Infrared spectra are recorded for Al+–(CH4)n n = 1–6 clusters in the CH stretch region (2800–3100 cm−1). The spectra, which are obtained by monitoring photofragmentation in a tandem mass spectrometer, are dominated by a single, narrow band corresponding to the totally symmetric C–H stretching mode of the CH4 subunits (rendered infrared active through the interaction with the Al+ cation). This band shifts progressively to higher wavenumber as the clusters becomes larger, concomitant with a weakening of the intermolecular Al+⋯CH4 bonds. Supporting ab initio calculations for the n = 1–6 clusters at the MP2/aug-cc-pVDZ level indicate that the Al+ cation is attached to each CH4 sub-unit in a face-bound η3 configuration and that when possible the methane molecules are adjacent to one another. Clusters built around an inserted [H–Al–CH3]+ core are also predicted to be stable but lie higher in energy than clusters built around an Al+ core; the latter species are the only ones observed experimentally.
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