Complex unimolecular reactions

These reactions are characterized by the concerted breakage and formation of multiple bonds with the formation of stable as well as radical products. Complex unimolecular reactions can be categorized into radical, molecular. 

In the preceding sections we have shown that at sufficiently high concentrations or for molecules sufficiently complex, unimolecular reactions may proceed in such a manner that the disturbance of the equilibrium distribution of reactive molecules does not significantly affect the rate of reaction. Under such conditions it is permissible to approximate the rate of a unimolecular reaction by its high-pressure rate. But such a pseudoequilibrium invites still another approach to the formulation of the theory of reactions. This approach, known as the transition-state method, was first outlined by Marcellin, was developed further by Rodebush " and Rice and Gershinowitz, and was put in its present form by Eyring and Evans and Polanyi. - ... 

Guo. Y.. Shalashilin. D. V.. Krouse. J. A. and Thompson. D. L.(1999) Intramolecular dynamics diffusion theory approach to complex unimolecular reactions, J. Chem. Phys. 110, 5521-5525. 

The introductory remarks about unimolecular reactions apply equivalently to bunolecular reactions in condensed phase. An essential additional phenomenon is the effect the solvent has on the rate of approach of reactants and the lifetime of the collision complex. In a dense fluid the rate of approach evidently is detennined by the mutual difhision coefficient of reactants under the given physical conditions. Once reactants have met, they are temporarily trapped in a solvent cage until they either difhisively separate again or react. It is conmron to refer to the pair of reactants trapped in the solvent cage as an encounter complex. If the unimolecular reaction of this encounter complex is much faster than diffiisive separation i.e., if the effective reaction barrier is sufficiently small or negligible, tlie rate of the overall bimolecular reaction is difhision controlled. 

Eyring H 1935 The activated complex in chemical reactions J. Chem. Phys. 3 107-15 Hofacker L 1963 Quantentheorie chemischer Reaktionen Z. Naturf. A 18 607-19 Robinson P J and Holbrook K A 1972 Unimolecular Reactions (New York Wiley)... 

Each term in this equation represents an independent pathway. The low-pH arm in the figure is equivalent to reaction (6-57), or one similar to it, in which the proton attacks the substrate directly. The high-pH pathway represents the unimolecular reaction of the substrate or else its reaction with water. As this discussion illustrates, a reaction whose pH profile shows upward bends can be analyzed in terms of separate pathways. A complex profile can be separated into regions at each upward bend each region is a distinct pathway. 

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

The Lindemann kinetics for unimolecular reactions [185] can be formally recovered if one subsumes the formation of a collision complex with the precursor complex steps R1-R2 <—> APC) into one corresponding to the excited reactant Rl. The excited... 

The formation of a complex between the propagating end and one or more molecules of monomer can have two extreme consequences. If the incorporation of a monomer molecule from the solvation shell of the cation is the growth-rate determining step, the propagation becomes a unimolecular reaction and the rate of polymerisation becomes of zero order with respect to monomer concentration. Such a model was developed by... 

If rates in solution and in gas phase are to be equal, the activity coefficient factor, i.e./A/B//x must be equal to unity. For unimolecular reactions, where the reactant and activated complex have similar structures and/A and /x do not differ widely, the rate of reaction in solution will be quite similar to that in the gas phase. 

The procedure and most of these comments apply also to the unimolecular reaction 3.33. There is only one difference The TST equation for relating k with the equilibrium between the reactant and the activated complex,... 

We have discussed in this chapter the thermal pyrolyses of a number of strained ring compounds. In most of the cases considered there is good evidence that the processes are unimolecular. Where possible we have tried to suggest plausible transition complexes, and reaction paths, based on a consideration of such factors as the kinetic parameters, stereochemistry of the reaction and effect of substituents. In reactions of this type, the description of the transition complex is fraught with difficulties, since the absence of such things as solvent effects (which can be so helpfrd in bimolecular reactions) limit the criteria on which such descriptions may be based. Often two types of transition complex may be equally good at accounting for the observed data. Sometimes one complex will explain some of the data while another is better able to account for the remainder. It is probable that in many cases our representation... 

Not all ionization methods rely on such strictly unimolecular conditions as El does. Chemical ionization (Cl, Chap. 7), for example, makes use of reactive collisions between ions generated from a reactant gas and the neutral analyte to achieve its ionization by some bimolecular process such as proton transfer. The question which reactant ion can protonate a given analyte can be answered from gas phase basicity (GB) or proton affinity (PA) data. Furthermore, proton transfer, and thus the relative proton affinities of the reactants, play an important role in many ion-neutral complex-mediated reactions (Chap. 6.12). 

Let us consider in more detail the concept of a free energy barrier. Transition state theory also uses the idea that there is such a barrier in the reaction path. What is special about TST is that it ascribes certain properties to the species at the top of the barrier, the activated complex. According to TST for a unimolecular reaction,... 

Fig. 8. (a) Energy diagrams for simple unimolecular decompositions (b) complex uni-molecular decompositions (c) simple bimolecular (metathesis) reactions (d) complex bimolecular reactions when one product channel is open and (e) when two product channels are open. 

Again the analysis of these reactions can be undertaken either directly by the use of TST or by considering their reverse, the complex fission reactions discussed earlier. Since unimolecular reactions were treated in considerable detail before, we will not repeat related issues here. 

These, and similar data for other systems, demonstrate the tremendous potential that the MICR technique has for the qualitative elucidation of potential energy surfaces of relatively complex organic reactions. Once implementation of the quadrupolar excitation technique has been effected to relax ions to the cell center, the technique will become even more powerful, in that the determination of highly accurate unimolecular decomposition lifetimes of chemically activated intermediates will also become possible. No other technique offers such a powerful array of capabilities for the study of unimolecular dissociation mechanisms and rates. 

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