THE B-PI.A-SIGMA COMPLEX C2H2-CENTER-DOT-CENTER-DOT-CENTER-DOT-CL-2 CHARACTERIZED BY ROTATIONAL SPECTROSCOPY AS AN INTERMEDIATE IN A REACTIVE MIXTURE OF ETHYNE AND CHLORINE

An intermediate C2H2 ... Cl-2 belonging to the b pi.a sigma 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 A(0), B-0 and C-0, quartic centrifugal distortion constants Delta(J), Delta(JK) and delta(J), and Cl-nuclear quadrupole coupling constants (chi aa)(Cl-x) and (chi bb)(Cl-x)-(chi cc)(Cl-x) (where x = i for inner or o for outer) were determined for the four isotopomers C2H2 ...Cl-35(2), C2H2 ...(ClCl)-Cl-37-Cl-35, C2H2 ...(ClCl)-Cl-35-Cl-37 and C2D2 ...Cl-35(2). Detailed analyses of the spectroscopic constants established unambiguously that the observed complex has a planar, T-shaped geometry of C-2v symmetry with Cl-2 as the stem of the T, that the extent of electric charge redistribution within Cl-2 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(sigma), is very small. The systematic shortening of the B ... Cl distance from B ... HCl to B ... Cl-2 identified previously was also observed for B = C2H2 and has been attributed to the 'snub-nosed' nature of molecular chlorine.