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Thursday, October 30, 2008

The Effect of the Contact Time on CP/MAS NMR Spectra

One parameter for CP / MAS data collection that must be set by the user is the contact time during which magnetization is transferred from the abundant nucleus (usually 1H) to the dilute nucleus (e.g. 13C). In the case of the 13C nuclei in organic samples, the build up of magnetization for each type of carbon depends on the extent of the dipolar coupling to the proton network. The extent of 13C - 1H dipolar coupling depends on both the degree of protonation for each type of carbon and any molecular motion (such as methyl group rotation) which may average the dipolar coupling. At longer contact times, the magnetization decays as a function of the T1(rho) of the protons. It should be noted that cross polarization is also affected by MAS. The length of the contact time should be chosen such that all types of carbons have had sufficient time to polarize yet not so long as to loose significant magnetization due to the proton T1(rho). For 13C CP/MAS an appropriate choice is usually between 1 and 10 ms. The figure below shows the effect of the duration of the contact time for the two 13C resonances of glycine. The 50 MHz 13C CP/MAS spectra were run as a function of contact time and plotted side by side. The intensities of each resonance are marked with color coded points. One can see that the carbonyl carbon builds up more slowly than the protonated carbon. An appropriate choice of contact time for glycine is 2 -3 msec.

3 comments:

Cortnie said...

This is for the alpha polymorph of glycine?

Glenn Facey said...

Hi Cortnie,

I'm not sure whether or not this is the alpha or gamma polymorph, however it seems to be a pure phase and not a mixture of polymorphs. See this link for more information on the polymorphs of glycine.

http://www.jointlab.nsu.ru/resources/education/chemistry/glycine_ascona_2004.pdf

Glenn

J.-F. Jacquinot said...

Dear Glenn,
may be it is worth mentionning that it is not always true that at long contact times magnetization decays as T1-rho. If the cross-polarisation time T-mix is longer than T1-rho, magnetization will grow with time constant T1-rho and decay with time constant T-mix.
(See I. Klur et al.:NMR cross-polarization when T-mix > T-1 rho; Examples from silica gel and calcium silicate hydrates, JOURNAL OF PHYSICAL CHEMISTRY B Volume: 104 Issue: 44 Pages: 10162-10167 Published: NOV 9 2000)

I find the blog well done and instructive.

Regards
Jacques-Fran├žois Jacquinot CEA Saclay FRANCE
jjacquinot@cea.fr