Chemically induced magnetic spin effect

A.R. Lepley, G.L. C loss. Eds.. Chemically Induced Magnetic Polarization, Wiley, New York, 1973 K.M. Salikhov, Yu.N. Molin. R.Z. Sagdccv, A.L. Buchachcnko, Spin Polarization and Magnetic Effects in Radical Reactions. Elsevier, Amsierdam, 1984. 

The ion pair spin multiplicity may be a valuable tool to affect the BET rates and to probe the ion pair dynamics via magnetic field effects [36], Even weak magnetic fields are known to influence relative probabilities of singlet and triplet reactions [34], Chemically induced dynamic nuclear polarization (CIDNP) is a particularly informative technique [12]. Many bond scission reactions and rearrangements in cyclic radical ions have been successfully explored using this approach. Both structural data (spin densities) and approximate kinetic informations are indirectly available from such experiments [12]. 

Chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance method based on the observation of transient signals, typically substantially enhanced, in either absorption of emission. These effects are induced as a result of magnetic interactions in radical or radical ion pairs on the nanosecond time scale. This method requires acquisition of an NMR spectrum during (or within a few seconds of) the generation of the radical ion pairs. The CIDNP technique is applied in solution, typically at room temperature, and lends itself to modest time resolution. The first CIDNP effects were reported in 1967, and their potential as a mechanistic tool for radical pair reactions was soon recognized [117, 118]. Nuclear spin polarization effects were discovered in reactions of neutral radicals and experiments in the author s laboratory established that similar eflects could also be induced in radical ions [119-121]. 

When a free radical is produced in a chemical reaction, the orientation of the electron spins with respect to the external magnetic field is random, so that in effect the electron spin states of the newly formed radicals are saturated. Thus if the newly formed radical is sufficiently short-lived, so that any nuclear polarization induced in the radical has not had time to decay before it becomes a diamagnetic molecule, when the n.m.r. signal is observable, an enhanced nuclear signal may be produced. Furthermore the relaxation of the electrons in the newly formed radicals may also induce nuclear spin polarization in other molecules in the solution. 

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). 

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