Composite-pulse and partially dipolar dephased multiCP for improved quantitative solid-state 13C NMR

Improved multiple cross polarization (multiCP) pulse sequences for quickly acquiring quantitative C-13 NMR spectra of organic solids are presented. Loss of C-13 magnetization due to imperfect read-out and storage pulses in multiCP has been identified as a significant mechanism limiting polarization enhancement for C-13 sites with weak couplings to H-1. This problem can be greatly reduced by composite 90 pulses with non-orthogonal phases that flip the magnetization onto the spin-lock field and back to the longitudinal direction for the H-1 repolarization period; the observed loss is <3% for over +/- 10 kHz resonance offset and up to 20% flip-angle error. This composite-pulse multiCP (ComPmultiCP) sequence consistently provides performance superior to that of conventional multiCP, without any trade-off. The longer total CP time enabled by the composite pulses allows for a wider amplitude ramp during CP, which decreases the sensitivity to Hartmann-Hahn mismatch by a factor of two, with a <7% root-mean-square deviation within a 1-dB range for Boc-alanine. In samples with very short T-1 rho, under-polarization of non-protonated carbons can be compensated by slight dipolar dephasing of CHn signals resulting from relatively weak decoupling during the Hahn spin echo period before detection. Quantitative spectra have been obtained by ComPmultiCP for low-crystallinity branched polyethylene at 4.5 kHz MAS, and in combination with partial dipolar dephasing for soil organic matter at 14 kHz MAS. (C) 2017 Elsevier Inc. All rights reserved.