Radical paramagnetic

Ph3C, R3C (free radicals) Paramagnetic species of varying stability... 

The superscript" is used in Scheme 12.2 and throughout this chapter to represent spin polarization, a term applied to situations for which a paramagnehc species possesses a population of spin states that is different from the Boltzmann distribution at the temperature of the experiment. Polarization disappears during the radical paramagnetic relaxation time, usually in the microsecond timescale. Here and below, we will... 

The term spin (unpaired electron) is commonly used to refer to any species, e.g., free radical, paramagnetic ion, which gives an ESR spectrum. 

The distinguishing characteristic of a free radical is the presence of an unpaired electron. Species with an unpaired electron are said to be paramagnetic. The relative stability of the triphenylmethyl radical allows it to be studied by magnetic susceptibility measurement, which involves weighing it both inside and outside a magnetic field. The unpaired electron makes the radical paramagnetic, so the sample is drawn into the magnetic field. By this technique the dissociation of hexaphenylethane to the triphenylmethyl radical was determined to occur to the extent of 2% in a 0.1 M sample. 

In systems with unpaired electrons (e.g., metals, radicals, paramagnetic transition metal complexes), a much larger range of chemical shifts is possible. Now the major magnetic interaction is between the nucleus of interest and the unpaired electron(s). The observed shift depends on the excess electron spin population and the coupling constant to the nucleus. The induced contact shifts or Knight shifts (in metals and conductors) often exceed 1000 ppm. 

For a sample to be EPR/ESR active, it must have one or more unpaired electrons. Stable free radicals, paramagnetic metal ions, and irradiated materials are some examples of such materials. The amount of sample required depends on the type of spectrometer (what band is used) and on the type of experiment—CW, pulsed, and double-resonance experiments like electron nuclear double resonance (ENDOR) (discussed in the following)—but in general, liquid and solid samples can be measured. Volumes of sample required range from 20 pL to 1 mL at concentrations of 10 nM to 20 mM for most experiments. 

In certain solid-phase materials, free radicals - paramagnetic species containing unpaired electrons - form upon irradiation. The concentration of these free radicals can be related to absorbed dose by electron paramagnetic resonance (EPR) analysis, sometimes referred to as... 

Smaller B, Remko J R and Avery E C 1968 Electron paramagnetic resonance studies of transient free radicals produced by pulse radiolysis J. Chem. Rhys. 48 5174-81... 

Closs G L, Forbes M D E and Norris J R 1987 Spin-polarized electron paramagnetic resonance spectra of radical pairs in micelles. Observation of electron spin-spin interactions J. Phys. Chem. 91 3592-9... 

It can be seen from Table 1 that there are no individual steps that are exothermic enough to break carbon—carbon bonds except the termination of step 3a of —407.9 kJ/mol (—97.5 kcal/mol). Consequentiy, procedures or conditions that reduce the atomic fluorine concentration or decrease the mobiUty of hydrocarbon radical intermediates, and/or keep them in the soHd state during reaction, are desirable. It is necessary to reduce the reaction rate to the extent that these hydrocarbon radical intermediates have longer lifetimes permitting the advantages of fluorination in individual steps to be achieved experimentally. It has been demonstrated by electron paramagnetic resonance (epr) methods (26) that, with high fluorine dilution, various radicals do indeed have appreciable lifetimes. 

Magnetic susceptibiUty measurements indicate that the organic part of the coal is diamagnetic, having traces of paramagnetic behavior resulting from free radicals or unpaired electrons (6). 

The reaction of bis(benzene)vanadium [12129-72-5] with TCNE affords an insoluble amorphous black soHd that exhibits field-dependent magnetization and hysteresis at room temperature, an organic-based magnet (12). The anion radical is quite stable in the soHd state. It is paramagnetic, and its intense electron paramagnetic resonance (epr) spectmm has nine principal lines with the intensity ratios expected for four equivalent N nuclei (13) and may be used as an internal reference in epr work (see Magnetic spin resonance). 

EPR studies of S-N radicals were reviewed in 1990. Many radicals containing the S-N linkage are persistent for more than several hours in solution at room temperature. Perhaps the best known example is the nitrosodisulfonate dianion [0N(S03)] , named as Fremy s salt. In the solid state this radical dianion dimerizes through weak N 0 interactions, but it forms a paramagnetic blue-violet monomer in solution. Although most chalcogen-nitrogen radicals dimerize in the solid state, a few heterocyclic C-S-N systems can be isolated as monomers (Section 11.3). 

The neutral radical 11.12 is an interesting heterocyclic analogue of 2,2 -bipyridyl. This paramagnetic (spin-bearing) ligand forms an N, -chelated complex with bis(hexafluoroacetylacetonato)cobalt(ir). ... 

The reactivity of NO towards atoms, free radicals, and other paramagnetic species has been much studied, and the chemiluminescent reactions with atomic N and O are important in assaying atomic N (p. 414). NO reacts rapidly with molecular O2 to give brown NO2, and this gas is the normal product of reactions which produce NO if these are carried out in air. The oxidation is unusual in following third-order reaction kinetics and, indeed, is the classic... 

The product is a black-brown solid that is very sensitive to oxygen. The same cation can be obtained by oxidation of S4N4 with AsFs and is unusual in being the only sulfur-nitrogen (paramagnetic) radical that has been obtained as a stable crystalline salt. X-ray diffraction analysis shows the structure to be a planar 5-membered ring with approximate... 

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