Solvent and Pressure Effects on Quenching by Oxygen of 9,10-Dimethylanthracene Fluorescence in Liquidn-Alkanes:  A Study on Solvent Cage Effects

Abstract

The fluorescence quenching by oxygen of 9,10-dimethylanthracene (DMEA) in liquid ethane and propane at pressures up to 60 MPa and 25 degrees C was investigated. The apparent activation volumes for the quenching rate constant, k(q),DeltaV++(q) , were 5.0 +/- 3.4 and 7.4 +/- 1.0 cm(3)/mol, whereas those for the solvent viscosity, eta,DeltaV++(eta) , were 190 +/- 22 and 42 +/- 1 cm(3)/mol in ethane and propane at 6.0 MPa, respectively. These results were discussed together with those in n-alkanes (C(4)-C(7)) and methylcyclohexane (MCH) that were previously reported, and it was found that DeltaV++(q) increases monotonically but DeltaV++(eta) decreases rapidly with increasing the number of carbon atoms in n-alkanes. The plot of ln k(q) against ln eta showed a leveling-off with decreasing eta. These observations were analyzed satisfactorily by the pressure dependence of the solvent viscosity on k(q) coupled with that of the radial distribution function, g(sigma), at contact with a hard sphere assumption. The apparent bimolecular rate constant, k(bim,0), for the quenching in the solvent cage was evaluated by extrapolating to g(sigma)eta = 0 in the plot of g(sigma)/k(q) against g(sigma)eta, and it was found that k(bim,0) decreased with increasing the radius of the solvent molecule. From the solvent size dependence of k(bim,0), the solvent cage effect was discussed phenomenologically.

Radical Cage Effects: The Prediction of Radical Cage Pair Recombination Efficiencies Using Microviscosity Across a Range of Solvent Types

“… − The “radical cage effect” successfully explains many aspects of radical chemistry, including magnetic isotope effects, , radical polymerization kinetics, − cobalamin reactivity, product distributions in photochemistry, stereochemistry of rearrangements, , the reactivity of halogenase analogues, and chemically induced dynamic nuclear polarization effects (CIDNP). , In addition to the solution phase, the cage effect has been observed to occur in solid state films, , inside micelles, as well as in compressed and supercritical gases . New reports of the radical cage effect appear frequently in the literature. − …”

Radical Cage Effects: Comparison of Solvent Bulk Viscosity and Microviscosity in Predicting the Recombination Efficiencies of Radical Cage Pairs

“… , Radical cage effects have an enormous impact on chemical reactivity in solution. , In particular, they are necessary to explain a host of kinetic observations and fundamental reaction phenomena. For example, cage effects are necessary to explain magnetic isotope , and CIDNP , effects, rate-viscosity correlations, variations in products and yields as a function of medium, , variations in quantum yields as a function of medium, and regio- and stereochemical control. − Examples of important reactions where cage effects are necessary to explain the kinetics include the initiation, propagation, and termination steps of radical polymerization reactions, − the reactions of coenzyme B 12 and its model complexes, ,− the reactions of hemes with O 2 , and various electron transfer reactions. − New observations of cage effects and their impact on reactivity are reported regularly. − …”