Spin, nuclear, and NMR

The Tl+ ion has been proposed as a probe for the behavior of K+ in biological systems. The two isotopes 203T1 and 205T1 (70.48%) have a nuclear spin, and nmr signals are readily detected both in solutions and in solids also the Tl1 (and Tlm) resonances are very sensitive to the environment and have large solvent-dependent shifts. For Tl+ it is possible to correlate shifts with solvating ability, hence the utility as a probe in biological systems. 

The two isotopes, 203T1 and 205T1 (70.48%), both have nuclear spin, and nmr signals are readily detected for thallium solutions or for solids. In solution both Tl1 and Tl111 resonances are markedly dependent on concentration and on the nature of anions present such data have shown that thallous perchlorate is highly dissociated, but salts of weaker acids and TIOH have been shown to form ion pairs in solution. 

Nitrogen has two stable isotopes N (relative atomic mass 14.003 07, abundance 99.634%) and (15.000 11, 0.366%) their relative abundance (272 1) is almost invariant in terrestrial sources and corresponds to an atomic weight of 14.00674(7). Both isotopes have a nuclear spin and can be used in nmr experiments. though... 

In some ways, it s surprising that carbon NMR is even possible. After all, 12Q the most abundant carbon isotope, has no nuclear spin and can t be seen b> NMR. Carbon-13 is the only naturally occurring carbon isotope with a nucleai spin, but its natural abundance is only 1.1%. Thus, only about 1 of ever) 100 carbons in an organic sample is observable by NMR. The problem of low abundance has been overcome, however, by the use of signal averaging anc Fourier-transfonn NMR (FT-NMR). Signal averaging increases instrument sensitivity, and FT-NMR increases instrument speed. 

We have seen earlier in this chapter that certain nuclei, other than carbon and hydrogen, also have nuclear spin and are NMR-active. The fact that there are other NMR-active nuclei leads to the question can we run spectra specifically to see these other nuclei The answer is yes. In Table... 

Table 1.1 shows that the nucleus of major interest in organic chemistry, 12C, does not have a nuclear spin and cannot be used as an NMR nucleus. In contrast, its isotope 13C, whose natural abundance is only 1.1%, has a nuclear spin of 7. 

Electron paramagnetic resonance (EPR) is also referred to as electron spin resonance (ESR). In many respects, it is similar to NMR and the corresponding principles, discussed in the previous section, apply. The critical difference is that an unpaired electron spin is detected in this method instead of a nuclear spin. The method applies only to paramagnetic systems. The electron spin is more readily detected than is a nuclear spin and magnets on EPR instruments are correspondingly smaller and less expensive. 

Edwards, Lusis, and Sienko have recently reported an ESR study (60) of frozen lithium-methylamine solutions which suggests the existence of a compound tetramethylaminelithium(O), Li(CH3NH2)4, bearing all the traits (60) of a highly expanded metal lying extremely close to the metal-nonmetal transition. Specifically, both the nuclear-spin and electron-spin relaxation characteristics of the compound, although nominally metallic, cannot be described in terms of the conventional theories of conduction ESR (6,15, 71) and NMR in pure metals (60, 96, 169). 

The Overhauser effect has been widely employed as an NMR analysis method in many disciplines ranging from medical to chemical sciences, and broadly refers to the motion-mediated transfer of spin polarization from a species with a higher gyromagnetic ratio (y) to one with a lower gyromagnetic ratio. Because molecular motion is critical for efficient transfer, the Overhauser effect is most commonly observed in liquid samples. The Overhauser effect can be divided into two categories the nuclear Overhauser effect (NOE), where both species are nuclear spins, and Overhauser DNP, where the higher y spin is an unpaired electron. As Overhauser DNP is the focus of this review, some of the terminology and equations are specific to the Overhauser DNP effect. 

Other nuclei, such as l3C, gF, 2H, and 3IP, also have nuclear spins and can be studied with NMR techniques. However, because y is different for these nuclei, they appear in a very different region of the spectrum from hydrogen and are not seen in a H-NMR spectrum. Both 12C and l60, which are very common in organic compounds, do not have nuclear spin and therefore have no NMR absorptions. 

The spin echo was discovered in 1950 by Erwin Hahn24 and is sometimes called a Hahn echo. The real significance of the spin-echo method lies not in its use to measure T2, but in the demonstration that an apparently irreversible dephasing of nuclear spins and decay of the FID (even to zero) can be reversed. As we see in Chapter 7, application of this concept is extremely important in obtaining narrow NMR lines in solids. We shall also encounter numerous examples in NMR of liquids where a spin echo is employed. 

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