Contact time cross-polarisation

So that the results from the delayed-contact and variable-contact data could be compared, they were normalized to give equal signal intensities at a contact time of 0.5 ms (zero delay in the delayed-contact experiment) after the variable-contact data had been adjusted to allow for the fact that full equilibration of proton and polarisation had not quite been reached after 0.5 ms. The difference in normalised signal intensity between the variable-contact and delayed-contact experiments, at a given time point t, is then a measure of the amount of 13C cross-polarising between 0.5 ms and t. 

In the case of the aromatic protonated carbons, cross-polarisation experiments can be performed. The t /2 cross-polarisation time (defined as the contact time required to produce half the maximum value of magnetisation that is possible by cross-polarisation) significantly increases with temperature, as shown in Fig. 17. 

Fig. 17 Cross-polarisation contact times, fi/2, for the aromatic carbons in the phenyl rings, in both the pure PET and the PET/TPDE blend O aromatic carbons in PET and aromatic carbons in the blend that contains 10% TPDE (from [12])...

Deconvolution of these peaks would yield a distribution of Q2 and Q3 sites as a function of temperature. However, cross-polarisation spectroscopy is not automatically a quantitative technique, since the signal s intensity depends -amongst others- upon the proximity of protons and upon the contact time, used in the experiment. 

A distinction between primary and secondary species can be made using 29Si CP MAS NMR. Primary species cause a chemical shift at -36 ppm (reference TMS), whereas secondary species cause a shift at -60 ppm. Following the procedures of Maciel and Sindorf,64,65 which take into account the dependence of the CP (Cross - Polarisation) line intensities with the CP contact time, the percentage of primary and secondary species is calculated. 

TCP, the cross polarisation relaxation time, is a measure of the strength of the dipolar contact between the 1H and the 13C nuclei. This strength depends both on the inter-nuclear distance and on the mobility of the chain or chain segments. The cross polarisation is less effective when the motion is fast, whereas rigid systems will cross-polarise rapidly. So a study of TcP will yield insight into the proximity of the nuclei and the mobility of the molecular chain or chain segments. 

This example is illustrated in Figure 13.9 in which the MAS spectrum (a) is represented with the spectrum obtained from cross polarisation (b). The comparison of the two clearly shows a relative increase in certain peaks these are the Si (OH) (Si)4, , n taking the values shown on the spectrum (b). The Si (OSi)4 signal is still present this is simply due to an extended contact time which permits the polarisation to be extended over 3 bonds. It may thus be necessary to vary the contact time in order to fully discriminate the atoms bearing hydroxyls, Quantitative analysis is not possible but the experiment can be used to determine the presence or absence of silanol groups. 

The H- Si cross-polarisation time constants (Ts,h) associated with the Q species of both the 5% V-MCM and the standard MCM-41 calcined samples have been derived from variable-contact-time experiments. These values can be viewed as an indication of how fast the transfer of polarisation from protons to silicons occurs. The value for the 5% V-MCM sample is of the order of 0.15 ms, whereas for the standard sample it is 20 times greater, about 3 ms. 

Cross polarisation from H to Mo has been shown to produce enhancements of 66-86% of the theoretical maximum and reliable second-order powder lineshapes in isotopically-enriched compounds such as (NH4)6Moy024.4H20 and (Bu4N)2Mo207 which contain suitable proton sources (Edwards and Ellis 1990). The optimum contact times were found to be 20-30 ms and the value of Tip for molybdenum is long by comparison with the other relaxation processes, making this a suitable candidate for CP. 

Cross-polarisation (CP), which involves a cross-pulsing sequence. Under the appropriate conditions, magnetisation flows from protons to C during the contact time, when resonant radio frequencies are apphed to both protons and the nuclei to be observed (e.g. C). 

Cross polarisation from protons to fluorine nuclei becomes feasible with the double-resonance probes developed for F- H work. Figure 6.6.3 shows a CP profile as a function of contact time. This experiment offers similar possibilities for discrimination within spectra that are traditional with C CPMAS spectroscopy. Thus, the dipolar dephasing pulse sequence (Fig. 

