Cross-polarization experiment with contact time

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.

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

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. 

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. 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...

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 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. 

Magic angle spinning NMR spectra with variable cross polarization contact times were obtained on the intact, non-extracted sediments. The time-dependent spectra reveal subtle differences in organic carbon with depth differences not observed in single contact experiments. Dlpolar-dephased spectra of these same sediments indicate the presence of substantial amounts of substituted aromatic/olefinic carbons which are rapidly altered with depth. 

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. 

The temperature-dependent NMR spectra of the PLA samples collected under the varying temperature from 20 to 80 °C are shown in Fig. 6. Cross polarization-magic angle spinning (CP-MAS) NMR experiments were carried out on a Varian 400 NMR system spectrometer operated at 100.56 MHz for resonance with a cross polarization contact time of 2 ms (Fawcett, 1996). A zirconium oxide rotor of 4 mm diameter was used to acquire the NMR spectra at a spinning rate of 15 kHz. Each sample was packed into a 4 mm cyUnder-type MAS rotor. A set of temperature-dependent NMR spectra were obtained under varying ambient temperature from 20 to 80 °C at every 20 °C step. The heating rate was approximately 10 °C per an hour. 

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