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.