Transfer Polarization

We have seen that the 180° 13C pulse reordered the 13C populations, but transfer of proton polarization to the 13C system occurred only when the and 13C 90° pulses were applied at time 4. We chose to use a matrix that represented the concurrent application of both pulses. However, these are independent, their Hamiltonians commute, and we could have considered instead the application of the two pulses separately—in either order. Here we examine the effect of applying the H pulse. 

We see that the 90y proton pulse has led to polarization transfer, as indicated by the magnitude of the diagonal elements. The role of the 13C 90° pulse (which can be applied equally well along x or y ) is then merely to generate the now enhanced 13C coherences that can be detected or allowed to evolve for further manipulation in ID or 2D experiments. 

The sensitivity of an atomic nucleus in the NMR experiment is related to its gyromagnetic ratio y. It determines the energy difference AE between the precession states in a magnetic field of flux density B0 (Figs. 2.1 and 2.35)  

The populations Ne and Nt of the excited (e) and the ground state (g) follow a Boltzmann distribution  

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Figure Bl.11.11. Polarization transfer from to (see the text). The inversion of one H transition also profoundly alters the C populations.

The remarkable stability and eontrollability of NMR speetrometers penults not only the preeise aeeiimulation of FIDs over several hours, but also the aequisition of long series of speetra differing only in some stepped variable sueh as an interpulse delay. A peak at any one ehemieal shift will typieally vary in intensity as this series is traversed. All the sinusoidal eomponents of this variation with time ean then be extraeted, by Fourier transfomiation of the variations. For example, suppose that the nomial ID NMR aequisition sequenee (relaxation delay, 90° pulse, eolleet FID) is replaeed by the 2D sequenee (relaxation delay, 90° pulse, delay i -90° pulse, eolleet FID) and that x is inereased linearly from a low value to ereate the seeond dimension. The polarization transfer proeess outlined in die previous seetion will then eause the peaks of one multiplet to be modulated in intensity, at the frequeneies of any other multiplet with whieh it shares a eoupling. 

Morris G A and Freeman R 1979 Enhancement of nuclear magnetic resonance signals by polarization transfer J. Am. Chem. See. 101 760-2... 

Doddrell D M, Pegg D T and Bendall M R 1982 Distortionless enhancement of NMR signals by polarization transfer J. Magn. Reson. 48 323-7... 

Muns ENDOR mvolves observation of the stimulated echo intensity as a fimction of the frequency of an RE Ti-pulse applied between tlie second and third MW pulse. In contrast to the Davies ENDOR experiment, the Mims-ENDOR sequence does not require selective MW pulses. For a detailed description of the polarization transfer in a Mims-type experiment the reader is referred to the literature [43]. Just as with three-pulse ESEEM, blind spots can occur in ENDOR spectra measured using Muns method. To avoid the possibility of missing lines it is therefore essential to repeat the experiment with different values of the pulse spacing Detection of the echo intensity as a fimction of the RE frequency and x yields a real two-dimensional experiment. An FT of the x-domain will yield cross-peaks in the 2D-FT-ENDOR spectrum which correlate different ENDOR transitions belonging to the same nucleus. One advantage of Mims ENDOR over Davies ENDOR is its larger echo intensity because more spins due to the nonselective excitation are involved in the fomiation of the echo. 

Robyr P, Gan Z and Suter U W 1998 Conformation of raoemo and meso dyads in glassy polystyrenes from C polarization-transfer NMR Macromolecules 31 8918- 23... 

Major advances m NMR have been made by using a second rf transmitter to irra diate the sample at some point during the sequence There are several such techniques of which we 11 describe just one called distortionless enhancement of polarization transfer, abbreviated as DEPT... 

Heteronuclear chemical shift-correlated spectroscopy, commonly called H-X COSY or HETCOR has, as the name implies, different and F frequencies. The experiment uses polarization transfer from the nuclei to the C or X nuclei which increases the SNR. Additionally, the repetition rate can be set to 1—3 of the rather than the longer C. Using the standard C COSY, the ampHtude of the C signals are modulated by the... 

With improvements in Instrument sensitivity and the use of techniques such as enhancement by polarization transfer (INEPT), it can be expected that natural abundance N NMR spectra will become increasingly Important in heterocyclic chemistry. The chemical shifts given in Table 10 illustrate the large dispersion available in N NMR, without the line broadening associated with N NMR spectra. 

DEPT (Section 13.18) Abbreviation for distortionless enhancement of polarization transfer. DEPT is an NMR technique that reveals the number of hydrogens directly attached to a carbon responsible for a particular signal. 

Techniques developed in recent years make it possible to obtain large amounts of information from l3C NMR spectra. For example, DEPT-NMR, for distortionless enhancement by polarization transfer, allows us to determine the number of hydrogens attached to each carbon in a molecule. 

Some of the most important 2D experiments involve chemical shift correlations between either the same type of nuclei (e.g., H/ H homonu-clear shift correlation) or between nuclei of different types (e.g., H/ C heteronuclear shift correlation). Such experiments depend on the modulation of the nucleus under observation by the chemical shift frequency of other nuclei. Thus, if H nuclei are being observed and they are being modulated by the chemical shifts of other H nuclei in the molecule, then homonuclear shift correlation spectra are obtained. In contrast, if C nuclei are being modulated by H chemical shift frequencies, then heteronuclear shift correlation spectra result. One way to accomplish such modulation is by transfer of polarization from one nucleus to the other nucleus. Thus the magnitude and sign of the polarization of one nucleus are modulated at its chemical shift frequency, and its polarization transferred to another nucleus, before being recorded in the form of a 2D spectrum. Such polarization between nuclei can be accomplished by the simultaneous appli-... 

Figure 2.7 Pulse sequence for a heteronuclear AX spin system representing polarization transfer from H to nuclei.

Suppose, for clarity, we add a common factor [(V2)jn + (V2)yc] to all four energy levels so the energy difference between them does not change. The values of these energy levels will then become y, yc, Jh, and yH + yc-Since yn is about four times yc, let us also assume that yn = 4 and yc = 1. The populations of the four energy levels will then be 0, 1, 4, and 5. The population difference between the two C spin states before the application of the H polarization transfer pulse corresponds to the lower energy state minus the upper energy state, i.e. 1—0=1 or 5 — 4=1. 

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. 

How does magnetization transfer from H to a coupled C nucleus (polarization transfer or population transfer) affect the signal intensity of the C nucleus ... 

What is the difference between nonselective and selective polarization transfer ... 

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