Evidence for a pre-reactive intermediate in a gaseous mixture of ethyne and chlorine monofluoride. Rotational spectrum of the Mulliken bπ· aσ complex C2H2⋯ClF

Abstract

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 was used in combination with a pulsed-nozzle, Fourier-transform microwave spectrometer to preclude chemical reaction of the components and to observe the spectrum of each of the isotopomers C₂H₂⋯³⁵ClF, C₂H₂⋯³⁷ClF and C₂D₂⋯³⁵ClF. Analysis of the spectrum led to the rotational constants A₀, B₀ and C₀, the centrifugal distortion constants Δ_J, Δ_JK and δ_J, the Cl-nuclear quadrupole coupling constants χ_aa(Cl) and χ_bb(Cl)–χ_aa(Cl), and the spin–rotation coupling constant ½[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 the 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 π-bond than F. The distance between the centre (*) of the π-bond and the Cl nucleus is r(*⋯Cl)= 2.783 (8)Å. The small intermolecular stretching force constant k_σ= 10.0 N m^–1 and the minor changes in the Cl nuclear qudrupole coupling constants of ClF on formation of the complex demonstrate that the complex is weakly bound and that the electric charge distribution of ClF suffers only a small perturbation. It is concluded that C₂H₂⋯ClF is best described formally as a complex of the Mulliken bπ· aσ weak outer type.