Spin selective population transfer

Enhancements between signals that are strongly spin-coupled to each other are best ignored as they are prone to another competing phenomenon, that of Selective Population Transfer (SPT). This makes it difficult to decide if any observed enhancement is down to a genuine NOE, or is merely an SPT. SPT... 

The rationale for INEPT can best be understood by looking first at a somewhat simpler continuous wave experiment for transferring polarization, selective population transfer (SPT). SPT can be understood simply in terms of the populations of energy levels, whereas INEPT requires consideration of coherent precessing magnetization. Figure 9.9 shows the energy levels and populations of an AX spin system, which we take to be H and 13C, respectively, in this example. At equilibrium the populations conform to a Boltzmann distribution. Because the H... 

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. 

The Selective Population Transfer (SPT) experiment is usually used in spin system analysis with a FT spectrometer. Normally the experimental SPT spectra are compared with calculated SPT spectra simulated using different combinations of coupling constant signs. In common with many textbooks the AMX spin system 2,3-dibromopropionic acid will be used to introduce the concepts behind the SPT experiment. The IH spin system parameters for 2,3-dibromopropionic acid are shown below. The only difference between Spin System A and Spin System B is the sign of the coupling constant J(H(2), H(3)), the results of SPT experiments will be used to distinguish between the two possible spin systems. 

In section II.A.2. we discussed how the rf pulse acts on the nuclear spins. We now discuss this topic further and apply the results to several examples having to do with selective excitation. There are many reasons for wanting to perform selective excitation in NMR. Some of them are to simplify complex spectra, to do selective population transfer in order to get the sign of spin-spin coupling, and to study cross relaxation. An important application of what we might call selective de-excitation is a notch in the irradiation pattern to suppress an unwanted resonance such as the solvent peak in a complex proton spectrum. Morris and Freeman (1978) have reviewed many aspects of such experiments. 

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... 

Applying rf fields of lower intensity to selectively perturb a single resonance or satellite line is the basis of the SPT or selective population transfer experiment. Perhaps the most interesting example of the utilization of SPT experiments, which form the basis for the INEPT, DEPT and other spectral editing experiments that have been developed, is found in the consideration of an AX heteronuclear spin system, e.g. or where the heteronuclear spin is insensitive relative... 

Significant progress in signal enhancement methods for the central transition has been achieved by the implementation of double frequency sweeps (DFS) [62]. The basic idea of DFS, applicable for both static and MAS experiments, is to invert simultaneously the STs so that the populations of the outer spin levels are transferred to the CT energy levels before they are selectively excited (Fig. 4). 

The phenomenon of electron spin polarization in photoexcited triplet molecules in solids has been known for quite some time (39,51,52). The mechanism is associated with the unequal populations of the triplet sublevels induced by the spin-selective nature of the spin-orbit coupling interactions which couple the excited singlet and triplet states during the intersystem crossing (ISC) process. In the presence of an external magnetic field the spin polarization in the molecular frame can be transferred to the laboratory frame for esr observation. Kim and Weissman (83,84) have recently demonstrated beautifully that the initial polarization following photoexcitation of the triplet molecules such as pentacene in dilute solid solution can be readily observed up to a temperature of 275°K. 

Spin population transfer experiments also need selective irradiation. As well as being used (136) to increase the intensity of quaternary carbon signals (p. 336), they have value for assignment by virtue of the... 

We will briefly consider in this section various aspects of homonuclear spin-de-coupling experiments and nuclear Overhauser effect (NOE) difference spectra. Obviously any detailed treatment is far beyond the size limitations of this chapter. Moving next to ID NMR techniques, we wiU briefly consider the utilization of selective spin-population transfer (SPT) and experiments which rely on these principles such as INEPT and DEPT, off-resonance proton decoupling techniques, decoupler gating experiments, and finally spin—lattice or Tj relaxation techniques. 

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. 

Demirplak and Rice developed the counter-diabatic control protocol while studying control methods that efficiently transfer population between a selected initial state and a selected target state of an isolated molecule [11-13]. The protocol has been studied for manipulation of atomic and molecular states [11, 12, 19] and spin chain systems [20, 21]. Experiments with the counter-diabatic protocol have been demonstrated for the control of BECs [22] and the electron spin of a single nitrogen-vacancy center in diamond [23]. The counter-diabatic field (CDF) protocol is identical with the transitionless driving protocol, independently proposed by Berry a few years later [24]. A discussion of the relationship between these approaches and several of the other proposed shortcuts to adiabaticity can be found in the review by Torrontegui and coworkers [10]. 

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