Dynamic nuclear-spin polarisation

For an understanding of ESR in crystals, a detailed discussion of the molecular fundamentals is necessary. We deal with this primarily in Sections 7.2 and 7.3. There, the spin quantisation in triplet states, magnetic dipole-dipole couphng, zero-field splitting, Zeeman spHtting and fine structure are explained. These fundamentals apply both to isolated molecules and to excitons (Sects. 7.4 and 7.5). In the two later Sects. 7.6 and 7.7 of this chapter, the so called optical spin polarisation in excited triplet states and dynamic nuclear spin polarisation will be treated. 

A well-known and important phenomenon in the area of nuclear-spin resonance (NMR) in gases, liquids, or solid samples is dynamic nuclear-spin polarisation (DNP) (see e.g. [M6]). This term refers to deviations of the nuclear magnetisation from its thermal-equilibrium value, thus a deviation from the Boltzmann distribution of the populations of the nuclear Zeeman terms, which is produced by optical pumping (Kastler [31]), by the Overhauser effect [32], or by the effet solide or solid-state effect [33]. In all these cases, the primary effect is a disturbance of the Boltzmann distribution in the electronic-spin system. In the Overhauser effect and the effet solide, this disturbance is caused for example by saturation of an ESR transition. Owing to the hyperfine coupling, a nuclear polarisation then results from coupled nuclear-electronic spin relaxation processes, whereby the polarisation of the electronic spins is transferred to the nuclear spins. 

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]. 

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. 

Dynamic nuclear polarisation (DNP) To increase sensitivity Magnetisation transferred from unpaired electrons to 13C spins via 1H spins if required. Sensitivity improvement of about 100-fold... 

Among the nuclear spins, those of the protons are most easily polarised. Nevertheless, the proton spin polarisation reaches only P(H) = 0.25 % in a field of 2.5 Tesla at T = 1 K in about one hour. The time to reach more favourable equilibria at lower temperatures increases dramatically to days and weeks. Proton spins are most easily aligned by dynamic nuclear polarisation (DNP), which is achieved by irradiating the sample with 4 mm microwaves at helium bath temperatures at 0.3 K in a magnetic field of 2.5 T in the presence of an organic radical (e. g. 

An alternative method, termed DNP (Dynamic Nuclear Polarisation), relies on the transfer of polarisation from unpaired electrons onto nuclear spins in the solid state but has been further developed to significantly enhance the sensitivity of solution-state NMR, as described briefly below. The technique has been applied to boost the polarisation levels for low-7 heteronuclear spins in particular and shows most promise to date for and N observation the initial report on the technique described a gain in signal-to-noise in excess of 10,000-fold for both these elements when compared to data collected at thermal equilibrium [115]. The hyperpolarisation of P and Si has also been demonstrated. 

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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