The bπ.aσ complex C2H2⋯Cl2characterised by rotational spectroscopy as an intermediate in a reactive mixture of ethyne and chlorine

An intermediate C2H2⋯Cl2 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 ΔJ, ΔJK and δJ, and Cl-nuclear quadrupole coupling constants χaa(Clx) and χbb(Clx)–χcc(Clx)(where x= i for inner or o for outer) were determined for the four isotopomers C2H2⋯35Cl2, C2H2⋯37Cl35Cl, C2H2⋯35Cl37Cl and C2D2⋯35Cl2. Detailed analyses of the spectroscopic constants established unambiguously that the observed complex has a planar, T-shaped geometry of C2v 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 Cli to Clo, and that the binding strength, as measured by the intermolecular stretching force constant kσ, is very small. The systematic shortening of the B⋯Cl distance from B⋯HCl to B⋯Cl2 identified previously was also observed for B = C2H2 and has been attributed to the ‘snub-nosed’ nature of molecular chlorine.