Radicals enthalpies of formation

Two recent reviews of hydrocarbon radical enthalpies of formation are J. Berkowitz, G. B. Ellison and D. Gutman, J. Phys. Chem., 98, 1744 (1994) and W. Tsang, in Energetics of Organic Free Radicals [Vol. 4 in file SEARCH Series (Eds. J. A. Martinho Simoes, A. Greenberg and J. F. Liebman), Blackie Academic Professional, an imprint of Chapman Hall, London, in press]. The enthalpies of formation of hydrocarbyl radicals is still a subject of some dispute. Admitting our prejudices, file values chosen in file current study are those recommended by the latter reference. 

A number of studies of H-atom transfer from hydrogen halides to free radicals, R + HX - RH + X, have been done by FPTRMS in which R was detected by photoionization, and its decay was monitored as a function of [HX] under pseudo-first-order conditions. When the rate coefficient is combined with determinations of the rate coefficient of the reverse reaction to obtain the equilibrium constant, the enthalpy of formation of the radical can be deduced. If the kinetics are accurately measured in isolation, this is a direct kinetic method which can be used to confirm (or otherwise) thermodynamic data obtained by classical, indirect kinetic methods which depend on correct mechanistic interpretation. In a number of instances free radical enthalpies of formation by these two different approaches have not been in good agreement. It is not the purpose of this short survey to discuss the differences, but rather to briefly indicate the extent to which the FPTRMS method has contributed to the kinetics of these reactions and to free radical thermochemistry. 

B. Experimental Data Measured Dissociation Energies and Radical Enthalpies of Formation... 

Table 15 shows the new values of bond dissociation energies and radical enthalpies of formation derived using the relationship... 

VIII. BOND ENERGIES AND RADICAL ENTHALPIES OF FORMATION... 

Bond Homolysis. A substantial number of gas-phase bond homolysis rate constants and free-radical enthalpies of formation have been determined (1,11) and a far greater number may be reliably estimated. However, two factors must be considered when applying gas-phase bond homolysis rate constants to condensed-phase systems. First, any selective solvation of product radicals will tend to increase (k /kg). However, solvation effects on free radical reaction rates are generally the... 

The values for the radical enthalpies of formation are listed in Table 19, together with references. 

Hildenbrand DL, Lau KH, Sanjurjo A (2003) Experimental thermochemistry of the SiCl and SiBr radicals enthalpies of formation of species in the Si—Cl and Si—Br systems. J Phys Chem A 107 5448-5451... 

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