Identification and characterisation of the gas-phase complex HCN⋯Cl2 by rotational spectroscopy

Abstract

The ground-state rotational spectra of three isotopomers, HC¹⁵N⋯³⁵Cl₂, HC¹⁵N⋯³⁷Cl³⁵Cl and HC¹⁵N⋯³⁵Cl³⁷Cl, of the linear complex HCN⋯Cl₂ have been observed by the pulsed-nozzle, Fourier-transform microwave technique and analysed to give the rotational constant B₀, the centrifugal distortion constant D_J, and the chlorine nuclear quadrupole coupling constants χ(Cl_i) and χ(Cl_o) in each case. A B₀ value has also been estimated for the isotopomer HC ¹⁴N⋯³⁵Cl₂. The distances r(N⋯Cl_i), r(N⋯Cl_o) and r(Cl—Cl) have been determined from the rotational constants (i denotes ‘inner’, o, ‘outer’). The small value of k_σ, the intermolecular stretching force constant, estimated from D_J, and the small difference Δχ(³⁵Cl)=χ(³⁵Cl_i)–χ(³⁵Cl_o) are interpreted in terms of only a small perturbation of the Cl₂ subunit on complex formation. A comparison of the properties of the four dimers B⋯Cl₂, where B = CO, HF, PH₃ and HCN, with those of the corresponding series B⋯HCl of hydrogen-bonded complexes indicates a parallel behaviour in the two series. The propensity of Cl₂ to form complexes with B, as measured by its gas-phase electrophilicity E, is compared with that of HCl.