Isolation of a Halogen-Bonded Complex Formed between Methane and Chlorine Monofluoride and Characterisation by Rotational Spectroscopy and Ab Initio Calculations.

A halogen-bonded complex formed between methane and chlorine monofluoride has been isolated in the gas phase before the reaction between the components and has been characterised through its rotational spectrum, which is of the symmetric-top type but only exhibits = 0 type transitions at the low effective temperature of the pulsed-jet experiment. Spectroscopic constants for two low-lying states that result from internal rotation of the CH subunit were detected for each of the two isotopic varieties HC···ClF and HC···ClF and were analysed to show that ClF lies on the symmetry axis with Cl located closer than F to the C atom, at the distance (C···Cl) ≅ 3.28 Å and with an intermolecular stretching force constant ≅ 4 N m.