Cross polarization time

Figure 2, Cross-polarization timing diagram ( see text for discussion), (Reproduced with permission from Ref, 40, Copyright 1982, Royal Society of London,)...

NMR in dichloromethane-d1. The spectrum was run quantitatively with broad band heteronuclear decoupling, b) 1JC CPMAS NMR of uncured PTEB. Cross-polarization time was 4.5ms the rotor speed was 2.3 kHz the cross polarization field was 50 kHz. Chemical shifts are relative to TMS. 

NMR relaxation measurements such as cross-polarization time constants (7ch) can also yield useful information regarding alkyl-chain mobility. Sindorf and Maciel [159] have characterized the relative mobility of low-density Cg and Cig stationary phases ( 2.6 and 1.7 p,mol/m, respectively) as a function of carbon position from the... 

Figure 4. The influence of magic angle spinning on the SL cross polarization time constant. In the PlP-cured epoxy the time constant for the nonprotonated carbons increases by about 50% for = 1-3 kHz while those of the protonated carbons are virtually unchanged over this range.

Figure 5. The rf pulse sequences for determining SL cross polarization time constant Ten, C-13 T,p and C-13 Tj under proton decoupling. Each experiment starts with a SL cross polarization.

The initial building-up of the carbon magnetization via polarization transfer is characterized with the spin-lock cross-polarization time Tcir It is sensitive to the static interactions, i.e. transfer of the polarization via static dipolar interactions (proportional to e J). Hence, the shortest TCH will have carbons a) in more rigid systems and b) with more directly attached protons, particularly the CH2 (or... 

Fig. 6. Cross-polarization timing diagram for the H-I3C spin system. Upper part, the H channel lower part, the 13C channel.

Io is the maximum intensity, TCp is the cross polarization time constant and Tiph and TlpC are the relaxation times for ll and 13C in the rotating frame. The cross polarization rate (l/TCp) depends on the square of the dipolar interaction while the relaxation time in the rotating frame provides... 

Octacalciumphosphate - for the chemical formula see Table 2 Cross-polarization time constant for the I-S model Proton diffusion time constant... 

Time period of forward CP in DCP - see text and Fig. 14 Depolarisation time period in DCP - see text and Fig. 14 P spin-lattice relaxation time in the laboratory frame Cross-polarization time constant for the I-P-S model H spin-lattice relaxation time in the rotation frame H spin-spin relaxation time in the laboratory frame Recycle delay... 

The cross-polarization time, Tch, for a given carbon atom is a function of the distance between the carbon atom and adjacent protons. In order that quantitative results be obtained from the cross-polarization experiment, these JcH values for all carbon atoms in the sample must be less than the experimental contact time, which in turn must be less than all Tj values, that is, the amount of time in the experiment when the Hartmann-Hahn condition exists must be less than the Tj values of all C and H atoms (see Palmer and Maciel, 1982, for a more complete discussion). If these conditions do not apply, then the integrated band areas will not be representative of the concentration of the various carbon atoms present in the sample. 

Figure 26 Proton-decoupled (top spectrum of each set) and proton-coupled (bottom spectrum of each set) 39.75-MHz Si CP-MAS NMR spectra of Fisher S-679 silica gel at six different magic-angle spinning speeds. Cross-polarization time, 5 ms. (a) 1.0 kHz, 1960 accumulations (b) 1.1 kHz, 3000 accumulations (c) 1.4 kHz, 720 accumulations (d) 1.6 kHz, 2000 accumulations (e) 1.8 kHz, 20(X) accumulations (f) 2.0 kHz, 2(XX) accumulations. (From Ref. 60.)...

Figure 28 39.75 MHz " Si CP-MAS NMR spectra of Fisher S-679 silica gel obtained with six different H- H dipolar-dephasing times (as indicated) prior to H- " Si crosspolarization. Cross-polarization time, 100 xs. Each spectrum is the result of 20,000 accumulations. Spectra in the left column have been scaled to the same peak height for the — 99 ppm peak, and spectra in the right column are plotted on the same absolute scale. (From Ref. 60.)...

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