Nuclear spin flip lines

Nuclear spin flip (Am/ = 1) lines occur in systems with anisotropic hyperflne couplings of smaller magnitude than the nuclear Zeeman energy. In this case satellite lines appear on each side of an often unresolved doublet of central lines. At X-band the satellites are separated approximately by Bn = 0.5 mT from the... 

Anisotropic A, with principal values b Apart from the spin flip lines due to weak dipolar coupling with distant H atoms of the matrix this case is rare at X-band and lower frequencies, but may become of interest for measurements at high microwave frequency bands. The situation is most likely to occur for species with anisotropic hyperfine couplings of moderate size due to nuclei with large nuclear g-factors like or F. The effective field acting on the nucleus is dominated by the applied field, so that a and the normal selection rule Ami = 0 applies. The splitting between the lines is AT = 1 A 1 and A g = 1 Ag 1 for isotropic and anisotropic g-factors, respectively. 

This expression is of the order of what one would calculate by squaring Equation (15) and summing to get the mean square field. The difference is due to the fact that two nuclear spins can simultaneously flip so as to conserve magnetic energy since they are coupled by the dipolar interaction. This process limits the lifetime of a spin state and gives rise to an additional line breadth. For two nonidentical nuclei such processes do not con-... 

Suppose we wish to measure T, for a nuclear spin that gives only a single NMR line. There are a number of ways in which this can be done, but one of the simplest and most accurate is the inversion-recovery method, in which a 180° pulse inverts the magnetization, and its recovery by spin—lattice relaxation is monitored by application of a 90° pulse. Figure 2.12 shows that an initial 180° pulse flips M... 

Fig. 4.19 Schematic X- and Q-band spectra with an anisotropic H hyperfine coupling of smaller magnitude than the nuclear Zeeman energy. Weak spin flip (Am/ = 1) lines separated by 23 i a, 1 (X) and 3.5 (Q) mT appear on the wings of the two main lines. The intensity...

If the proton spins could be driven to flip at a rate that is rapid compared to the static C- H dipolar interaction, which occurs naturally in mobile polymer solutions, then the resonance lines observed in solid-state NMR spectra would likewise no longer be broadened by these hetero-nuclear, spin-dipolar interactions. The NMR spectrum of PBT shown in Fig. 20.12(b) was recorded by applying an rf field B at the resonance frequency of protons, with a field strength of 50 kHz, in a direction perpendicular to the applied field Bo (analogous to the broadband H scalar-/... 

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