agreement with the observed spectra. The calculated valence force constants, including k(C—C)= 7.905 and k(C—Cl)= 5.7109 mdyn Å^–1, differ considerably from those of an earlier normal-coordinate analysis and are in harmony with those of related molecules.