Structures of F-(CH4) and Cl -(CH4)n (n = 1,2) Anion clusters elucidated through Ab initio calculations and infrared spectra

Ab initio calculations are performed at the MP2/aug-cc-pVTZ level for F--CH4 and Cl--CH4, to show that the dimers have C3v symmetry with the CH4 sub-unit attached to the halide anion by a single hydrogen bond. This geometry is consistent with infrared spectra of F--CH4 and Cl--CH4 recorded in the CH-stretch region. The calculations also indicate substantial anharmonicity in the H-bonded CH stretch of F--CH4. Infrared spectra of the F--(CH4)2 and Cl--(CH4)2 trimer clusters are consistent with structures that have two equivalent CH4 sub-units H-bonded to the halide core. Additional bands in the F--(CH4)2 spectrum are assigned as transitions to CH4 bending overtone and combination levels, gaining infrared intensity from Fermi interaction with the H-bonded CH stretch.