Radical-molecule complexes

Let us emphasize that when it is claimed that the reactant complexes are lower in energy than the isolated reactant, this statement is made on terms of enthalpy or ZPE corrected electronic energies. On the other hand, the Gibbs free energy associated with step 1, i.e. the formation of the reactant complex (AGi = Grc-Gr), for the formation of radical-molecule complexes is always positive in a wide range of temperatures, around 300 K. The expression for the equilibrium constant can... 

Radical-molecule complexes offer the opportunity to examine bimolecular processes without a third body present. For example, ground state oxygen atoms complexed to HCl molecules might be used to examine the O + HCl OH + Cl reaction (e.g., by vibrationally exciting the HCl moiety in an O-HCl complex), which is thermoneutral, with a barrier to reaction of 3000 cm Such experiments wait in the wings. 

The recombination of iodine atoms in an excess of HCl, DCl, and HBr has been studied over a wide range of temperature, using flash photolysis techniques. The results suggest that reaction is dominated at least at lower temperatures, by the radical-molecule complex mechanism. It appears that I—(HX) are the unstable intermediates, in which the iodine atom is interacting with both H and X, rather... 

There is considerable evidence for the formation of a somewhat stable radical molecule complex for the recombination of iodine atoms in the presence of I2 [109]. The rate coefficient is reported [110, 111] to be... 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

The intermediacy of ion-neutral complexes is neither restricted to even-electron fragmentations nor to complexes that consist of a neutral molecule and an ion. hi addition, radical-ion complexes and radical ion-neutral complexes occur that may dissociate to yield the respective fragments or can even reversibly interconvert by hydride, proton or hydrogen radical shifts. Many examples are known from aliphatic alcohols, [180-183] alkylphenylethers, [184-187] and thioethers. [188]... 

Figure 8B shows /(c) and K(t) for ethane formed by the association of two methyl radicals [using complex 3, Table I, to evaluate the 22 P(e rf) ] and for ethane thermally activated at 300°C. The average energy of the ethane molecule formed by methyl radical association is 3.6 kcal. mole-1 higher than for the molecule formed by thermal activation. Therefore, if... 

Fig. 39 llustration of (a) a symmetry-allowed HT process and (b) a symmetry-forbidden HT process. Both reactions take place within a dimeric ethene radical cation complex. Both dimers possess C2v symmetry. For the symmetry-forbidden reaction, (a), the two ethene molecules lie in perpendicular planes consequently, the reactant and product have different electronic state symmetries, Bx and B2, respectively, and Vel is therefore zero. For the allowed process, (b), the two ethene groups lie in parallel planes and both reactant and product have identical state symmetries, Bp, thus, Vel is non-zero. For non-adiabatic HT, where Vel is very small ( 25 cm-1), the allowed and forbidden processes have nearly identical free energies of activation. 

As shown in Table 4.1, formation of the mixed adduct is favored over homodimerization of 8a with the simple styrene 13a, but this selectivity is inverted for the case of the more bulky dienophile tra 5-[3-methylstyrene 13b, presumably due to steric effects. Although the overall reaction is highly exothermic on the radical cation surface, the reaction is not insensitive to steric effects. Chemoselectivity in the radical cation cycloaddition is largely a consequence of a substrate s ability to stabilize the radical cation of the oxidized species through the formation of a weakly bound ion-molecule complex. Such complexes have been known for a long time in gas-phase... 

Many radical-molecule reaction mechanisms have been shown to be complex and to involve a fast equilibrium between the reactants and the pre-reactive complex, followed by the irreversible formation of products. An example of this mechanism for toluene is shown in Reactions Ibi and Ibii, respectively,... 

Scheme 4 Proposed catalytic mechanism of PHM and D/3M showing the reactive ternary complex. Proposed structure of the intermediate formed after reaction of Cub(H)-02 with substrate to form a substrate-derived free radical and Cub(11)-OOH. This illustrates a possible pathway for electron transfer from QiaCI) to Cub(H)-OOH throngh the solvent-filled cleft and the changes in copper ligation that accompany oxidation. With the exception of reactive intermediates, the water molecules complexed to the copper sites have been omitted. (Ref 27, Reproduced by permission of American Society for Biochemistry and Molecular Biology)...

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