The a1Δg → X3Σg- transition in molecular oxygen:: Interpretation of solvent effects on spectral shifts

Ab initio computational methods have been used to investigate the effect of solvent on the a(1)Delta(g) --> X(3)Sigma(g)(-) transition in molecular oxygen. For a given solvent molecule, M, energies have been obtained for the M-O-2-(a(1)Delta(g)) complex that is in equilibrium with its surrounding outer solvent and for the M-O-2(X(3)Sigma(g)(-)) complex that is not in equilibrium with the outer solvent (i.e., the Franck-Condon state populated in the a --> X transition). These energies depend principally on the quadrupolar and higher order coupling terms between the complex and the outer solvent and only minimally on the dipolar coupling term and on dispersion interactions. Upon averaging over multiple M-O-2 orientations, each with an unique transition probability, differences between the calculated O-2(a(1)Delta(g)) and O-2(X(3)Sigma(g)(-)) energies correlate well with the experimental spectral data. Thus, previously published correlations between experimental a-X spectral shifts and the solvent polarizability cannot be ascribed solely to a dispersion interaction, but rather simply reflect the general importance of the solvent's electronic response in the oxygen-solvent interaction.