Solvation Enthalpies of Free Radicals: O−O Bond Strength in Di-tert-butylperoxide

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Abstract
The photolysis reaction of di-tert-butylperoxide was studied in various solvents by photoacoustic calorimetry (PAC). This technique allows the determination of the enthalpy of this homolysis reaction, which by definition corresponds to the O−O bond dissociation enthalpy of the peroxide in solution, D (O−O). The derived value from these experiments in benzene, 156.7 ± 9.9 kJ mol-1, is very similar to a widely accepted value for the gas-phase bond dissociation enthalpy, DHo(O−O) = 159.0 ± 2.1 kJ mol-1. However, when the PAC-based value is used together with auxiliary experimental data and Drago's ECW model to estimate the required solvation terms, it leads to 172.3 ± 10.2 kJ mol-1 for the gas-phase bond dissociation enthalpy. This result, significantly higher than the early literature value, is however in excellent agreement with a recent gas-phase determination of 172.5 ± 6.6 kJ mol-1. The procedure to derive the gas-phase DHo(O−O) was tested by repeating the PAC experiments in carbon tetrachloride and acetonitrile. The average of the values thus obtained was DHo(O−O) = 179.6 ± 4.5 kJ mol-1, confirming that the early gas-phase result is a lower limit. More importantly, the present study questions the usual assumption that the solvation terms of homolysis reactions producing free radicals in solution should cancel, and suggests a methodology to estimate solvation enthalpies of free radicals.

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