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Wednesday, October 23, 2013

FSLG CP HETCOR

Solid-state 1H MAS NMR spectra with resolution comparable to that obtained for liquids, are difficult (if not impossible) to obtain. The main problem is that magic angle spinning is unable to average the homonuclear 1H dipolar coupling interaction to zero.  The combined use of MAS and multiple pulse decoupling schemes (CRAMPS) can be used to improve the resolution.  In this case, the 1H FID is sampled during windows of the multiple pulse decoupling scheme where pulses are not being delivered, however the attainable resolution is still much less that that observed for liquids where the rapid molecular tumbling reduces the homonuclear dipolar interaction to zero.  Furthermore, CRAMPS experiments can be difficult to setup and run.  An alternative method of obtaining "high resolution" solid-state 1H NMR spectra (with resolution comparable to that of a CRAMPS spectrum) is a frequency switched Lee-Goldburg cross polarization heteronuclear correlation experiment (FSLG CP HETCOR) where the 1H spectrum is obtained in the indirect dimension of a 2D experiment.

In this pulse scheme, used in conjunction with MAS, 1H magnetization is aligned at the magic angle and subjected to FSLG decoupling where it is forced to precess about a field oriented at the magic angle by using 2π pulses with carefully chosen offset frequencies.  The ideal effect is to average the homonuclear dipolar coupling to zero.  The FSLG decoupling train serves as the evolution time (t1) in a 2D data collection scheme.  During the variable evolution period the 1H chemical shifts evolve while the heteronuclear dipolar coupling is averaged by MAS and the homonuclear dipolar coupling is averaged by both the MAS and the FSLG pulse train.  The 1H magnetization is then returned to the transverse axis and cross polarization (CP) is used to transfer the frequency encoded proton magnetization to 13C.  The 13C FID is observed while 1heteronuclear decoupling is applied.  If CP contact times are chosen sufficiently short, one obtains a 2D 13C-1H dipolar correlation map with correlations present between carbon resonances and the protons to which they are most strongly dipolar coupled.  If longer contact times are used, more correlations will appear resulting from longer range dipolar couplings and 1H spin diffusion.  In either case, the 1H projection of the data represents a high resolution 1H spectrum of the sample with resolution comparable to or better than a CRAMPS spectrum.  The figure below shows FSLG 13C-1H CP HETCOR spectra for Dianin's compound acquired on a 200 MHz spectrometer using a spinning speed of 5 kHz.

The spectrum on the right was acquired with a 50 µsec contact time and shows the aromatic carbon resonances correlated to aromatic proton resonances and the aliphatic carbon resonances correlated with the aliphatic proton resonances.  The spectrum on the left was acquired with a 300 µsec contact time and shows all of the 13C resonances correlated to all of the 1H resonances.  In both cases the 1H projection is a high resolution 1H NMR spectrum.

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