Selective polarization transfer

Consider the 13C— H bond as a two-spin system. CH coupling occurs between one nucleus with small population difference (13C) and another one with large polarization (1H). Fig. 2.43(a) illustrates this situation by the number of dots on the energy levels. Population inversion of the proton levels 1 and 3 connected by the transition 1H1 is achieved by an appropriate 180° pulse, which turns the double cone of precession shown in Fig. 2.1 upside down. Thereafter, the inverted proton population difference controls both carbon-13 transitions (Fig. 2.43(b)). This is the polarization or population transfer making up an enhanced absorption signal for one transition (e.g. 13Ci in Fig. 2.43 (b)) and an enhanced emission on the other (e.g. 13C2 in Fig. 2.43(b)). 

Selective population or polarization transfer (abbreviation SPT) was first achieved for chloroform [51] by brief irradiation of one of both 13C satellites in the proton NMR spectrum before recording the 13C—1H doublet as FID signal. In this experiment, the decoupling pulse inverts the population of the proton precession states connected by the irradiated satellite transition (Fig. 2.43(b, c)). The two SPT experiments shown for the 13C —H doublet of chloroform in Fig. 2.43 (b, c) give an impression of the signal enhancement achievable by population transfer in comparison to a normal 13C NMR spectrum 

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

Nonselective polarization transfer, as implied by the term, represents a process that allows simultaneous polarization transfer from all protons to a//X nuclei. In sefectmepolarization transfer, however, the population of just one nucleus is inverted at any one time. The selective polarization transfer sequence therefore cannot be used to generate a proton-decoupled C-spectrum containing all sensitivity-enhanced C resonances. 

Selective polarization transfer A polarization transfer experiment in which only one signal is enhanced selectively. 

The idea of frequency selectivity is certainly not restricted to J-coupling mediated polarization transfer. Furthermore, frequency selective polarization transfer can also be realized by DQ recoupling techniques. For the technique of shift-evolution-assisted selective homonuclear recoupling (SEASHORE), which employs POST-C7 to construct an effective DQ coupling [82, 83], frequency selective polarization can be achieved when the transmitter offset is set to the... 

Table 2.1. Intensities of CH multiplets in Coupled 13C NMR Spectra with Boltzmann Distribution of Nuclei (a) and after Selective Polarization Transfer (b).

Pulse sequences for non-selective polarization transfer, not only useful for signal enhancement but also for multiplicity selection, are referred to as INEPT [54], abbreviated from Insensitive Nuclei Enhanced by Polarization Transfer . An improved method denoted as Distortionless Enhancement by Polarization Transfer or DEPT" [55] permits the cleanest multiplicity selection known so far, with full enhancement and low sensitivity to individual CH coupling constants. In addition, fully enhanced and undistorted coupled spectra can be recorded. Finally, subspectra for CH, CH2 and CH3 groups can be generated. 

Modern techniques.. /-Modulated spin-echo ( APT ), non-selective polarization transfer ( DEPT ) experiments and two-dimensional carbon-proton, proton-proton ( COSY ) and carbon-carbon ( 2D-INADEQUATE ) shift correlations are in use (see the following section) and will be used in order to clarify doubtful signal assignments (see Sections 2.9 and 2.10). 

B. Non-selective Polarization Transfers (INEPT, DEPT, PENDANT,... 

The physics behind SPI (Selective Polarization Inversion)77 or SPT (Selective Polarization Transfer)78,79 experiments is described and explained in every current NMR textbook, since it provides a nice introduction to understanding some of the more common current experiments (e.g. INEPT or 2D homo- and heteronuclear correlation experiments). The two names, SPI and SPT, are used indiscriminately for the same experiment, although in general SPI might be considered a special case of an SPT experiment with maximum polarization transfer achieved by inversion78. 

Wu X, Zhang S, Wu X (1988) Selective polarization transfer in solid state high-resolution CP MAS NMR. J Magn Reson 77 343-347... 

Consider the mechanism of the selective polarization-transfer (SPT) experiment using a spin system [45]. Such a weakly coupled spin system has four energy... 

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