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 ΔJ, ΔJK and δJ, and halogen nuclear quadrupole coupling constants χaa(X) and {χbb(X)-χcc(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 C2v symmetry in which ethyne acts as the bar of the T. This geometry, with the zero-point distance r(*···I)=3.115(1) A 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 χaa(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 kσ=12.2(1) Nm-1 determined for the isotopomers C2H2···I35Cl and C2H2···I37Cl from ΔJ 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.