Selective population inversion

Phenolic and etherified softwood lignin model compounds were also successfully crosslinked at the meta positions. Special care was taken to assign the structure of the dimer (22) (Fig. 4C). With NMR-SPI (selective population inversion) techniques it was unevocably proved that the methylene linkage was situated at the 6-positions (13). 

Fig. 2.43. Top Energy levels of a 13C —1H pair and their populations according to a Boltzmann distribution (a) and after population inversion of levels 1 and 3 (b) and 2 and 4 (c), respectively. Differences of 13C populations arc neglected. Bottom. 13C— H doublet of chloroform with Boltzmann distribution of nuclei (a) and after selective population inversion according to (b) and (c) [51],...

A correction of structure from (13) to the azaphilone structure (14) for monochaetin, a metabolite elaborated by the fungus Monochaetia compta, was made possible by examination of the long-range H—l3C connectivity pattern as determined by the heteronuclear selective population inversion (SPI) NMR technique <86JCS(P1)1975>. 

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. 

Experiments with various decoupling techniques [5.1], with solvent presaturation [5.1], or selective population inversion of a singlet transition [5.1]... 

A fundamentally different approach to signal excitation is present in polarization transfer methods. These rely on the existence of a resolvable J coupling between two nuclei, one of which (normally the proton) serves as a polarization source for the other. The earliest of these type of experiments were the SPI (Selective Population Inversion) type (19>) in which low-power selective pulses are applied to a specific X-satellite in the proton spectrum for an X-H system. The resultant population inversion produces an enhanced multiplet in the X spectrum if detection follows the inversion. A basic improvement which removes the need for selective positioning of the proton frequency was the introduction of the INEPT (Insensitive Nucleus Excitation by Polarization Transfer) technique by Morris and Freeman (20). This technique uses strong non-selective pulses and gives general sensitivity enhancement. 

Detailed NMR studies of 11, using extensive homonuclear decoupling, 13C- H selective population inversion (SPl) and selective decoupling... 

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... 

The sensitivity limitations inherent in the observation of proton decoupled N signals become more significant if undecoupled spectra are desired. The method of selective population inversion (M 31i), which is capable of noticecibly enhancing the line intensities of coupled spectra, therefore seems very attractive. In this method a tt pulse is applied selectively to a given component (or an appropriate sub-multiplet) of the satellite proton spectrum pertaining to the molecules which contain the N isotope. Just after this selective inversion, a non selective... 

Another useful technique to increase X receptivity is to use a Selective Population Inversion (SPI) pulse sequence (15) on one proton transition. A long (150 to 250 ms), soft pulse is applied via the decoupler to excite and invert one transition of the coupled proton spectrum selectively. The pulse sequence is visualized in Figure 16 together with a SPI experiment... 

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