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

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, DHsin(degrees)(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, DH(degrees)(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 DH(degrees)(O-O) was tested by repeating the PAC experiments in carbon tetrachloride and acetonitrile. The average of the values thus obtained was DH(degrees)(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.

Solvent Effects on the Energetics of the Phenol O−H Bond: Differential Solvation of Phenol and Phenoxy Radical in Benzene and Acetonitrile

“…Although there are several experimental studies in which the energetics of PhOH‚‚‚S intermolecular bonds were determined, 12 we have found that the so-called ECW method, developed by Drago and co-workers, 13 provides an alternative and direct way of estimating that bond enthalpy in many solvents. 8,14,15 Regarding the enthalpy of solvation of the hydrogen atom, it is usually identified with the enthalpy of solvation of H 2 , which is nearly constant for organic solvents (1.2 ( 0.2 kcal mol -1 ). 8 However, it is important to stress that this value becomes redundant when the PhO-H gas-phase bond dissociation enthalpy relies on DH°s ln (PhO-H) obtained from photoacoustic calorimetry (PAC) experiments.…”

No Fear of Perfluorinated Peroxides: Syntheses and Solid‐State Structures of Surprisingly Inert Perfluoroalkyl Peroxides

“…Earlier experimental activation energies determined from rate constants of the thermal decomposition of [(H 3 C) 3 CO] 2 (163 AE 2kJmol À1 ) [11] and 1a (149 AE 4kJmol À1 ) [15] were questioned by am ore recent photoacoustic calorimetry study,w hich recommended ah igher BDE of 179.6 AE 4.5 kJ mol À1 for [(H 3 C) 3 CO] 2 . [34] Thel ow activation energy obtained for the thermal decomposition of these non-rigid tert-butyl derivatives might not be ag ood approximation for thermodynamic OÀOb ond energies,a nd ar edetermination of the rather low experimental BDE for 1a also needs to be considered ( Table 2).…”

Keine Scheu vor perfluorierten Peroxiden: Synthese und Strukturen von überraschend inerten Perfluoralkylperoxiden

“…Wirh aben Gasphasen-UV/Vis-Spektren dieser Peroxide im Wellenlängenbereich von 200 bis 800 nm aufgenommen. In Übereinstimmung mit früheren Studien [34] liefert die Photolyse von 1a mit einer 1000 WX enon-Hochdrucklampe im Quarzglas quantitativ die Zersetzungsprodukte (F 3 C) 2 CO und C 2 F 6 . Der Übergang mit der niedrigsten Anregungsenergie für( F 3 CO) 2 liegt unterhalb 200 nm, [36] während 1a und 1b schwächere,r ot-verschobene UV-Übergänge bei 253 beziehungsweise 250 nm zeigen.…”

“…Diese energetisch niedrigsten Anregungen lassen sich, vorläufigen TD-DFT-Berechnungen zufolge (Tabelle S3.2), nahezu ausschließlich den HOMO-LUMO-(n-p*)-Übergängen zuordnen. In Übereinstimmung mit früheren Studien [34] liefert die Photolyse von 1a mit einer 1000 WX enon-Hochdrucklampe im Quarzglas quantitativ die Zersetzungsprodukte (F 3 C) 2 CO und C 2 F 6 .…”

“…CO] 2 eine grçßere BDE von 179.6 AE 4.5 kJ mol À1 ergab,i nF rage gestellt [34]. Die Hochtemperatur-Aktivierungsenergie fürd en thermischen Zerfall dieser flexiblen tert-butylsubstituierten Peroxide liefert mçglicherweise keine verlässlichen Werte fürd eren thermodynamische O-O-Bindungsenergie,s odass auch eine Überprüfung der relativ geringen experimentellen BDE von 1a in Betracht gezogen werden sollte (Tabelle 2).…”