Radicals thermochemistry

D. Griller, D. D. M. Wayner. Radical Thermochemistry and Organic Reactions. Pure Appl. Chem. 1989, 61, 717-724. 

In this chapter, we look closely at the performance of several ab initio techniques in the prediction of radical thermochemistry with the aim of demonstrating which procedures are best suited in representative situations. We restrict our attention to several areas in which we have had a recent active interest, namely, the determination of radical heats of formation (AHf), bond dissociation energies (BDEs), radical stabilization energies (RSEs), and selected radical reaction barriers and reaction enthalpies. We focus particularly on the results of our recent studies. 

There is a wide variety of ab initio techniques available for the study of radical thermochemistry, ranging from quite cheap and approximate methods to much more expensive and accurate approaches. The quality of results yielded by these procedures depends on the size of the basis set used and on the degree of electron correlation included. In practice, it is necessary to strike a balance between the required accuracy and the computational cost that can be afforded. 

The amount of experimental information available regarding the thermochemistry of radicals is limited because of the inherent instability of such species. Therefore, theory has a potentially useful complementary role to play. However, the theoretical determination of radical thermochemistry is not without its own difficulties, and thus a careful assessment of accuracy needs to be carried out before theoretical procedures can be used routinely in this area. Steps in this direction are described in this chapter. 

Table I. Recommended Values for Bond Dissociation Energies and Radical Thermochemistry in Aromatic Oxidation...

Warbentin (19) has indicated how the radical thermochemistry involved can assist in assigning a mechanism for the thermal decomposition of phenyl oxalate. Exclusive initial fission of either the one C—C or the two C—O bonds would lead, via PhOCO or OCCO intermediates, to high C02 or high CO yields. In experiments lasting about 75 hours at 500°K. in diphenyl ether, comparable amounts of CO (13%) and C02 (9% ) are formed. The following steps, possibly concerted ... 

All the radicals concerned are important in oxidation processes of aromatic molecules, and this paper aims to offer a starting point for the thermochemical dissection of such oxidation processes. It is also hoped that it may stimulate further investigations of radical thermochemistry, especially in the aromatic field. Areas for fruitful work on bond energies include the formate and carbonate esters, including phenyl formate and phenyl carbonate, and the bond strengths in formic acid itself and in benzaldehyde. 

A modification of G2 by Pople and co-workers was deemed sufficiently comprehensive tliat it is known simply as G3, and its steps are also outlined in Table 7.6. G3 is more accurate titan G2, witli an error for the 148-molecule heat-of-formation test set of 0.9 kcal mol . It is also more efficient, typically being about twice as fast. A particular improvement of G3 over G2 is associated with improved basis sets for tlie third-row nontransition elements (Curtiss et al. 2001). As with G2, a number of minor to major variations of G3 have been proposed to either improve its efficiency or increase its accuracy over a smaller subset of chemical space, e.g., the G3-RAD method of Henry, Sullivan, and Radom (2003) for particular application to radical thermochemistry, the G3(MP2) model of Curtiss et al. (1999), which reduces computational cost by computing basis-set-extension corrections at the MP2 level instead of the MP4 level, and the G3B3 model of Baboul et al. (1999), which employs B3LYP structures and frequencies. 

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. 

P. M. Mayer, C. J. Parkinson, D. M. Smith, and L. Radom,/. Chem. Phys., 108,604 (1998). An Assessment of Theoretical Procedures for the Calculation of Reliable Free Radical Thermochemistry A Recommended New Procedure. 

Several other noteworthy features of resonance-stabilized radical thermochemistry are ... 

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