Modified spectral editing methods for C-13 CP/MAS experiments in solids

The spectral editing approach of Zilm and coworkers utilizes polarization, polarization inversion, and spin depolarization methods for enhancing or su ppressing NMR spectral lines in solids. The proposed pulse sequences allow nonprotonated C, CH, CH2, and CH3 types of carbon resonances to be separate d from one another and identified accordingly. The former method tentativel y 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 organ ic molecules, but exceptions do exist that could constitute a serious drawb ack in a few instances. The new approach separates the nonprotonated C and CH3 carbon peaks unequivocally using modified pulse sequences similar to th ose of Zilm. Further, both the CH only and CH2 only spectra, respectively, can be acquired directly from combining so called (+) and (-) sequences usi ng 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 spectr a, combined with the previously reported CH3 and nonprontonated C only spec tra, offer a complete spectral editing technique for solid samples. Example s of these spectral editing methods are provided for 3-methylglutaric acid, fumaric acid monoethyl ester, and two complex natural products: methyl o-m ethylpodocarpate and 10-deacetylbaccatin III. (C) 2000 Academic Press.