Reactivity in radical abstraction reactions: Application of the curve crossing model

Abstract

The curve crossing model was applied to a series of hydrogen abstraction reactions from a family of alkanes, RH (R = methyl, ethyl, isopropyl, tert‐butyl) by alkyl, hydrogen and chlorine radicals. The analysis was based on quantitative data obtained from an ab initio MO study. Schematic reaction profiles for the reaction of RH with alkyl and hydrogen radicals are built up from just two configurations: reactant, DA, and product D3* A. For the Cl atom reaction, however, a significant contribution of D+ A−, a charge‐transfer configuration, is also shown to be present. A simple explanation for differences in the intrinsic barrier for the identity radical abstraction reaction based on the initial gap size between DA and D3* A configurations is provided. The influence of the D+ A− configuration on the nature of the transition state of the Cl atom reaction and its intrinsic barrier is described. It is the D+ A− configuration that is responsible for the polar character often observed in radical abstraction and addition reactions.

α-Hydrogen Abstraction by •OH and •SH Radicals from Amino Acids and Their Peptide Derivatives

“…This, coupled with electrophilic attacking radicals (e.g., •Cl), results in a destabilization of the α-transition structure. 7 Although α--abstraction reactions are, arguably, of less biological relevance than side--chain reactions because they occur less frequently, they are of interest from the perspective of quantum chemistry computations. The α--radicals (and the transition structures that lead to their formation), with their direct interactions with both the amino and carboxyl substituents, represent a system that may potentially be challenging for theoretical methods.…”

Accurate ab initio prediction of propagation rate coefficients in free-radical polymerization: Acrylonitrile and vinyl chloride

“…For instance, high-level ab initio molecular orbital calculations of small prototypical systems have provided an insight into the mechanism of radical reactions such as addition to C@C bonds [15,16] and C@S bonds [17,18], hydrogen atom abstraction reactions [19][20][21], and bond dissociation energies and associated radical stabilities [22,23]. The mechanistic insights from these studies are applicable, at least in a qualitative manner, to the polymeric versions of these reactions (e.g., propagation, addition fragmentation in the RAFT process, chain transfer), and indeed more directed studies of model polymeric systems have been used to identify reaction mechanisms and model and interpret structure-reactivity trends in conventional and controlled radical polymerization processes [6,.…”

Radical‐type reactivity of the methylenedimethylsulfonium ion, (CH3)2S+CH2˙

“…the abstraction of a hydrogen atom from methane by the methyl radical has an energy barrier of 63 kJ mol-'. 39 Hence, the observation of slow reaction rates for the distonic ion, as compared to cationic reactions (e.g. hydrogen atom abstraction by ionized ethyl methyl sulfide), are in agreement with a radical mechanism.…”