Correlating solvent dynamics and chemical reaction rates using binary solvent mixtures and two-dimensional infrared spectroscopy

Two-dimensional infrared (2D-IR) spectroscopy was performed on Vaska's complex (VC) and its oxygen adduct (V C-O₂) in binary solvent mixtures of chloroform or benzyl alcohol in d₆-benzene. The second order rate constants for oxygenation were also measured in these solvent mixtures. The rate constant in chloroform mixtures is linear with mole fraction within the error of the measurements but changes nonlinearly in benzyl alcohol mixtures, displaying a preference for the alcohol over benzene. The rate constants were compared with FTIR spectra of the carbonyl ligand and the frequency-frequency correlation function of this mode determined by 2D-IR. The line shape broadening mechanisms of the linear spectra of the CO bound to VC and VC-O₂ are similar to those previously reported for VC-I₂. There is a particularly strong correlation between rate constants and homogeneous linewidths of the carbonyl vibration on the VC-O₂ product state. Concurrently, the FTIR spectra and spectral diffusion observed by 2D-IR corroborate an increase in solvent heterogeneity around the product. We interpret these results in the context of the potential role of solvent dynamics in facilitating chemical reactivity.