Modified Spectral Editing Methods for 13C CP/MAS Experiments in Solids

Abstract

The spectral editing approach of Zilm and coworkers utilizes polarization, polarization inversion, and spin depolarization methods for enhancing or suppressing NMR spectral lines in solids. The proposed pulse sequences allow nonprotonated C, CH, CH2, and CH3 types of carbon resonances to be separated from one another and identified accordingly. The former method tentatively separates the nonprotonated C and CH3 peaks with a cutoff shift of 35 ppm. This shift is a reasonable demarcation shift for a preponderance of organic molecules, but exceptions do exist that could constitute a serious drawback in a few instances. The new approach separates the nonprotonated C and CH3 carbon peaks unequivocally using modified pulse sequences similar to those of Zilm. Further, both the CH only and CH2 only spectra, respectively, can be acquired directly from combining so called (+) and (−) sequences using different spectral delay periods and pulse parameters. The (+) and the (−) pulse sequences produce signals for the nonprotonated and methyl carbons that have essentially the same amplitude but opposite phases. These spectra, combined with the previously reported CH3 and nonprontonated C only spectra, offer a complete spectral editing technique for solid samples. Examples of these spectral editing methods are provided for 3-methylglutaric acid, fumaric acid monoethyl ester, and two complex natural products: methyl o-methylpodocarpate and 10-deacetylbaccatin III.