Vibrational Spectroscopy of Fe3+(CH4)n (n = 1-3) and Fe4+(CH4)4.

Vibrational spectra are measured for Fe3+(CH4)n (n = 1-3) and Fe4+(CH4)4 in the C-H stretching region (2650-3100 cm-1) using photofragment spectroscopy, monitoring loss of CH4. All of the spectra are dominated by an intense peak at around 2800 cm-1 that is red-shifted by ∼120 cm-1 from free methane. This peak is due to the symmetric C-H stretch of the η3 hydrogen-coordinated methane ligands. For clusters with three iron atoms, the peak becomes less red-shifted as the number of methane ligands increases. For clusters with one methane ligand per iron atom, the red shift increases in going from Fe2+(CH4)2 (88 cm-1) to Fe3+(CH4)3 (108 cm-1) to Fe4+(CH4)4 (122 cm-1). This indicates increased covalency in the binding of methane to the larger iron clusters and parallels their increased reactivity. Density functional theory calculations, B3LYP, BPW91, and M11L, are used to identify possible structures and geometries and to predict the spectra. Results show that all three functionals tend to overestimate the methane binding energies. The M11L calculations provide the best match to the experimental spectra.