Cross-polarization experiment contact time

The variation of the 2 Si CPMAS spectrum was examined as the contact time for the cross polarization experiment was changed. The most intense CP signal was obtained with relatively short contact times, indicative of short silicon-hydrogen internuclear distances. Maximum CP intensity in the -103 ppm peak occurred at about 1 to 2 ms, suggesting to 2gSi distances of a few angstroms or less for this type of silicon. 

Fig. 1 SSNMR spectra of ibuprofen, 75.6 MHz. (A) Bloch decay experiment (single pulse), no decoupling, static, 240-sec pulse delay, 100 scans, 400-min experiment time. (B) Same as (A) but with decoupling ( 60 kHz). (C) Same as (B) but with 5-kHz MAS. (D) Cross-polarization experiment, with H decoupling ( 60 kHz), 5-kHz MAS, 1.5-msec contact time, 3-sec pulse delay, 100 scans, 5-min experiment time. (E) Same as (D) with the TOSS pulse sequence applied to suppress spinning sidebands. Note Asterisk ( ) denotes spinning sidebands sharp ( ) denotes spectrometer background artifact.

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. 

A variety of pulse sequence programs were employed. A conventional cross polarization, single contact program was used to obtain spectra of Intact sediments. Contact times were varied from 200-3000 psec with a three sec recycle time. For dephaslng delay experiments, a 50 psec delay was Inserted prior to data collection. This delay consisted of two 25 ysec Intervals separated by a 10 psec, 180° refocusing pulse. Data was collected In 2K of memory, exponentially multiplied with 50 Hz of line broadening, and expanded to 8 K prior to Fourier transformation. All spectra are the result of 5000 accumulations. 

G and 9.8 G respectively satisfying the Hartmann-Hahn condition. A single contact sequence comprised 5.0 rrjs contact time and a recycle time of 4.0 s. The corresponding parameters for the 29Si-NMR cross-polarization experiments are 29Si (39.7 MHz) and (200.0 MHz) rf fields were 39.3 and 9.8 G respectively, with a contact time of 10.0 ms and a recyle time of 3.0 s. 

Fig. 4. Timing sequence of cross-polarization experiment (a) polarization of H in rotating frame (b) spin locking of in rotating frame (c) "C- H contact under Hartmann-Hahn conditions and (d) observation of free induction decay. Left timing sequence of CP experiment. Right behaviour of nuclear spins. (Reprinted with permission from Miknis , 1995, Kluwer Academic Publishers.)...

Fig. 7. Idealized behaviour of the carbon magnetization as a function of the experimental contact time in the cross-polarization experiment.

Fig. 4. Schematic representation of a cross-polarization experiment. The introduction of a time delay after the contact period leads to a dipolar dephasing of the resonating /-spins...

Figure 1. Pulse diagram of the single-contact cross-polarization experiment with a contact time, CT and acquisition time ACQ.

Long contact times are useless the intensity of resonances decreases due to the proton spin relaxation (in the spin-locking conditions, i.e. in rotating frame, it is the proton spin relaxation time T.p ). Usually, a contact time in the range 2-6 ms was the most frequently applied in cross-polarization experiments. However, according to the plot of intensity versus tc illustrated in Figure 12-8, the optimal value for chromanol is ca. 10 ms. 

Figure 12-8 Selected results of the variable-contact cross-polarization experiments. The plot of signal intensity versus contact time for chromanol carbons.

Fig. 8.21. The change in carbon magnetization with contact time for the cross-polarization experiment. The initial rise is due to the cross-polarization contact time, Tch. and the relaxation decrease is governed by the...

The contact times, ti/2, necessary to obtain half of the maximum equilibrium polarization in cross-polarization experiments using very short contact times have been determined at 298 K. At the frequencies involved, i.e. 10" -10 Hz, the methylene unit adjacent to the terphenyl moiety has a rigid-lattice behaviour, whereas the next-nearest CH2 group undergoes oscillations on the valence cone of approximatey 20"" about one equilibrium conformation. The following CH2 group performs oscillations of large amplitude, or more likely jumps between two equilibrium conformations. 

Cross-polarization is based on the notion that the vast proton spin system can be tapped to provide some carbon polarization more conveniently than by thermalization with the lattice (7). Advantages are two-fold the carbon signal (from those C nuclei which are indeed in contact with protons) is enhanced and, more importantly, the experiment can be repeated at a rate determined by the proton longitudinal relaxation time Tin, rather than by the carbon T c (I)- There are many variants (7) of crosspolarization and only two common ones are described below (12,20). 

The former is a protein of 14.7 kDa involved in the multienzyme nucleotide excision repair (NER) pathway with a determined NMR solution structure . In this protein, the Zn + possesses rather a structural than a catalytic role. Zn NMR spectra were acquired using a rather sophisticated probe (for details, see Reference 87) and operating at temperatures 5-250 K. Data acquisition was performed with the application of spin-echo methods for enhanced sensitivity . Specifically, experiments were carried out at 25 K using a combination of CP (cross-polarization) and spikelet echo pulse sequences which provide a considerable increase in signal-to-noise ratio (of the order of 30) relative to a classical quadrupole echo pulse sequence. The proton field strength applied to the above measurements was 60 kHz with a matching field of 20 kHz for zinc and a contact time... 

In order to conceive the contributions of the various species to every band, the spectra are deconvoluted. After deconvolution of the recorded spectrum into a sum of Gaussian curves, the different contributing signals can be integrated separately. For quantitative assessment some contact time and cross-polarization criteria have to be met. Optimal conditions are obtained from a variable contact time experiment. Full details have been worked out by Pfleiderer23 and Caravajal et al.21,22 In general, quantitative correlations should only be made for species within one spectrum and not... 

In order to study the molecular aggregation during the volume relaxation of network epoxies, CP/MAS carbon-13 (natural abundance) NMR was utilized. The Hartman-Hahn cross-polarization technique 129) was used with a cross contact time of 1 mi llisecond for transfer of proton polarization to carbon nuclei. The protondecoupling was achieved at the radio frequency of 56.4 MHz. Carbon-13 14.2 MHz spectra were measured in a 1.4 Tesla magnetic field. Room temperature (23 °C) experiments were performed at 54.7° MAS at 1 KHz. The spinner was constructed using an Andrew-type rotor driven by compressed air. 

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