Water-Assisted Vibrational Relaxation of a Metal Carbonyl Complex Studied with Ultrafast 2D-IR

Water is capable of assisting exceptionally rapid vibrational relaxation within dissolved solute species. Although ultrafast dynamics of metal carbonyl complexes have long served as models for vibrational relaxation, all reports to-date have investigated nonaqueous solutions due to the insolubility of the vast majority of metal carbonyl complexes in water. Using the water-soluble complex [RuCl2(CO)(3)](2), which belongs to a class known as carbon monoxide (CO) releasing molecules (CORM), we report the first ultrafast vibrational relaxation measurements of a metal carbonyl complex in water and compare this relaxation with polar organic solvents, namely, methanol. The vibrational relaxation, measured by two-dimensional IR (2D-IR) spectroscopy, is an order of magnitude faster in H2O (3.12 +/- 0.29 ps) than in methanol (42.25 +/- 3 ps). The accelerated relaxation times of the coupled CO units in H2O and D2O is interpreted as resulting from the enhancement of intramolecular relaxation pathways through additional coupling induced by the solvent. In addition, the vibrational lifetime shows a significant isotope dependence: in D2O the relaxation time is 4.27 +/- 0.27 ps, a difference of roughly 30%. We interpret these measurements in terms of a nonresonant channel primarily arising from water's reorientational dynamics, which occur primarily through large angular jumps, as well as a resonant transfer of vibrational energy from the carbonyl bands to the libration-bend combination band. These measurements indicate that metal carbonyls, which are among the strongest IR transitions, are exquisitely sensitive to the presence of water and hold promise as IR analogs of EPR spin labels.