Homolytic models

An experimental probe for the presence of radical intermediates resulting from thermally induced homolytic cleavage of the N-0 bond was derived by incorporating an alkene into a model substrate to act as a potential intramolecular radical trap (Scheme 6.25) [11]. In a control experimental, thermal reaction of 73 gave the desired product 74 in 66% isolated yield. On the other hand, thermal rearrangement of the unsaturated compound 75 under our typical conditions gave the desired hydroxypyrimidinone 76 in only 38% isolated yield. When the vinyl ami-doxime mixture 75Z/E was heated in o-xylene at 125 °C in the presence of a... 

The overall degradation of (103) assisted by the cluster [(Cp )2 M o2Co2S3(CO)4] (Cp = CH3C5H4) is the model reaction that best resembles the heterogeneous counterparts, particularly those classified as Co/Mo/S phase,158 in terms of both structural motif and HDS activity.229 Morever, the Co/Mo/S cluster has successfully been employed to show that the C—S bond scission in the desulfurization of aromatic and aliphatic thiols occurs in homolytic fashion at 35 °C and that thiolate and sulfido groups can move over the face of the cluster as they are supposed to do over the surface of heterogeneous catalysts.230... 

The kinetics of the electron transfer reaction leading to the homolytically dissociating primary radical is also a question of interest. It may be modeled using the Morse curve for the reactant and the Morse curve shown in Fig. 10 representing the homolytic dissociation of the primary radical. This point will be discussed in detail in Section 5. 

PARABOLIC MODEL OF BIMOLECULAR HOMOLYTIC REACTION 4.4.1 Main Equations of IPM... 

A SET process has been postulated between Rh(III) oxidative adducts and an NAD(P)H model compound (cf. Section 18.2.4) [91]. Oxidative adducts formed by Sn2, SNAr, or inner-sphere SET pathways may produce radicals by homolytic M-C bond cleavage [130, 155, 176, 199]. 

This chapter assesses the performance of quantum chemical models with regard to the calculation of reaction energies. Several different reaction classes are considered homolytic and heterolytic bond dissociation reactions, hydrogenation reactions, isomerization reactions and a variety of isodesmic reactions. The chapter concludes with a discussion of reaction energies in solution. 

Local density models also provide a poor account of homolytic bond dissociation energies. The direction of the errors is the opposite as noted for Hartree-Fock models (reaction energies are too large), but the magnitudes of the errors are comparable. In fact, the average of Hartree-Fock and local density homolytic bond dissociation energies is typically quite close to the experimental energy. ... 

It is clear that proper description of the energetics of homolytic bond dissociation requires models that account for electron correlation. Are correlated models also needed for accurate descriptions of relative homolytic bond dissociation energies where the relevant reactions are expressed as isodesmic processes A single example suggests that they may not be. Table 6-15 compares calculated and measured CH bond dissociation energies in hydrocarbons, R-H, relative to the CH bond energy in methane as a standard ... 

With due attention to the noted failures, it is evident that relative homolytic bond dissociation energies, unlike absolute homolytic bond dissociation energies, can be reasonably well described with simple and practical models. 

Overall, the performance of Hartree-Fock models is very poor. In most cases, activation energies are overestimated by large amounts. This is not surprising in view of previous comparisons involving homolytic bond dissociation energies (see Table 6-2), which were too small. In terms of mean absolute deviation from the standard (MP2/6-311+G ) calculations, STO-3G yields the poorest results and 3-2IG the best results. 6-3IG and 6-311+G models provide nearly identical activation energies (just as they did for transition-... 

Table A6-1 Homolytic Bond Dissociation Energies. Hartree-Fock Models... 

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