Free radical intramolecular interactions

Distortion of the chair conformation by substitution occurs in 2,2,6,6-tetramethyl-piperidin-4-one hydrochloride (45) (71AX(B)932), in which steric interaction between axial methyl groups leads to flattening of the ring. The resultant C(2)—C(6) intramolecular distance is 3.2 A, to be contrasted with 2.4 A in the non-methylated compound. A similar effect is to be found in other 2,2,6,6-tetramethyl derivatives (81AX(B)1771). In the related free radical nitroxide (46) the steric interactions are reduced by adoption of a symmetric twist chair form (74AX(B)790). 4-Chloro-5-methylamino-2,3,6-pyridinetrione monohydrate... 

Nonconjugated dienes and polyenes have triplet photochemistry which may be considered to arise from intramolecular interaction of one excited double bond with an isolated ground-state double bond. For example, the photocyclization of enrfo-dicyclopentadiene can be effected using acetone as a sensitizer.286 Other more flexible 1,5-dienes, when sensitized to triplet states, cross couple to yield bicyclo[2.1.1]-hexane structures. For instance, triplet mercury atoms convert both 1,5-hexadiene and 1,5-cyclooctadiene to such structures.267 Irradiation of the cyclooctadiene in the presence of cuprous chloride produces the tricyclo derivative in good yield266 but recent evidence again indicates that this latter reaction may proceed via free-radical intermediates.269... 

It is quite easy for open-chain systems to undergo intramolecular cyclization because of their many rotational degrees of freedom. More rigid systems undergo efficient cyclization only if the free-radical center and the n system are held in close proximity, as in the first example below. Where the molecular geometry is fixed in such a way as to prevent effective interaction between the free-radical center and the n system, cyclization is inefficient and reduction predominates. Cyclization in the second example is an obvious impossibility ... 

This type of coiled conformation, in which a certain number of monomer units are present in a certain number of turns, would cause the terminal carbon atom of a free radical, formed by the scission of a carbon-carbon bond in the polymer chain backbone, to be in close proximity to, and to interact with, a specific carbon atom, or a hydrogen atom linked to a specific carbon atom, in the turn. Thus, in an isotactic polyethylene molecule, which contains three monomer units per turn and is represented by the structure in Fig. 2, the scission of the Qg - Cm bond would bring C 6 in close proximity to or C 2 or the hydrogen atoms linked to any one of them. If Qe containing the unpaired electron attacks the C i, a six-membered ring may be formed feq. (15)]. It has been observed that in an intramolecular cyclization, the fastest reactions are those which proceed via six-membered rings However, cyclohexane (I) may lose one or more hydrogen atoms to a free radical and form hexene, hexadiene, hexane, and other compounds as shown below. 

As isotactic polystyrene has a similar crystal structure to that of polyethylene, the concept of the intramolecular cydization may be applied to the degradation of the former to account for the formation of the products of degradation. Let the three-unit one turn radical form the cyclic compound (VIII) by the interaction of the Cn and CK atoms. The cyclic compound (VIII) may lose hydrogens to other free radicals to form 1,3,5-triphenyl-benzene (IX). 

The magnetic properties of free radicals are represented by the parameters describing their interaction with external magnetic fields and the parameters of intramolecular hyperfine interactions. For many free radicals, in particular 5-atoms and radicals in liquid or solid environments where the orbital and rotational components of angular momentum are quenched, the parameters are conveniently given by the elements of the tensors g and of the spin-Hamiltonian... 

The following section addresses the impact of ionic dissociation of MAA on the propagation kinetics in aqueous solution at different monomer concentrations. The primary intention of these studies is to find out whether the preceding kinetic analysis, which assumes intramolecular rotn-tional mobility of the TS structure and thus the pre-exponential factor being affected by strong intermolecular interactions, is also suitable for interpreting free-radical propagation of ionized MAA in aqueous solution. 

Intramolecular Interactions of Acylperoxy Initiators and Free Radicals... 

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