Vibrational dynamics of iron pentacarbonyl [Fe(CO)5] in liquid alkanes revisited

We explore the solvent-dependence of the vibrational dynamics of Fe(CO)5 (IPC) using a novel 2D-IR setup. A 320 pixel MCT camera allows us to achieve high spectral resolution within a large detection window, which can be matched in excitation by scanning coherence times of up to 250 ps. The dynamics of the IR active modes of IPC in the C≡O stretching range are probed in a series of alkanes of different chain lengths and viscosities. 2D-IR maps at short waiting times reveal the detailed anharmonic structure of the modes at play: we determined the cross-anharmonicity between the A2″ and E' modes (δ = -0.8 ± 0.1 cm-1), and we are able to differentiate the diagonal anharmonicity of the doubly degenerate mode (ΔE ≃ 18 cm-1) from its non-diagonal anharmonicity (ΔEE ≃ 11 cm-1). Our analysis of the polarization dependence of the 2D-IR signals strongly confirms an exchange mechanism between the IR-active modes due to Berry pseudo-rotation, which shows very little dependence on solvent viscosity, in contrast to the anisotropy loss. The implications of our findings for the exchange mechanism are discussed.