Solute−Solvent Complex Kinetics and Thermodynamics Probed by 2D-IR Vibrational Echo Chemical Exchange Spectroscopy

Abstract

The formation and dissociation kinetics of a series of triethylsilanol/solvent weakly hydrogen bonding complexes with enthalpies of formation ranging from -1.4 to -3.3 kcal/mol are measured with ultrafast two-dimensional infrared (2D IR) chemical exchange spectroscopy in liquid solutions at room temperature. The correlation between the complex enthalpies of formation and dissociation rate constants can be expressed with an equation similar to the Arrhenius equation. The experimental results are in accord with previous observations on eight phenol/solvent complexes with enthalpies of formation from -0.6 to -2.5 kcal/mol. It was found that the inverse of the solute-solvent complex dissociation rate constant is linearly related to exp(-DeltaH0/RT) where DeltaH0 is the complex enthalpy of formation. It is shown here, that the triethylsilanol-solvent complexes obey the same relationship with the identical proportionality constant, that is, all 13 points, five silanol complexes and eight phenol complexes, fall on the same line. In addition, features of 2D IR chemical exchange spectra at long reaction times (spectral diffusion complete) are explicated using the triethylsilanol systems. It is shown that the off-diagonal chemical exchange peaks have shapes that are a combination (outer product) of the absorption line shapes of the species that give rise to the diagonal peaks.