Stepwise dissociation energies

In a few favourable cases these dissociation energies are known quite accurately. The two dissociation energies in the stepwise dissociation of the w ater molecule, D(HO—H) and D(H—O) are example. The most accurate result for D 0—H) is probably the spectroscopic evaluation of a dissociation limit for an excited state of OH, leading to Z (0—H) -- 102-37 kcal/mole, at 25 From the relationship 

Combined wnth A / /(HgOg, g) - —32-53 0-04 keal/ mole this leads to A /i (OH, g) = 7-8 kcal/mole. 

A similar approach has also been made already to D(HS—H) and D(H—S). In this ca,se the value J f (HS,g) = 38-2 kcal/mole gives 7)(H—S) — 80 kcal/mole, and from Zl H/(H2S, g) == — 4-8kcal/ mole, the value i (HS—H) 95 kcal/mole was derived. 

The case of ammonia is slightly more complex since here there arc three stepwise dissociation energies i)(H2N-—H), i)(HN—H), and 7)(lSr—H). Determination of the heat of formation of the NHg 

44 i 2 kcal/mole, referred to previously, 11-09 lead to D(H2N—H) = 107 kcal/ 

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Fig. 5. Stepwise dissociation energies for the xenon hexafluoride cation. 

The stepwise dissociation energies Z)(FO—F) and 2)(0— ) were calculated from the data as 40 1 and 50.8 2.0 kcal. These are in excellent agreement with the recent figures obtained by Cl)me and Watson. Houser has expressed scepticism for the kinetic data for this pyrolysis reaction as deduced from the shock-tube experiments of Lin and Bauer. Houser places more trust in the kinetic data obtained from earlier studies using conventional techniques. 

A yet more complicated situation is presented by the stepwise dissociation energies of methane, for although D(CHg—H) is well established, the same cannot be said for the dissociation energies D(CH2—H) and D(C—H). Bepresenting the successive dissociation energies by Dj, Dg, and one can equate the sum of the dissociation energies to the heat of formation of CII4 from the atoms, at 25 ""C,... 

Intrinsic or absolute bond energies refer bond strength to the valence states of atoms in molecules normally, the valence state will differ from the atomic ground state. The relevance of intrinsic bond strength becomes apparent when examining the stepwise dissociation energies, Z>i, Z>2> D in MX , each of which may differ from another. 

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) ... 

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. 

Like N4, cubic Ng is expected to have a substantial dissociation barrier since the orbital correlation of Ns with 4N2 shows a crossing of occupied and unoccupied orbitals [4], Recent theoretical studies [3,60-63] have confirmed that the energy barrier for dissociation into N2 molecules is predicted to be 10-22 kcal/mol depending on the level of theory, and the pathway is likely to be a stepwise dissociation reaction such as Ng - N2 +... 

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 ... 

In the case of hydration, an asymmetric stepwise low energy pathway to dissociative adsorption is generally accepted. The mechanism initially involves molecular adsorption as a precursor state. A secondary interaction through hydrogen bonding between neighbor dimer OH groups, that seems to affect further surface reactions of adsorbed OH and H, has been illustrated by theoretical and experimental studies. 

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)... 

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