Effect of Solvent on the O 2 (a 1 Δ g ) f O 2 (b 1 Σ g + ) Absorption Spectrum: Demonstrating the Importance of Equilibrium vs Nonequilibrium Solvation Niels Dam, ² Tama ´ s Keszthelyi, ²,§ Lars K. Andersen, ² Kurt V. Mikkelsen, ‡ and Peter R. Ogilby* ,² Department of Chemistry, UniVersity of Aarhus, Langelandsgade 140, DK-8000 Århus, Denmark, and Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5 DK-2100, Copenhagen, Denmark ReceiVed: January 14, 2002 In a time-resolved infrared spectroscopic study, the a 1 Δ g f b 1 Σ g + 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 O 2 (a 1 Δ g ) f O 2 (X 3 Σ g - ) 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 X 3 Σ g - ,a 1 Δ g , and b 1 Σ g + states of oxygen. The data indicate that the bandwidths of the O 2 (a 1 Δ g ) f O 2 (b 1 Σ g + ) and O 2 (a 1 Δ g ) f O 2 (X 3 Σ g - ) 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 Introduction In recent years, it has become increasingly apparent that the effect of solvent on radiative transitions in dissolved oxygen provides an informative system to investigate mechanisms by which a host medium can perturb a solute. 1-3 Oxygen is particularly important in this regard because transitions between the three lowest electronic states, the X 3 Σ g - ,a 1 Δ g , and b 1 Σ g + states, are formally forbidden as electric dipole processes. Thus, for dissolved oxygen, the solvent plays an integral role in defining both the transition probability as well as the transition energy. This point has become quite evident, as increasingly sophisticated spectroscopic experiments have been able to quantify, with greater accuracy, the response of a given transition in oxygen to solvent perturbation. Furthermore, oxygen and many common solvent molecules are sufficiently small that high-level computational tools can be employed to model experimental results. This latter point is a critical component of ultimately establishing accurate mechanisms by which solvent influences the behavior of what is arguably one of nature’s most ubiquitous solutes; molecular oxygen. There are three electronic transitions in dissolved oxygen for which solvent-dependent spectra can be “readily” obtained (Scheme 1). An extensive amount of data on the a f X phosphorescent transition at ∼7850 cm -1 has been compiled over the years by a number of research groups. 4-7 The effect of solvent on the b f a fluorescence spectrum at ∼5200 cm -1 has likewise been studied. 8 In these latter experiments, however, the range of solvents from which data can be obtained is limited due to the comparatively short lifetime of the b 1 Σ g + state in solvents that contain C-H and O-H bonds (τ < 100 ns). 9 Finally, we have recently established that the a f b absorption spectrum at ∼5200 cm -1 can be recorded in time-resolved experiments. 2,10,11 In this case, the comparatively long solvent- * To whom correspondence should be addressed. ² Department of Chemistry, University of Aarhus. § Present address: Surface Spectroscopy Group, Chemical Research Centre, P.O. Box 17, H-1525 Budapest, Hungary. ‡ Department of Chemistry, University of Copenhagen. SCHEME 1 5263 J. Phys. Chem. A 2002, 106, 5263-5270 10.1021/jp0200876 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/02/2002