O-2(a(1)Delta(g)) absorption and O-2(b(1)Sigma(+)(g)) emission in solution: Quantifying the a-b Stokes shift

In a nanosecond time-resolved infrared spectroscopic study of dissolved oxy gen, O-2(a(1)Delta (g)) absorption, i.e., a(1)Delta (g) --> b(1)Sigma (+)(g ), and O-2(b(1)Sigma (+)(g)) emission, i.e., b(1)Sigma (+)(g) --> a(1)Delta (g), were monitored at similar to 5200 cm(-1) in a number of solvents. The maxima of the respective spectra depend significantly on the solvent, indi cating that the O-2(a(1)Delta (g)) and O-2(b(1)Sigma (+)(g)) energy levels likewise depend significantly on the solvent. The corresponding Stokes shif ts, however, are small. The latter, recorded as the difference between the absorption and emission maxima, do not exceed the uncertainty limits that d erive from the step-scan Fourier transform spectroscopic measurements (simi lar to+/-13 cm(-1)). Nevertheless, the data clearly indicate that the diffe rence between the equilibrium and nonequilibrium solvation energies for the O-2(a(1)Delta (g)) and O-2(b(1)Sigma (+)(g)) states is not large. Within t he error limits, it is not possible to ascertain if the Stokes shifts are s olvent dependent. Ab initio computational methods were used to model the da ta, and the results indicate that both long- and short-range interactions b etween oxygen and the perturbing solvent must be considered to adequately d escribe spectroscopic transitions in dissolved oxygen. The computational re sults indicate that a 1:1 complex between oxygen and the perturbing molecul e embedded in a dielectric continuum appears to provide a sufficiently accu rate model that can be used to probe subtle solvent-oxygen interactions.