Rotational spectroscopy of mixtures of ethyne and iodine monochloride: isolation and characterisation of the π-type complex C2H2···ICl

Abstract

The ground-state rotational spectrum of a complex formed by ethyne and iodine monochloride was observed by using pulsed-nozzle, Fourier-transform microwave spectroscopy. A fast-mixing nozzle was utilised to avoid chemical reaction of the component gases prior to their supersonic expansion. Rotational constants A₀, B₀ and C₀, quartic centrifugal distortion constants Δ_J, Δ_JK and δ_J, and halogen nuclear quadrupole coupling constants χ_aa(X) and {χ_bb(X)-χ_cc(X)}, where X=Cl or I, were determined for the three isotopomers C₂H₂···I³⁵Cl, C₂H₂···I³⁷Cl and C₂D₂···I³⁵Cl. Detailed interpretation of the rotational constants established that the equilibrium geometry of the complex has a planar, T-shape of C_2v symmetry in which ethyne acts as the bar of the T. This geometry, with the zero-point distance r(*···I)=3.115(1) Å between the centre of the π-bond of ethyne and the nearest halogen atom I, establishes that the interaction in this complex is between ethyne as a π electron donor and I of ICl as the electron acceptor. The halogen nuclear quadrupole coupling constants χ_aa(X) were interpreted on the basis of a simple model to show that, on complex formation, fractions δ₁=0.026 and δ₂=0.056 of an electronic charge are transferred from the ethyne π bond to I and from I to Cl, respectively, leading to a net decrease of 0.030e at I. The complex is weakly bound, according to the intermolecular stretching force constant k_σ=12.2(1) Nm^-1 determined for the isotopomers C₂H₂···I³⁵Cl and C₂H₂···I³⁷Cl from Δ_J values. The opportunity is taken to compare the properties of C₂H₂···ICl established here with those similarly determined for the series C₂H₂···XY, where XY=Cl₂, ClF, or BrCl.