Infrared Spectra and Ab Initio Calculations for the F-−(CH4)n (n = 1−8) Anion Clusters

Z.M. Loh, R.L. Wilson, Duncan Wild, E.J. Bieske, J.L. Lisy, B. Njegic, M.S. Gordon

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Infrared spectra of mass-selected F-−(CH4)n (n = 1−8) clusters are recorded in the CH stretching region (2500−3100 cm-1). Spectra for the n = 1−3 clusters are interpreted with the aid of ab initio calculations at the MP2/6-311++G(2df 2p) level, which suggest that the CH4 ligands bind to F- by equivalent, linear hydrogen bonds. Anharmonic frequencies for CH4 and F-−CH4 are determined using the vibrational self-consistent field method with second-order perturbation theory correction. The n = 1 complex is predicted to have a C3v structure with a single CH group hydrogen bonded to F-. Its spectrum exhibits a parallel band associated with a stretching vibration of the hydrogen-bonded CH group that is red-shifted by 380 cm-1 from the ν1 band of free CH4 and a perpendicular band associated with the asymmetric stretching motion of the nonbonded CH groups, slightly red-shifted from the ν3 band of free CH4. As n increases, additional vibrational bands appear as a result of Fermi resonances between the hydrogen-bonded CH stretching vibrational mode and the 2ν4 overtone and ν2 + ν4 combination levels of the methane solvent molecules. For clusters with n ≤ 8, it appears that the CH4 molecules are accommodated in the first solvation shell, each being attached to the F- anion by equivalent hydrogen bonds.
Original languageEnglish
Pages (from-to)13736-13743
JournalThe Journal of Physical Chemistry Part A
Issue number51
Publication statusPublished - 2006


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