The bπ.aσ complex C₂H₂Cl₂ characterised by rotational spectroscopy as an intermediate in a reactive mixture of ethyne and chlorine

An intermediate C₂H₂Cl₂ belonging to the bπ.aσ class of complex defined by Mulliken has been isolated in a reactive mixture of ethyne and molecular chlorine and characterised by means of its ground-state rotational spectrum, as observed with a fast-mixing nozzle incorporated in a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants A₀, B₀ and C₀, quartic centrifugal distortion constants Δ_J, Δ_JK and δ_J, and Cl-nuclear quadrupole coupling constants χ_aa(Cl_x) and χ_bb(Cl_x)–χ_cc(Cl_x)(where x= i for inner or o for outer) were determined for the four isotopomers C₂H₂³⁵Cl₂, C₂H₂³⁷Cl³⁵Cl, C₂H₂³⁵Cl³⁷Cl and C₂D₂³⁵Cl₂. Detailed analyses of the spectroscopic constants established unambiguously that the observed complex has a planar, T-shaped geometry of C_2v symmetry with Cl₂ as the stem of the T, that the extent of electric charge redistribution within Cl₂ on formation of the complex is equivalent to a transfer of only 0.02e from Cl_i to Cl_o, and that the binding strength, as measured by the intermolecular stretching force constant k_σ, is very small. The systematic shortening of the BCl distance from BHCl to BCl₂ identified previously was also observed for B = C₂H₂ and has been attributed to the ‘snub-nosed’ nature of molecular chlorine.