Chemically induced dynamic nuclear polarization electron spin resonance

Chemically induced dynamic nuclear polarization is a spectroscopic technique that takes advantage of the coupling between electron and nuclear spins to detect products of radical recombinations by nuclear resonance. It is suited to investigation of the dynamics of radical processes, particularly the events just preceding radical recombinations. First observed in 1967 by Bargon and Fischer32 and independently by Ward and Lawler,33 the phenomenon consists of... 

A more profound knowledge of the photolytic processes of a drug substance under UV-VIS photon exposure can be obtained utilizing this method of experimental photochemistry. The irradiation of many drug substances having photosensitizing properties results in the formation of free radicals as part of their primary photochemical processes (17). The formation of these radicals can be detected and monitored by means of electron spin resonance (ESR) or nuclear resonance spectrometry, especially utilizing the chemically induced dynamic nuclear polarization technique (8). 

The presence of speeies able to initiate polymerization as observed by other spectroscopic tools such as electron spin resonance spectroscopy (ESR), emission spectroscopy, transient absorption spectroscopy or chemically induced dynamic nuclear polarization (CIDNP). 

In the case of transient species with unpaired electrons such as free radicals, and the triplet states of carbenes or nitrenes, electron spin resonance (ESR) spectroscopy can provide unique evidence about the structure of the intermediate. Useful information about intermediates in reactions involving radical pair coupling can also be obtained by a technique known as chemically-induced dynamic nuclear polarization (CIDNP). However, detailed discussions of ESR and CIDNP are outside the scope of this book and for further information suitable text books on physical organic chemistry or the references given in the Further Reading section should be consulted. 

There are a variety of techniques for the determination of the various parameters of the spin-Hamiltonian. Often applied are Electron Paramagnetic or Spin Resonance (EPR, ESR), Electron Nuclear Double Resonance (ENDOR), Electron Electron Double Resonance (ELDOR), Nuclear Magnetic Resonance (NMR), occassionally utilizing effects of Chemically Induced Dynamic Nuclear Polarization (CIDNP), Optical Detection of Magnetic Resonance (ODMR), Atomic Beam Spectroscopy and Optical Spectroscopy. The extraction of the magnetic parameters from the spectra obtained by application of these and related techniques follows procedures which may in detail depend on the technique, the state of the sample (gaseous, liquid, unordered solid, ordered solid) and on spectral resolution. For particulars, the reader is referred to the general references (D). 

Current abbreviations will be used BP for benzoyl peroxide, DTBP for di-/-butyl peroxide, AIBN for azo-bis(isobutyronitrile). In for initiation, esr for electron spin resonance, and cidnp for chemically induced dynamic nuclear polarization. Furthermore, we have chosen to adopt homogeneous abbreviations for the radicals discussed. The precursor radicals will be called A, generated from AH, AX,..., and the two possible radicals resulting from intramolecular addition will be called Cy (for cyclized) followed by a number for the size of the ring. 

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