Fig. 6.6.3. Dependence of intensity (at 188.29 MHz) on contact time for a sample of poly(trifluoroethylene). MAS rate 12.6 kHz. The plots can be fitted to a simple equation with effective Tip (incorporating both H and relaxation) of 14 ms and cross-polarisation times of 1.3 and 0.4 ms for the CF2 and CFH fluorines, respectively.

Blends of PVDF and PMMA were studied by Tekely et al. [79] using MAS NMR, with cross polarisation from protons. They obtained data on PMMA magnetisation as a function of contact time for the... 

Fig. 18.22. Evolution of the F magnetisation as a function of the contact time in a standard cross-polarisation experiment for a commercial Viton sample.

The solid state NMR measurements on the hard coke concentrates were carried out at 25 MHz on a Bruker DSX spectrometer with MAS at 4.5-5.0 kHz to give spectra in which the sideband intensities are only ca. 6-7% of the central aromatic bands. A contact time of 1 ms was used for the cross polarisation (CP) measurements and the H decoupling and spin-lock field was ca. 60 kHz. The FIDs were processed using a Lorentzian linebroadening factor of 50 Hz. To determine the fraction of protonated and non-protonated carbon, four delay periods between 1 and 100 is were employed in dipolar dephasing experiments. 

Cross polarisation with variable contact time... 

The NMR method makes use of measurements of the dependence of to Si cross polarisation efficiency as a function of CP contact time to determine the distances between different silicon atoms in the ordered complex to H atoms in /luru-xylene molecules in the pores. The experiment has to be performed where the molecules show little translational motion, and under conditions where any molecular re-orientation is well understood (from wideline NMR, for example see Section 7.2.3). To give additional information, the experiments were performed separately on selectively deuterated and dg- i-xylene molecules. Plots of the Si signal intensity from different crystallographic T-sites against the contact time show an increase to a maximum value. 

GA, GB and GC resins where examined by freeze drying without curing, while the GA -I- pMDl reaction product was first hardened and then examined. Solid state CP-MAS (cross-polarisation/magic angle spinning) C-NMR spectra were recorded on a Bruker Avance 11 MSL 300 MHz spectrometer at a frequency of 75.47 MHz. Chemical shifts were calculated relative to tetramethylsilane (TMS). The rotor was spun at 12 kHz on a 4 mm Bruker probe. The spectra were acquired with 5 s recycle delays, a 90° pulse of 5 ps and a contact time of 1 ms. The number of transients was 5000. 

Figure 3.14 The pulse sequence for cross-polarisation. The sequence is initiated with a 90° proton pulse followed by spin locking (the contact time) of the carbon and proton magnetisations. This leads to the efficient polarisation of the carbons via the protons when the carbon and proton field strengths are matched. The signals are then observed with high-power proton decoupling.

Figure 3.19 Intermolecular cross-polarisation in polymer blends. The cross-polarisation intensity as a function of contact time is shown for (a-d) a mechanical mixture and (e-h) a miscible blend prepared by annealing. Reprinted with permission from ref. 21.

Figure 4.4 The cross-polarisation pulse sequence. The contact time (CT) is of the order of ms. DC designates decouple...

Figure 4.15. Cross-polarisation dynamics for S0.3MHz spectra of a copolymer of nylon-6 and 40% polyether/polyesteramide (see the text) [14] variation of peak height S with contact time CT A, nylon-6-carbons (diamonds indicate the C4/CS peak, squares are for C2, and circles are for Cl, see III) B, polyether carbons (the symbols are for the peaks at the following chemical shifts triangles 18.2 ppm, circles 75.8 ppm, squares 73.9 ppm, diamonds 71.2 ppm). S is expressed relative to the peak height of the most intense signal, extrapolated to zero CT. A theoretical curve is shown for Cl in A...

A number of observations should be made here. Firstly, the typical rate of cross polarisation for carbons in organic materials is such as to require contact times of between 1 and 10 ms for complete equilibrium to be achieved. For nuclei with no directly bonded protons the longer times are appropriate. Secondly, having carried out one contact and acquired an FID (total time required T + aq possible in principle to... 

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