Effect of Solvent on the O₂(aΔ_g) → O₂(b¹Σ_g⁺) Absorption Spectrum:  Demonstrating the Importance of Equilibrium vs Nonequilibrium Solvation

Abstract

In a time-resolved infrared spectroscopic study, the a1Δg → b1Σg+ absorption spectrum of molecular oxygen at ∼5200 cm-1 was recorded in 19 solvents using a step-scan Fourier transform infrared spectrometer. Solvent-dependent changes in the full width at half-maximum of this absorption band covered a range of ∼30 cm-1 and solvent-dependent changes in the position of the band maximum covered a range of ∼55 cm-1. When considered along with solvent-dependent O2(a1Δg) → O2(X3Σg-) emission data, the current results identify features that must be incorporated in computational models of the interaction between oxygen and the surrounding solvent. In particular, data presented herein clearly demonstrate the importance of considering the influence of equilibrium and nonequilibrium solvation when interpreting the effect of solvent on transitions between the X3Σg-, a1Δg, and b1Σg+ states of oxygen. The data indicate that the bandwidths of the O2(a1Δg) → O2(b1Σg+) and O2(a1Δg) → O2(X3Σg-) transitions principally reflect the effects of equilibrium solvation, whereas the associated solvent-dependent spectral shifts reflect the effects of both equilibrium and nonequilibrium solvation. These general conclusions make it possible to resolve some long-standing problems associated with early attempts to interpret the effect of solvent on electronic transitions in oxygen