Polarization of nuclear spins

Clearly, if a situation were achieved such that exceeded Np, the excess energy could be absorbed by the rf field and this would appear as an emission signal in the n.m.r. spectrum. On the other hand, if Np could be made to exceed by more than the Boltzmann factor, then enhanced absorption would be observed. N.m.r. spectra showing such effects are referred to as polarized spectra because they arise from polarization of nuclear spins. The effects are transient because, once the perturbing influence which gives rise to the non-Boltzmann distribution (and which can be either physical or chemical) ceases, the thermal equilibrium distribution of nuclear spin states is re-established within a few seconds. 

Radicals escaping from a radical pair become uncorrelated as approaches zero. In the free (doublet) state they are detectable by e.s.r. spectroscopy. However, just as polarization of nuclear spins can occur in the radical pair, so polarization of electron spins can be produced. Provided that electron spin-lattice relaxation and free radical scavenging processes do not make the lifetime of the polarized radicals too short. 

Figure 12.2. OvQview of the principles of NMR spectroscopy. Polarization of nuclear spins by a magnetic field is perturbed by plication of a radiofiequency (rt) pulse. The resultant signal is Fourier transformed, to yield a spectrum reflecting the numbCT and environments of nuclei in the sample.

The dye-sensitized photo-oxidation of phenols has been examined using CIDNP techniques.148 It is concluded from the observed polarizations of nuclear spin that sensitization by xanthene dyes (such as Rose Bengal) is the result of reversible hydrogen abstraction by the triplet dye molecule from the phenolic hydroxyl group. Any resulting photochemical reactions (e.g. with oxygen) arise from irreversible reactions of the phenoxy-radicals produced, rather than from direct reactions of the phenols with 102. Dye-sensitized photo-oxidation of p-hydroxyphenylpyruvic acid (94) in solution at pH 7 (conditions under which... 

Despite considerable progress in the field, NMR spectroscopy still has two significant limitations the intrinsically low sensitivity, due to the low Boltzmann polarization of nuclear spins in thermal equilibrium, and the low dispersion of observed frequencies, due to small differences in nuclear shielding by surrounding electrons for nuclei of the same kind. The first problem is continuously... 

High polarization of nuclear spins through dynamic nuclear polarization in liquid state has enabled the direct monitoring of metabolites in vivo. The design and testing of the compressed sensing suited for a flyback 3D-MRSI sequence are presented. Phantom tests validated the accuracy of the compressed sensing approach and initial mouse experiments demonstrated in vivo feasibility. 

Another technique for the study of reactions that is highly specific for radical processes is known as CIDNP, an abbreviation for chemically induced dynamic nuclear polarization." The instrumentation required for such studies is a normal NMR spectrometer. CIDNP is observed as a strong perturbation of the intensity of NMR signals in products formed in certain types of free radical reactions. CIDNP is observed when the normal population of nuclear spin states dictated by the Boltzmann distribution is disturbed by the presence of an unpaired electron. The intense magnetic moment associated with an electron causes a polarization of nuclear spin states, which is manifested by enhanced absorption or emission, or both, in the NMR spectrum of the diamagnetic product of a free radical reaction. The technique is less general than EPR spectroscopy because not all free radicals can be expected to exhibit the phenomenon. 

Carver TR, Slichter CP (1953) Polarization of nuclear spins in metals. Phys Rev 92 212-213... 

The observation of P HIP effects in the heterogeneous hydrogenation reaction over the Pd /SILP/ACF catalyst proves that the catalyst is able to perform pairwise hydrogen addition. For normal hydrogen, it is possible that the main reaction route involves the addition of random H atoms to the substrate, and thus yields no polarization of nuclear spins of the product molecule. 

By examining the expression for Q ( equation (B1.16.4)). it should now be clear that the nuclear spin state influences the difference in precessional frequencies and, ultimately, the likelihood of intersystem crossing, tlnough the hyperfme tenn. It is this influence of nuclear spin states on electronic intersystem crossing which will eventually lead to non-equilibrium distributions of nuclear spin states, i.e. spin polarization, in the products of radical reactions, as we shall see below. 

