Asynchronous MASSLF spectroscopy: a convenient method for assigning solid-state carbon-13 CPMAS spectra

Abstract

A two-dimensional solid-state NMR technique is presented which permits assignment of {sup 13}C CPMAS spectra on the basis of the number of protons bonded to a carbon center. The method is similar to several other methods that have become popular for accurately determining C-H or N-H bond distances in solids and for following molecular motions in polymers. These techniques, referred to collectively as MASSLF spectroscopy, rely on the use of MAS NMR to resolve dipolar coupled patterns that would normally overlap in static one-dimensional spectra. The dipolar coupled patterns generated by these techniques are sufficiently sensitive that small variations in C-H distances in organic solids can be accurately measured. The patterns are, however, much more sensitive to the number of protons directly bonded to the carbon center. Under conditions of fast magic angle spinning, methylenes typically exhibit twice the number of sidebands as methines, and rapidly rotating methyl groups and nonprotonated carbons usually exhibit only one set of sidebands. The method presented here determines the number of methylenes, methines, and methyl plus nonprotonated carbons contributing to a resonance by taking advantage of the fact that the dipolar patterns have characteristic and predictable relative sideband intensities. A new MASSLF pulse sequencemore » is used in this work that differs from previous sequences by not requiring synchronization with the MAS rotation. In addition the sequence conveniently allows for a much wider bandwidth in the dipolar dimension and produces the effect of quadrature detection in this frequency domain without the need for multiple data sets.« less