Polar molecules, reactions with ions free radical

The phenomenon of chemically induced dynamic nuclear polarization (CIDNP) consists of the manifestation of unusual line intensities and/or phases of signals of radical reaction products in the NMR spectrum when reaction takes place directly in the probe of the spectrometer. These anomalous lines (enhanced absorption or emission of NMR signals), which reflect the populations of nuclear spin states deviating from the Boltzmann condition, are observed within the time range of nuclear relaxation times of the diamagnetic molecules (T, ), which are as a rule, several seconds to tens of seconds. Subsequently, the NMR spectrum re-acquires its usual form. In 1967, two research groups in Europe (J. Bargon, H. Fischer, and U. Johnson) and the USA (H. Ward and R. Lawler) discovered independently that this phenomenon is directly associated with the free radicals involved in the process. Later on, it was shown that this also pertains to radical ions and triplet excited states of molecules. 

Although sulfur vulcanization has been studied since its discovery in 1839 by Goodyear, its mechanism is not well understood. Free-radical mechanisms were originally assumed but most evidence points to an ionic reaction [Bateman, 1963]. Neither radical initiators nor inhibitors affect sulfur vulcanization and radicals have not been detected by ESR spectroscopy. On the other hand, sulfur vulcanization is accelerated by organic acids and bases as well as by solvents of high dielectric constant. The ionic process can be depicted as a chain reaction involving the initial formation of a sulfonium ion (XI) by reaction of the polymer with polarized sulfur or a sulfur ion pair. The sulfonium ion reacts with a polymer molecule by hydride... 

In polar solvents, such as acetonitrile, organic donor-acceptor systems such as those listed in Table 6.2 show only the fluorescence due to A no new fluorescence appears as in exciplex formation. Flash spectroscopy shows absorption spectra characteristic of the hydrocarbon radical anion and the amine radical cation. The product in these solvents is either an ion-pair or two free ions, stabilised no doubt by solvation, and the reaction is a complete transfer of an electron from one molecule to another, rather than exciplex formation. The reaction goes effectively to completion, and so (with only one fluorescence lifetime to be considered) the kinetic equations for the intensity and lifetime reduce to the simple Stem-Volmer forms (Equations (6.16) and (6.19)). The rate constants for the reactions of aromatic hydrocarbons with various amines in acetonitrile are found to be correlated with the free-... 

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