Methods of disturbing the Boltzmann distribution of nuclear spin states were known long before the phenomenon of CIDNP was recognized. All of these involve multiple resonance techniques (e.g. INDOR, the Nuclear Overhauser Effect) and all depend on spin-lattice relaxation processes for the development of polarization. The effect is referred to as dynamic nuclear polarization (DNP) (for a review, see Hausser and Stehlik, 1968). The observed changes in the intensity of lines in the n.m.r. spectrum are small, however, reflecting the small changes induced in the Boltzmann distribution. 

The origin of postulate (iii) lies in the electron-nuclear hyperfine interaction. If the energy separation between the T and S states of the radical pair is of the same order of magnitude as then the hyperfine interaction can represent a driving force for T-S mixing and this depends on the nuclear spin state. Only a relatively small preference for one spin-state compared with the other is necessary in the T-S mixing process in order to overcome the Boltzmann polarization (1 in 10 ). The effect is to make n.m.r. spectroscopy a much more sensitive technique in systems displaying CIDNP than in systems where only Boltzmann distributions of nuclear spin states obtain. More detailed consideration of postulate (iii) is deferred until Section II,D. 

There is a completely analogous development for a system of nuclear spins interacting with a time-dependent magnetic field polarized along the x axis. 

The product of escape from the cage (3) is formed with an excess population of nuclear spin state a. Figure A1.6 shows the upper state with enhanced population. With this inverted polarization, the spin system will emit energy and a negative peak will be observed. There is a net effect in the direction of emission, E. 

Let us consider the hyperfine depolarization effect caused by the presence of nuclear spin that is not oriented or aligned by light but acts as a randomly oriented flywheel to reduce the average polarization of fluorescence. The emission process can be effected by the presence of nuclear spin J, even if the hyperfine structure is unresolved by the detection apparatus. For this purpose one has to multiply each polarization moment fq in Eq. (5.34) by a coefficient g(K which is equal to [177, 402]... 

In Section 11.8 we saw how polarization (departure from the equilibrium distribution) of nuclear spin state populations by a nearby unpaired electron can have a profound effect on NMR signal intensity. It turns out that in a l3C 1H experi-... 

The NOE is an example of polarization transfer (or cross polarization), because polarization of one set of nuclear spin states (here, saturation of the H nuclei) results in the polarization of another set (here, the 13C nuclear spin states). The maximum Overhauser signal enhancement (q) is given by... 

A detailed consideration of all these steps in the Overhauser triplet mechanism operating in the photoreduction of qulnone systems has been given by Adrian et al. (10). With some reasonable approximations (a pseudo-first-order reaction of the polarized radicals and steady-state conditions), it is possible to show that the rate of production of nuclear spin polarized products is... 

Illustration of nuclear spin-spin interaction transmitted via polarization of bonding electrons. The two electrons about each carbon will tend to be parallel, since this arrangement minimizes the electron-electron repulsion (Hund s rule for electrons in degenerate orbitals). 

In CIDNP studies the pattern of nuclear spin polarizations is used to deduce the structures of free radical intermediates, and if there is a simultaneous NOE this can affect the conclusions drawn. (344) In the photolysis of [10] to yield [12] the biradical intermediate [11] should not cause significant polarization of the olefinic protons of [10] but in fact these are observed to have weak emission. This raises the possibility that the intermediate is really the biradical [13], but a homonuclear double resonance experiment which destroyed the cyclopropyl spin polarization of [10] eliminated the olefinic emission which was evidently solely due to the NOE. Thus it is confirmed that [11] is indeed the intermediate in the reaction. (344)... 

Much of the technical development of NMR over the past half century has focused on improving sensitivity. The fundamental problem is the low starting Boltzmann polarization that arises from the low energies of nuclear spin transitions. Several methods have been developed to improve the sensitivity or S/N in NMR. One major approach is through pulse sequence development to optimize the efficiency and information content of NMR spectra through manipulating the spin physics some of the more important experiments for small molecules were described above. 

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