Models, for radicals

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

The transition-state model for these cyclizations (Scheme 34) differs fundamentally from the well-established Beckwith-Houk transition model for radical cyclizations [130,146-148]. Thus, while both models invoke chairlike transition states, without excluding the possibility of twist boatlike systems in some instances, the Beckwith-Houk model involves full conformational... 

In order to estimate kinetic constants for elementary processes in template polymerization two general approaches can be applied. The first is based on the generalized kinetic model for radical-initiated template polymerizations published by Tan and Alberda van Ekenstein. The second is based on the direct measurement of the polymerization rate in a non-stationary state by rotating sector procedure or by post-effect in photopolymerization. The first approach involves partial absorption of the monomer on the template. Polymerization proceeds according to zip mechanism (with propagation rate constant kp i) in the sequences filled with the monomer, and according to pick up mechanism (with rate constant kp n) at the sites in which monomer is outside the template and can be connected by the macroradical placed onto template. This mechanism can be illustrated by the following scheme ... 

We begin by describing the current understanding of the kinetics of polymerization of classical unsaturated monomers and macromonomers in the disperse systems. In particular, we note the importance of diffusion-controlled reactions of such monomers at high conversions, the nucleation mechanism of particle formation, and the kinetics and kinetic models for radical polymerization in disperse systems. 

Om A, Kim JH. 2008. A quantitative structure-activity relationship model for radical scavenging activity of flavonoids. J Med Food 11 29-37. 

Transient intermediates arising by radical additions to the amide bond have been studied recently in an effort to explain the complex processes occurring in electron-capture dissociation of multiply charged peptide and protein cations [192]. The amino(hydroxy)methyl radical, HC(OH)NH2 (64), is a prototypical species that represents the simplest model for radical additions to peptide bonds. Radical 64 was generated from cation 64+ which in turn was prepared by... 

Constructing increasingly sophisticated structural models for radicals in proteins remains a major challenge. To develop truly predictive capabilities, computational chemists must work to incorporate more realistic models of intermolecular contacts and to model the effects of molecular dynamics on... 

A "diatomic model for radical-radical recombination seems to be a good approximation as well. Therefore, lor such reactions the maximum of the effective potential energy (8.IV), including a centrifugal potential, allows us to define a transition state (or "activated complex). This provides the possibility for an application of either the colli-sional or statistical formulations of the theory of chemical reaction rates these formulations will be compared in the following sections. 

One of the arguments for the stabilization of radicals by hyperconjugation is that the C-H homolytic bond dissociation energies for alkanes vary as shown in Table 5.6. However, the C-F homolytic dissociation energies do not follow the same trend. Are these results consistent with the hyperconjugation model for radical stability ... 

Tanko JM, Friedline R, Suleman NK, Castagnoli N (2001) tert-Butoxyl as a model for radicals in biological systems caveat emptor. J Am Chem Soc 123 5808-5809... 

For type II mechanism, Rrei increases with [T] to the critical concentration of the template, c (the concentration at which template macromolecules start to overlap) and then remains stable. It is worth noting that c (concentration in moles per volume) depends on the molecular weight of the template. The most complete kinetic model for radical-initiated template polymerization has been developed and described by Tan and Alberda van Ekenstein (12). 

Greenwald EE, North SW, Georgievskii Y, Klippenstein SJ. (2005) A two transition state model for radical-molecnle reactions A case study of the addition of OH to C2H4. J. Phys. Chem. A. 109 6031-6044. 

A two transition state model for radical molecule reactions was postulated long ago by Singleton and Cvetanovic in their analysis of the reaction of 0( P) atoms with olefins.Unfortunately, their analysis focused on the canonical implementation and numerous subsequent ab initio implementations of the two transition state model have followed their lead. The canonical implementation can yield as much as a factor of 6 overestimate of the rate coefficient for temperatures near the crossover temperature (the temperature at which the inner and outer rate coefficients are equivalent), as illustrated in Fig. 4.7 for the OH-I-C2H4 reaction. Notably, the E/J resolved two transition state model predictions deviate significantly from the single transition state results over the full temperature range from 20 to 300 K. 

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