EVIDENCE FOR A PRE-REACTIVE INTERMEDIATE IN A GASEOUS MIXTURE OF ETHYNE AND CHLORINE MONOFLUORIDE - ROTATIONAL SPECTRUM OF THE MULLIKEN B-PI-CENTER-DOT-A-SIGMA COMPLEX C2H2-CENTER-DOT-CENTER-DOT-CENTER-DOT-CLF

A complex of ethyne and chlorine monofluoride has been identified and characterised in a supersonically expanded mixture of the two gases by means of its ground-state rotational spectrum. A fast-mixing nozzle w as used in combination with a pulsed-nozzle, Fourier-transform microwa ve spectrometer to preclude chemical reaction of the components and to observe the spectrum of each of the isotopomers C2H2...(ClF)-Cl-35, C 2H2...(ClF)-Cl-37 and C2D2...(ClF)-Cl-35. Analysis of the spectrum led to the rotational constants A(0), B-0 and C-0, the centrifugal distor tion constants Delta(J), Delta(JK) and delta(J), the Cl-nuclear quadru pole coupling constants chi(aa)(Cl) and chi(bb)(Cl)-chi(aa)(Cl), and t he spin-rotation coupling constant 1/2[M(bb)(Cl) + M(cc)(Cl)] in each case. Arguments based on nuclear-spin statistical weight effects and a detailed interpretation of the spectroscpic constants revealed that t he detected pre-reactive intermediate has a planar T-shaped geometry, with C2H2 acting as the bar of the T, and ClF as the stem but oriented so that the Cl atom was closer to the ethyne pi-bond than F. The dist ance between the centre () of the pi-bond and the Cl nucleus is r(*.. .Cl) = 2.783 (8) Angstrom. The small intermolecular stretching force c onstant k(sigma) = 10.0 N m(-1) and the minor changes in the Cl nuclea r quadrupole coupling constants of ClF on formation of the complex dem onstrate that the complex is weakly bound and that the electric charge distribution of ClF suffers only a small perturbation. It is conclude d that C2H2...ClF is best described formally as a complex of the Mulli ken b pi . a sigma ao weak outer type.