Energy, bond radicals

Examine the energies of radicals resulting from hydrogen atom abstraction in 3-ethylpentane. Which radical is the lowest energy Is there a relationship between the CH bond lengths in 3-ethylpentane and the stabilities of the radicals resulting from bond dissociation Elaborate. 

In sharp contrast to the stable [H2S. .SH2] radical cation, the isoelectron-ic neutral radicals [H2S.. SH] and [H2S. .C1] are very weakly-bound van der Waals complexes [125]. Furthermore, the unsymmetrical [H2S.. C1H] radical cation is less strongly bound than the symmetrical [H2S.. SH2] ion. The strength of these three-electron bonds was explained in terms of the overlap between the donor HOMO and radical SOMO. In a systematic study of a series of three-electron bonded radical cations [126], Clark has shown that the three-electron bond energy of [X.. Y] decreases exponentially with AIP, the difference between the ionisation potentials (IP) of X and Y. As a consequence, many of the known three-electron bonds are homonuclear, or at least involve two atoms of similar IP. 

These methods can give us useful information on radicals in a manner similar to that for closed-shell systems, provided the exploitation is correct. Of course, in expressions for total energy, bond orders, etc., a singly occupied orbital must be taken into account. One should be aware of areas where the simple methods give qualitatively incorrect pictures. The HMO method, for example, cannot estimate negative spin densities or disproportionation equilibria. On the other hand, esr spectra of thousands of radicals and radical ions have been interpreted successfully with HMO. On the basis of HMO orbital energies and MO symmetry... 

Influence of the Neighboring rr-Bonds on the Activation Energy of Radical Addition... 

The activation energy of radical abstraction is influenced by the so-called triplet repulsion in the transition state. This influence is manifested by the fact that the stronger the X—R bond towards which the hydrogen atom moves in the thermally neutral reaction X + RH, the higher the activation energy of this reaction. The triplet repulsion is due to the fact that three electrons cannot be accommodated in the bonding orbital of X—C therefore, one electron... 

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. 

Obviously, species with weak bonds like will dissociate more easily than Sg. At lower temperatures the activation energy of radical formation is considerably larger, which has been explained by the influence of impurities, X, which react with the radicals according to... 

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

Figure 6.5. Schematic rationalization for lower energy (vis or near-IR) electronic transitions observed for open-shell species with singly occupied bonding (radical cations, left) or antibonding orbitals (radical anions, right).

Such a process is structurally very unlikely. It would be expected to have a very small A factor as well as a fairly high activation energy. If we rule this type of process out then we are forced to assume that the CH2 attacks the H atom initially in very much the same way that it might for a metathetical process. If we follow this process through, then we see that if the H-atom abstraction is successful, we have still a transition state in which two radicals, the newly formed methyl and the residual alkyl, arc in very close proximity, being essentially H-bonded. However, we have already noted as an empirical fact that the cross sections for alkyl radical recombinations are extremely high. We should thus like to propose that the H-bonded radical pair simply rotates into the favorable position for C-C bond formation before separation can take place. The pathway for the insertion reaction would then take the form ... 

During coal conversion, the coal structure influences both thermal and catalytic reactions. Thermal reactions of solid coals initiate the breakage of weak bonds at rates proportional to their bond dissociation energies. The radicals thus produced require stabilization by hydrogenation or addition of small molecules otherwise the radicals couple to produce much more thermally stable bonds, which eventually leads finally to the formation of infusible and insoluble coke. 

The approach taken in our laboratory combines both of these trends. Specifically, we have developed a new experiment that allows us to study, for the first time, the photodissociation spectroscopy and dynamics of an important class of molecules reactive free radicals. This work is motivated in part by the desire to obtain accurate bond dissociation energies for radicals, in order to better determine their possible role in complex chemical mechanisms such as typically occur in combustion or atmospheric chemistry. Moreover, since radicals are open-shell species, one expects many more low-lying electronic states than in closed-shell molecules of similar size and composition. Thus, the spectroscopy and dissociation dynamics of these excited states should, in many cases, be qualitatively different from that of closed-shell species. 

If hydrogen transfer is under thermodynamic control, then the vitamin will experience cleavage of the weakest CH (or OH) bond. Compare energies of radicals derived from hydrogen abstraction at different positions from a model a-tocopherol (R = CH3). Which radical is most stable Are there alternative radicals of similar stability ... 

Of fundamental importance to free-radical chemistry are bond dissociation energies and radical heats and entropies of formation. Bond dissociation energy is defined as the energy required to break a particular bond to form two radicals. More precisely, bond dissociation energy of the R—X bond, D(R—X), is the enthalpy change of Reaction 9.7.37... 

Table 9.3 Selected Bond Dissociation Energies and Radical Heats of Formation at 25°C (kcal mole-1)0...

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