Vibrational relaxation of M(CO)6 (M=Cr, Mo, W): Effect of metal mass on vibrational cooling dynamics and non-Boltzmann internal energy distributions

The vibrational relaxation dynamics following the dissociation of C≡O from M(CO)6 (M=Cr, Mo, W) have been studied with picosecond transient absorption spectroscopy. After dissociation of C≡O, the pentacarbonyl species forms a complex with a solvent molecule. The cooling of these solvated pentacarbonyl complexes was monitored from 1 ps to 1 ns and different rates for vibrational relaxation were found for each of these three compounds. The W(CO)5⋅S (S=cyclohexane) vibrationally relaxes in 35 ps, whereas Mo(CO)5⋅S relaxes twice as quickly, 18 ps. This result is surprising because the higher density of states in W(CO)5⋅S would be expected to lead to faster cooling of the hot solvated complex. The primary cooling of Cr(CO)5⋅S is completed in 18 ps just as in Mo(CO)5⋅S, but a slower cooling of approximately 150 ps is also present. This component is assigned to vibrational relaxation of the C≡O stretching mode. From comparisons with other studies, it appears that the existence of this slower cooling component is present only in first row transition metal carbonyls.