Cross-polarization experiment

I would have thought that a reasonable fraction of the polymer was very much immobilized with the carbon black. If so why do you not see it when cross polarization experiments with spinning are carried out ... 

Polymer Dynamics. 13C spin-lattice relaxation times (Ti) were determined with either an inversion-recovery sequence (16) (for carbons observed by direct polarization) or with a modified cross-polarization experiment (17). 13C rotating-frame relaxation times (Tip(C)) were derived from measurements of the carbon signal that remained after a Tjp(C) hold time of... 

Much of the power of MAS NMR has come from the ability of exploiting crosspolarization for facile signal detection and spectral editing for insensitive and rare-spin nuclei. To date, protons have almost invariably constituted the abundant-spin reservoir for cross-polarization experiments, although, most recently, 19F has found increasing use. 

Cross-polarization is a well-known method for enhancement of low-/ nuclei signals in solids (CP MAS experiments)145. The theory and first cross-polarization experiment in liquid were presented by Hartmann and Hahn in the same paper that laid the foundation of the famous CP experiments in solids in 196274. It took almost two decades until CP experiments in liquids were investigated in more detail by Ernst and coworkers146 and Bertrand and coworkers147. 

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. 

The cross polarization experiment can be described through the following steps ... 

The benefits are primarily an intensity enhancement of the dilute spin signal by a factor of 7 abundant/7 dilute and a reduction of the recycle time between experiments since the ratedetermining relaxation time is now that of the abundant species, rather than that of the rare spins. Usually, the relaxation of the abundant spins are much faster than the dilute spin relaxation (13). The cross polarization experiment may thus be repeated with much shorter intervals, leading to a further increase of the signal-to-noise ratio of the rare spin NMR spectmm within a given period of time. The effectiveness of the cross polarization experiment depends on the strength of the dipolar interaction between the abundant and rare spin systems, i.e. on the distance between the actual nuclei (proportional to r, where r is the distance between the abundant and the dilute nuclei) (11). The efficiency of magnetization transfer decreases extremely fast as the distance between the abundant and rare spins increase. One should emphasis, that under normal conditions, the CP experiment does not provide quantitative results. Finally, the cross polarization sequence does not influence the line width. 

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. 

Finally, in this section, the simple Hartmann-Hahn cross polarization experiment, and in particular, the JH-19F CP build up curve was used to determine the residual JH-19F dipolar coupling constant due to motion in an ethylene/tetrafluoroethylene co-polymer.31 This curve for the crystalline... 

Fig. 4 The top diagram represents the pulse sequence for the cross-polarization experiment, whereas the bottom diagram describes the behavior of the H and 13C spin magnetizations during the sequence. (From Ref. 11.)...

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. 

Additional problems arise from cross-polarization experiments being rather complicated by the low number of protonated carbon atoms. H-NMR-examina-tions are not very instructive due to the large width and the in parts extremely low intensity of signals. 

Recall that from the definition of spin temperature a low temperature implies large polarization of the magnetic moments. Thus, low carbon spin temperature is what is desirable just before inducing an FID. Because in the basic cross polarization experiment as described you can gain S/N according to the ratio Tp/T, the improvement depends on how well the protons are cooled initially. Some of the methods commonly used to do this will be described later in this section. 

What has been described so far is a single contact cross polarization experiment. Although we have not even said how the carbon FID will be induced, when it does occur it leaves the carbon spin temperature very high. Ordinary spin-lattice relaxation process (via the line joining the carbon reservoir to the lattice) has been assumed to be quite slow so that it... 

At the time of this writing, a rapidly growing area of research within NMR is the high resolution study of solids brought about by the development of certain multiple pulse sequences (such as the WAHUHA and MREV-8) on the one hand and an adaptation of the Hartmann and Hahn cross polarization experiment to obtain spectra of dilute spins on... 

Pulse sequences can be used to manipulate information in other ways, as well. The chemical shift concertina uses it to manipulate the relative importance of two terms in the Hamiltonian, the chemical shift term and the spin-spin coupling term. Echo sequences can be viewed as causing the system to evolve back in time to enable us to view information that existed earlier but was obscured by instrumental problems. The same is true of the ZTR experiment to observe the FID at zero time. The cross polarization experiments enable us to exploit the thermodynamic properties of a spin system to enhance the spectrum S/N of a second spin system. 

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