CO(v=1) population lifetimes of metal–carbonyl cluster compounds in dilute CHCl3 solution

Abstract

Tunable infrared picosecond pulses in the 5 μ region have been used for time‐resolved pump–probe measurements of the population relaxation lifetime (T₁) of CO(v=1) stretching vibrations in a series of metal–carbonyl cluster compounds in room temperature chloroform solution. T₁ was the same for symmetric (ν=2084 cm⁻¹, 90±10 ps) and antisymmetric (2014 cm⁻¹, 87±10 ps) modes of the dicarbonyl Rh(CO)₂(C₅H₇O₂); T₁ was the same for the B₁ (2092 cm⁻¹, 710±130 ps) and B₂ (2036 cm⁻¹, 750±90 ps) modes of Rh₂(CO)₄Cl₂. Similarly long T₁ times were found for Rh₄(CO)₁₂ (2075 cm⁻¹, 610±65 ps) and Rh₆(CO)₁₆ (2077 cm⁻¹, 700±100 ps). The molecule Co₄(CO)₁₂ has also been compared to the corresponding rhodium analog and it exhibits an initially fast relaxation of 47±5 ps followed by a slower 396±70 ps decay. The transient response of the more complex systems to the single frequency experiment is found to be sensitive to frequency and can exhibit bleaching, absorption, and a combination of these effects. Such behavior is attributed to overlap of the IR pulse with v=1 to v=2 and higher transitions in the M₄(CO)₁₂ (M=Rh or Co) and Rh₆(CO)₁₆ molecules. The long CO(v=1) T₁ values for the metal cluster molecules suggest relaxation via multiquantum transfer of vibrational energy to adjacent M–C stretch and M–C–O bend vibrations; energy transfer to vibrational or electronic states of the central metal core seems unimportant in determining T₁ for these systems.