Population transfer experiments

Population transfer experiments may be selective or nonselective. Selective population transfer experiments have found only limited use for signal multiplicity assignments (SSrensen et al, 1974) or for determining signs of coupling constants (Chalmers et al., 1974 Pachler and Wessels, 1973), since this is better done by employing distortionless enhancement by polarization transfer (DEPT) or Correlated Spectroscopy (COSY) experiments. However, nonselective population transfer experiments, such as INEPT or DEPT (presented later) have found wide application. 

For an effective measurement of "/(SiH) (1 < n < 4) values in silylated silyl enol ethers, a selective population transfer experiment was modified so that selective decoupling was applied during acquisition. 1H-29Si heteronuclear COSY confirmed the presence of silylated groups in the molecule6. 

FIG. 6. Selective population transfer experiments, C FT spectra of-NC region of CH jNC. (A) Normal fully coupled spectrum. (B) With continuous weak saturating irradiation of C satellite at lowest frequency in proton spectrum. (C) With irradiation of satellite at next lowest frequency. (D) With irradiation of satellite at highest frequency. From ref 127,... 

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... 

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. 

A new alkaloid (7) is the first example of a Gelsemium alkaloid having an A a-methoxyindole moiety in the molecule. This alkaloid may be an early biogenetic intermediate to the A a-methoxyoxindole alkaloids and their related compounds. Full assignments of the IH- and l C-NMR spectra of A7 a-methoxy-19(Z)-anhydrovobasinediol (7) were conducted mainly by CSCM ID (decoupled selective population transfer experiment) (18) and selective INEPT (insensitive nuclei enhanced by polarization tansfer) (19) experiments. The structure was finally determined by single crystal X-ray analysis (11). 

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... 

Among the population transfer experiments, only HS and RAPT (or FSG) have been used to acquire MAS spectra for proteins [37] and pep-... 

Similarly, a negative sign of J N- F in 2-fluoropyridine has been determined by this technique (J 1). However, the usefulness of selective population transfer experiments for routinely obtaining coupled spectra is rather limited. A first requirement consists in precisely positioning the double irradiation frequency upon usually unobservable satellite proton transitions associated with the nitrogen-15 isotope. Unless the coupling constants have already been obtained by... 

One of the most useful proton nuclear magnetic resonance ( H-NMR) one-dimensional (a single-frequency axis) techniques to be applied in structure determination is difference spectroscopy. This approach involves subtracting a spectrum obtained under the conditions of some perturbation from a normal, unperturbed spectrum. The resulting difference spectrum reveals the spectral changes caused by the perturbation, whereas those signals that remain unaffected are nulled. The perturbation can be a decoupling irradiation, a nuclear Overhauser enhancement, or even a population transfer experiment (135, 328). 

To date, only IR photon energies in the H-stretching range have been used in IR-induced population transfer experiments, which for most systems results in excitation far above the isomerization barrier, impeding an experimental determination of the isomerization barriers directly. Zwier and coworkers presented an elegant alternative, where stimulated emission pumping (SEP) is combined with HF or PTS methods, see Fig. 2d. This combination allows one to probe the barrier to conformational isomerization [56]. The method consists of two steps a pump-dump SEP followed by a probe laser interrogation to determine the new cmifomiational distribution. In the early part of the molecular beam expansimi, SEP prepares the... 

Fig. 7 Set-up top panel) and principle lower panel) of population transfer experiments SEP or IR excitation causes a conformational selective excitation in the expansion (Steps I and II), isomerisation take place (step III) and products are ultimately probed by UV spectroscopy in the late expansion (step IV). Extracted from [149]...

Finally, a variant of the population transfer experiment (see Sect. 4.1) can be proposed to monitor the presence of dark conformers, missing in the UV spectra. Although not detected by R2PI in the nanosecond regime, the presence of such dark conformers could be revealed through their IR excitation in the supersonic expansion. 

---