Methylene-only subspectra in C-13 CPMAS using a new double quantum filtering sequence

Methodology for the assignment of C-13 CPMAS spectra is still in its infanc y. Previous methods of CPMAS spectral editing have utilized differences in the strength of the C-13-H-1 dipolar interaction or the rate and spin therm odynamics of crosspolarization from protons to carbon, to differentiate bet ween quaternary, tertiary, and methylene carbons. We introduce a different approach, which is based on the fact that double-quantum coherence develops between the protons of a methylene group considerably faster than between most other proton spin pairs in an organic solid. We generate this coherenc e, filter it, convert it back to single quantum, and then crosspolarize sel ectively to carbon, followed by a short period of reversed crosspolarizatio n to null out unwanted coherence generated from longer distance spin pairs. The sequence has been named DQCP. While the signal-to-noise of this method is poorer than ordinary CP, it is comparable to previous methods for gener ating methylene-only spectra, and the technique is straightforward and easy to implement, (C) 1999 Academic Press.