Selective spin population inversion

The earliest of the magnetization transfer experiments is the spin population inversion (SPI) experiment [27]. By selectively irradiating and inverting one of the 13C satellites of a proton resonance, the recorded proton spectrum is correspondingly perturbed and enhanced. Experiments of this type have been successfully utilized to solve complex structural assignments. They also form the basis for 2D-heteronuclear chemical shift correlation experiments that are discussed in more detail later in this chapter. 

Figure 12.12a depicts the coupled C-H spin system of HCC13, a composite of Figures 12.3 and 9.2a. There are two 13C transitions (vCi < vC2), each with intensity proportional to population difference 2AC, and two H transitions (vHi < vH2), each with intensity proportional to population difference 2Ah (Ah = 4Ac). In the selective population inversion (SPI) experiment, we will irradiate only one specific hydrogen... 

The experiment described above is termed selective population transfer (SPT), or more precisely in this case with proton spin inversion, selective population inversion, (SPI). It is important to note, however, that the complete inversion of spin populations is not a requirement for the SPT effect to manifest itself. Any unequal perturbation of the lines within a multiplet will suffice, so, for example, saturation of one proton line would also have altered the intensities of the carbon resonance. In heteronuclear polarisation (population) transfer experiments, it is the heterospin-coupled satellites of the parent proton resonance that must be subject to the perturbation to induce SPT. The effect is not restricted to heteronuclear systems and can appear in proton spectra when homonuclear-coupled multiplets are subject to unsymmetrical saturation. Fig. 4.20 illustrates the effect of selectively but unevenly saturating a double doublet and shows the resulting intensity distortions in the multiplet structure of its coupled partner, which are most apparent in a difference spectrum. Despite these distortions, the integrated intensity of the proton multiplet is unaffected by the presence of the SPT because of the equal positive and negative contributions (see Fig. 4.19d). Distortions of this sort have particular relevance to the NOE difference experiment described in Chapter 8. 

Only a selection of available pulses is presented with the emphasis on compensation for resonance offset effects and on sequences of short total duration. Individual pulses are presented in the form XXyy where XX represents the nominal pulse flip angle and yy its relative phase, either in units of 90° (x, y, —x, —y) or directly in degrees where appropriate. Sequences are split into those suitable for (A) population inversion (act on Mz) or (B) spin-echo generation (act on Myy). 

This can be illustrated on the Cr ion in a strong crystal field. Its energy level scheme is given in Figure 3.31. Upon irradiation into the A2 - T2 transition with high intensity, population inversion between the and A2 levels can be obtained. This is due to the fact that the T2 level empties rapidly into the level which in turn has a very long life time (ms) in view of the spin selection rule. 

The experiment described above is termed selective population transfer (SPT), or more precisely in tbis case with proton spin inversion, selective population inversion, (SPl). It is important to note, however, that the complete inversion of spin populations is not a requirement for the SPT effect to manifest itself. Any unequal perturbation of the lines... 

Figure 5 Simulated NMR spectra for a nucleus with spin 5/2 (such as Mg) in a single crystal, in the case of (A) and (B) populations corresponding to thermal equilibrium, with non-selective excitation ( hard pulse) in (A) and CT-selective excitation ( soft pulse) in (B). For (C) and (D) populations achieved after saturation of STs, with non-selective excitation (C) and CT-selective excitation (D). For (E) and (F) Populations achieved after complete inversion of the satellite transitions (in the order first, inversion of STl and ST4 and then inversion of ST2 and ST3), with non-selective excitation (E) and CT-selective excitation (F).The numbers at the right-hand side of the spectra in (B), (D) and (F) indicate the corresponding enhancement factors of the CT resonance.

Figure 12.12. (a) Equilibrium spin state population for a coupled H-13C spin system and the two resulting doublets, (b) Effect on spin state populations of selective inversion of the vH1 transition and the resulting two doublets. 

Figure 4A1. Polarisation transfer in a two-spin system, (a) Populations (in grey) and population differences (in bold) for each transition at equilibrium and (b) the corresponding spectra obtained following pulse excitation illustrating the four-fold population difference (see text), (c) The situation after selective inversion of one-half of the proton doublet and (d) the corresponding spectra showing the enhanced intensity of the carbon resonances.

In the INEPT sequence (Eigure 12), the final 90" pulse sets up selective inversion of the populations of a pair of levels within the coupled AX ( H-i C) spin system. The 90" pulse then generates two antiphase magnetization vectors of relative intensity +4 and -4 (relative to equilibrium magnetization) along the x-axes owing to magnetization (polarization) transfer from aris-... 

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