Identification and characterisation of the gas-phase complex HCN⋯Cl2by rotational spectroscopy

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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 DJ, and the chlorine nuclear quadrupole coupling constants χ(Cli) and χ(Clo) in each case. A B0 value has also been estimated for the isotopomer HC 14N⋯35Cl2. The distances r(N⋯Cli), r(N⋯Clo) and r(Cl—Cl) have been determined from the rotational constants (i denotes ‘inner’, o, ‘outer’). The small value of kσ, the intermolecular stretching force constant, estimated from DJ, and the small difference Δχ(35Cl)=χ(35Cli)–χ(35Clo) 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.