GROUND-STATE BENZENE OXYGEN COMPLEX

The Drago-Rose/Benesi-Hildebrand spectrophotometric method has been used to quantify ground-state complex formation between benzene and oxygen in the gas phase at 295 K. Absorption to the benzene-oxygen charge-transfer state was the experimental parameter monitored. Assuming a 1:1 reaction stoichiometry, the data yield a free energy for complex formation (DELTA-G295K) of -1.2 +/- 0.3 kcal/mol and a complex extinction coefficient of 14 +/- 4 cm-1 M-1 at 218 nm. Independent experiments based on Dalton's law of partial pressures, however, indicate that the Drago-Rose/Benesi-Hildebrand results most likely overestimate the stability of the benzene-oxygen complex and that a more accurate DELTA-G value is approximately -0.8 kcal/mol. In an analogous experiment, the methanol-oxygen charge-transfer absorption was quantified in a series of aqueous solutions at 295 K. In this case, the data do not indicate the presence of a weakly bound complex but rather are more consistent with Mulliken's definition of a "contact" complex.