Vibrational Spectroscopy and Theory of Fe+(CH4)n (n=1-4)

Vibrational spectra are measured for Fe+(CH4) (n = 1-4) in the C-H stretching region (2500-3200 cm(-1)) using photofragment spectroscopy. Spectra are obtained by monitoring CH4 fragment loss following absorption of one photon (for n = 3, 4) or sequential absorption of multiple photons (for n = 1, 2). The spectra have a band near the position of the antisymmetric C-H stretch in isolated methane (3019 cm(-1)), along with bands extending >250 cm(-1) to the red of the symmetric C-H stretch in methane (2917 cm(-1)). The spectra are sensitive to the ligand configuration (eta(2) vs eta(3)) and to the Fe-C distance. Hybrid density functional theory calculations are used to identify possible structures and predict their vibrational spectra. The IR photodissociation spectrum shows that the Fe+(CH4) complex is a quartet, with an eta(3) configuration. There is also a small contribution to the spectrum from the metastable sextet eta(3) complex. The Fe+(CH4)(2) complex is also a quartet with both CH4 in an eta(3) configuration. For the larger clusters, the configuration switches from eta(3) to eta(2). In Fe+(CH4)(3), the methane ligands are not equivalent. Rather, there is one short and two long Fe-C bonds, and each methane is bound to the metal in an eta(2) configuration. For Fe+(CH4)(4), the calculations predict three low-lying structures, all with eta(2) binding of methane and very similar Fe-C bond lengths. No single structure reproduces the observed spectrum. The approximately tetrahedral C-1 ((4)A) structure contributes to the spectrum; the nearly square-planar D-2d (B-4(2)) and the approximately tetrahedral C-2 ((4)A) structure may contribute as well.