Steroid Reaction Mechanisms by D. N. Kirk, M. P. Hartshorn

By D. N. Kirk, M. P. Hartshorn

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This can also be a very efficient mechanism, and the linewidths of many such nuclei can be hundreds or even thousands of hertz wide. 3. Whilst the direct observation of these nuclei may not be routine for many organic chemists, their observation can, at times, prove very enlightening for specific problems, and the indirect effects they have on the spectra of spin-½ nuclei should not be overlooked. Quadrupolar nuclei possess an electric quadrupole moment in addition to a magnetic dipole moment. This results from the charge distribution of the nucleus deviating from the usual spherical symmetry associated with spin-½ nuclei and becoming ellipsoidal in shape.

To illustrate these points, we consider one of the fundamental building blocks of numerous NMR experiments, the spin-echo. Consider first two groups of chemically distinct protons, A and X, that share a mutual coupling JAX, which will be subject to the simple two-pulse sequence in Fig. 14. For simplicity we shall consider the effect of chemical shifts and couplings separately, starting with the chemical shifts and again assuming the reference frequency to be that of the A spins (Fig. 15). The initial 90°x creates transverse A and x-magnetisation, after which the X vector precesses during the first time interval, D.

However, magnetisation in the x–y plane corresponds to deviation from the equilibrium spin populations and, just like any other chemical system that is perturbed from its equilibrium state, the system will adjust to re-establish this condition, and so the transverse vector will gradually disappear and simultaneously grow along the z axis. This return to equilibrium is referred to as relaxation, and it causes the NMR signal to decay with time, producing the observed FID (Fig. 11). The process of relaxation has wide-ranging implications for the practice of NMR and this important area is also addressed in this introductory chapter.

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Steroid Reaction Mechanisms by D. N. Kirk, M. P. Hartshorn
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