Orphan spin operator diagonal suppression

Abstract

We report the suppression of diagonal signals in (H)N(H)(H)NH spectra through the use of untransferred signals (orphan spin operators) from cross polarization steps. In contrast to most diagonal suppression methods, there is no loss of sensitivity for cross-peaks at ultrafast MAS. For the proton-detected (H)N(H)(H)NH sequence, recorded with 55 kHz MAS, we observed approximately 50% suppression of diagonal signal after 1.08 ms mixing. A similar construction for CHHC spectra at 13 kHz MAS suppresses both the diagonal and cross-peaks, and simulations suggest proton assisted recoupling (PAR) is responsible. Alternatively, a combined PAR+CHHC spectrum can be recorded.