Spin population inversion experiment

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. 

Fig. 5 Naj Li (left side) and Naj Li (right side) SEDOR results on the model compound LiNaS04. Solid curves show fits to the equation shown, where the inversion factor f takes into account the incomplete spin population inversion caused by strong quadrupolar splittings. The dotted curve shown on left is the theoretical prediction for f=1.00 in the Naj Li SEDOR experiment. Reproduced from [56]...

Figure 13. Energy levels and transitions for an AX system. At the bottom of the figure, a population inversion experiment (SPI) is shown. If an rf pulse at exactly the resonance frequency is applied to one of the proton lines (A2) at a power level low enough to affect only A2, then the populations of the energy levels 2 and 4 are inverted. The C spins sharing these levels will show an enhanced XI line and an inverted X2 line.

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. 

One complicating factor associated with the implementation of composite pulses arises from the fact that many composite sequences have been designed with a particular initial magnetisation state in mind and may not perform well, or give the expected result, when the pulses act on other states. The two principal applications are the use of composite 180° pulses for population inversion, for example in the inversion-recovery experiment, or in the ubiquitous spin-echo for refocusing, in which they act on longitudinal and transverse... 

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... 

Repeating these experiments using the YAG laser fundamental (1064nm, 1.17 eV), adjusting the energy to achieve the same calculated temperature jump, gave essentially identical (Q, J, A)-state distributions. The inversion of population in the two spin-orbit levels, the population plateau for internal energies below 300 cm and the rapid fall-off of rotational population for... 

Even if the optimisation of the use of DFS, RAPT or adiabatic inversion pulses is not straightforward for nuclei with low sensitivity, it is nonetheless worth applying one of these methods to improve sensitivity. As long as there is no influence on the CT resonance, these techniques are likely to produce an enhanced CT signal compared to standard spin-echo experiments. Therefore, for Mg (as well as for other insensitive half-integer spin quadrupolar nuclei such as S, K and Ca), it is always advisable to apply some population transfer technique before the excitation of the CT signal. 

The REDOR experiment has been proposed for spin-1/2 pairs. For quadmpolar nuclei-spin-1/2 pairs, the use of this experiment is limited. The main reason behind this is the fact that a significant quadmpolar broadening (in order of several MHz) makes the Sn pulses inefficient for the inversion of S populations. For such systems REDOR experiments can be applied if the n pulses are apphed to the spin-1/2 nucleus and only central transition is observed for a halfinteger spin. The transfer of population in double resonance (TRAPDOR) or rotational-echo adiabatic passage double resonance (REAPDOR) experiment offer a much better choice for the measurement of quadmpolar nuclei-spin-1/2 pair dipolar couplings. 

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