Vibrational and Structural Dynamics of Mn(CO)₅Br and Re(CO)₅Br Examined Using Nonlinear Infrared Spectroscopy

Vibrational and structural dynamics of two transition metal carbonyl complexes, Mn(CO)₅Br and Re(CO)₅Br were examined in DMSO, using ultrafast infrared pump-probe spectroscopy, steady-state linear infrared spectroscopy and quantum chemistry computations. Two carbonyl stretching vibrational modes (a low-frequency A₁ mode and two high-frequency degenerate E modes) were used as vibrational probes. Central metal effect on the CO bond order and force constant was responsible for a larger E-A₁ frequency separation and a generally more red-shifted E and A₁ peaks in the Re complex than in the Mn complex. A generally broader spectral width for the A₁ mode than the E mode is believed to be partially due to vibrational lifetime effect. Vibrational mode-dependent diagonal anharmonicity was observed in transient infrared spectra, with a generally smaller anharmonicity found for the E mode in both the Mn and Re complexes.