JOURNAL OF PHYSICAL CHEMISTRY A, vol.114, no.42, pp.11322-11329, 2010 (SCI-Expanded)
Vibrational spectra are measured for Fe+(CH4)n (n = 1−4) in the C−H stretching region (2500−3200 cm−1) using photofragment spectroscopy. Spectra are obtained by monitoring CH4fragment 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 (η2 vs η3) 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 η3 configuration. There is also a small contribution to the spectrum from the metastable sextet η3 complex. The Fe+(CH4)2 complex is also a quartet with both CH4 in an η3 configuration. For the larger clusters, the configuration switches from η3 to η2. 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 η2 configuration. For Fe+(CH4)4, the calculations predict three low-lying structures, all with η2 binding of methane and very similar Fe−C bond lengths. No single structure reproduces the observed spectrum. The approximately tetrahedral C1 (4A) structure contributes to the spectrum; the nearly square-planar D2d (4B2) and the approximately tetrahedral C2 (4A) structure may contribute as well.