Bond dissociation energy stepwise

Next, supervised-learning pattern recognition methods were applied to the data set. The 111 bonds from these 28 molecules were classified as either breakable (36) or non-breakable (75), and a stepwise discriminant analysis showed that three variables, out of the six mentioned above, were particularly significant resonance effect, R, bond polarity, Qa, and bond dissociation energy, BDE. With these three variables 97.3% of the non-breakable bonds, and 86.1% of the breakable bonds could be correctly classified. This says that chemical reactivity as given by the ease of heterolysis of a bond is well defined in the space determined by just those three parameters. The same conclusion can be drawn from the results of a K-nearest neighbor analysis with k assuming any value between one and ten, 87 to 92% of the bonds could be correctly classified. 

However, if we take the CH4 molecule apart stepwise, we get four different results that we may call the bond dissociation energies D (the nomenclature is arbitrary) ... 

High-valent oxo-complexes, isolated or in situ-generated, interact most often with electron-rich n -systems 1 or suitable C-H bonds with low bond dissociation energy (BDE) in substrates 3 (Fig. 2). These reactions may occur concerted via transition states 1A or 3A leading to epoxides 2 or alcohols 4. On the other hand, a number of epoxidation reactions, such as the Jacobsen-Katsuki epoxidation, is known to proceed by a stepwise pathway via transition state IB to radical intermediate 1C [39]. Similarly, hydrocarbon oxidation to 4 can proceed by a hydrogen abstraction/S ... 

TABLE 3. Stepwise bond dissociation energies (BDE, kcal mol 1) for MI I ... 

Seminal work by Saveant and his group [39] has contributed to our understanding of the dynamics of dissociative ET, compared with the corresponding stepwise process [40], Obviously, bond-dissociation energy, rather than bond dissociation free energy, represents the important contribution to the intrinsic barrier [41]. One must, moreover, take into account that dissociative ET can occur in the adiabatic (e.g. tert-butyl bromide) and in the non-adiabatic regime (di-tert-butyl peroxide)... 

We adopt AjH (BCl2, g> 298.15 K) = -19.0 3.0 kcal mol . This value also leads to a realistic progression in the stepwise bond dissociation energies for BCl(g), BClgCg) and BClg(g). Using all JANAP data (2), we calculate Dj(B-Cl) - 127.9, DgCBCl-Cl)... 

Figure 1 Schematic representation of the coupled multiphoton excitation and unimolecular reaction CF3I — CF3 -h l( P3/2). Stepwise excitation occurs at the large wavenumber of the V (CF3 symmetric stretching) mode, 1070 cm", left-hand side in principle there might be a direct multiphoton proce.s.s to threshold (centre). The right-hand part shows the potential for the Cl bond, with a small fundamental wavenumber of 286 cm", a bond dissociation energy of about 220 kJ mol" and the bond breaking reaction (horizontal arrow) ...

There is at present no convenient, self-consistent source of all bond energies. The standard work is Cottrell. T. L. The Strengths of Chemical Bonds, 2nd ed. Butter-worths London, 1958, but it suffers from a lack of recent data. Darwent (National Bureau of Standards publication NSRDS-NBS 31, 1970) has summarized recent data on dissociation energies but did not include some earlier work or values known only for total energies of atomization rather than for stepwise dissociation. Three useful references of the latter type are Brewer, L. Brackett, E. Chem. Rev. 1961,61,425 Brewer, L. et al. Chem. Rev. 1963, 63, 111 Feber, R. C. Los Alamos Report LA-3164, 1965. The book by Darwent mentioned above also lists bond energy values for some common bonds. 

When some polychlorobenzene, such as chlorobenzene, 1,4-dichlorobenzene, and 1,2,4-trichlorobenzene, was employed as the electron acceptor, it was found that the ELT proceeds from the NpD(T/ ), in which the dissociative ELT is not operative. Since the C—Cl bond dissociation rate of chlorobenzene radical anion was reported to be 1.8 x 109 to >2 x 10los-1, the radical ion pair should be formed after the ELT from NpD(T2) [95, 96]. For the stepwise mechanism (Scheme 2.6), the dissociative ELT theory for the concerted dissociation cannot be applied [55]. The Marcus theory is adequate for the present system. According to the Marcus ELT theory, the free energy change for the ELT can be given by Eq. (10) ... 

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