Chemically induced dynamic nuclear polarisation

Radical pair theory states that the rate of 5 - To mixing is directly related to the nuclear spin configuration through the hyperflne interaction, which in turn determines the recombination yield. The nuclear spin polarisation generated in both the recombined and escaped products is known as chemically induced dynamic nuclear polarisation [6, 31-33]. 

Net effect, one radical is observed in emission (E) and the other radical in enhanced absorption (A) (requiring Ag 0 and AgBo being much greater than the hyperfine interaction). 

To illustrate how nuclear spin polarisation arises, consider a radical pair initially triplet correlated with one only magnetic nucleus (with hyperfine constant a ). The rates of 5 - To mixing for the two nuclear spin orientations is then given by the expression (assuming Ag 0 and ai 0) 

This difference in the Lamor frequency of the electron spin gives the recombination product an excess of a nuclear spin which will be observed as emission in the CIDNP spectrum. The converse is true for escaped products, which will have an excess of p nuclear spins and wiU be observed as enhanced absorption in the CIDNP spectram. Obviously, for a singlet correlated radical pair the opposite is true with an excess of p and a nuclear spins observed on the recombination and escaped products respectively. 

Theoretical treatment Assuming that the wavefunction of the radical pair can be described as (f) = C5 (f) 5, xn + CTnif) To, x ) (where x is the nuclear spin wavefunction with magnetic quantum number nti = 1/2), then using the time dependent Schrodinger equation (with boundary conditions that Csn(0) = 1 and 

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In aniline derivatives (458) the mechanism of this reaction is still not fully settled (459-461). However, the latest results seem to favor a pathway that, applied to 2-nitraminothiazole, would give Scheme 138, where the key step is the formation of a radical ion (223). Reexamination of the original reports on this reaction (16, 374, 378. 462) with EPR and Chemically Induced Dynamic Nuclear Polarisation techniques could be fruitful. 

Since the heroic early mechanistic investigations, there have been two developments of major significance in radical chemistry. The first was the advent of electron spin resonance (ESR) spectroscopy (and the associated technique of chemically induced dynamic nuclear polarisation, CIDNP) [24], which provided structural as well as kinetic information the second is the more recent development of a wide range of synthetically useful radical reactions [20]. Another recent development, the combination of the pulse radiolysis and laser-flash photolysis techniques, is enormously powerful for the study of radicals but beyond the scope of this book. 

CIDNP chemically induced dynamic nuclear polarisation... 

The fundamental processes involved in the physical formation of a radiation track and in its subsequent evolution by diffusion and reaction are stochastic in nature. Every track is unique and even identical tracks may evolve differently. Thus most recent simulation methods [5-8] are stochastic in these senses (i.e. for the underlying track and for the diffusion and reaction of the reactive particles that can take place). Unfortunately, these methods ignore the spin-dynamics because of the complexity it introduces. As most radicals in radiation chemistry are paramagnetic species, there is a possibility of spin-controlled reactions and other spin effects such as quantum beats [9], chemically induced dynamic nuclear polarisation (CIDNP) [10-13] and chemically induced dynamic electron polarisation (CIDEP) [11, 12], which would... 

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