The Cl−–CH4 anion dimer: mid infrared spectrum and ab initio calculations

The Cl−–CH4 dimer has been investigated using infrared vibrational predissociation spectroscopy (2800–3080 cm−1 range), and through ab initio calculations at the MP2(full)/aug-cc-pVTZ level. The infrared spectrum features parallel and perpendicular bands, associated with excitation of C–H stretch vibrations localized on the CH4 core. Spectroscopic and theoretical data are consistent with a C3v proton-bound minimum energy configuration for the complex, although internal rotation of the CH4 sub-unit is not completely quenched. The calculated barrier for tunneling between equivalent proton-bound minima is 603 cm−1. Comparisons are made between the properties of the isoelectronic Cl−–H2O, Cl−–NH3, and Cl−–CH4 complexes.