Donor/acceptor complexes, halogen

Unified Mulliken Correlations of Donor/Acceptor Complexes with Halogen Derivatives... 

Henrici-Olive and Olive were the first to put forward the hypothesis that complexes are sometimes formed between the active centre and the monomer and or/solvent [45], As only the complex with monomer is capable of propagation, part of the centres is inhibited and the polymerization rate is reduced. This theory was found to be valid with styrene [46], but not with MMA [47]. Burnett called attention to the important circumstance that radicals solvated in various ways may react differently, or at least at different rates [47]. His conclusions were based on kinetic studies of MMA polymerization in various halogenated aromatics. In the copolymerization of butyl vinyl ether with methacrylates, complex formation between the active centre and condensed aromatics prior to monomer addition was observed by Shaik-hudinov et al. [48], The growing polymer forms a stable donor-acceptor complex with naphthalene, described by the formula. 

The first step of the reaction is a single-electron oxidation of the aromatic substrate to form a cation-radical [193,194]. Oxidizing agents for this reaction include Bronsted acids, oxidative Lewis acids, halogens (i.e., Br2), metal salts (e.g., Ti(CF3C02)3), electron-donor-acceptor complexes, irra-... 

Pi-complexes, also called donor-acceptor complexes, are often a weak association of an electron-rich molecule with an electron-poor species. The donor is commonly the electron cloud of a pi bond or aromatic ring the acceptor can be a metal ion, a halogen, or another organic compound. In the absence of solvent, as can occur in an enzyme cavity, the cation-pi interaction can be stronger than hydrogen bonding. The cation snuggles into the face of the aromatic pi cloud (see aromaticity. Sections 1.9.3 and 12.3). 

This again demonstrates an enhanced intersystem crossing from singlet to triplet levels in donor-acceptor complexes of halogenated materials due to spin-orbit coupling. 

Iminophosphines can form donor-acceptor complexes with JV-heterocyclic car-benes as illustrated by Burford et al. [253,254]. Iminophosphines of the form X-P=N=R (R = 2,6-di-terf-butyl-4-isopropylphenyl or 2,6-diisopropyl, X = halogen or triflate) have all been complexed with ImMe2fPr2 through the phosphorus center (Figure 15.44). A similar complex has also been derived fi om a phosphe-tidine precursor via dissociation of the dimer [253]. 

The theory of atoms in molecules (AIM) of Bader [59, 60] offers criteria enabling the existence and the nature of bonds to be determined from the topological analysis of the electron density. This theory has not only provided new insights into the understanding of intramolecular bonds but has also been successful in the field of intermolecular bonds, such as hydrogen bonds [61], halogen bonds [62], van der Waals bonds [63] and more strongly bound donor/acceptor complexes [64]. 

Haima, M.W (1968) Bonding in donor-acceptor complexes. I. Electrostatic contributions to the ground-state properties of benzene-halogen complexes. J. Am. Chem. Soc., 90, 285-291. 

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