Selective H-H recoupling via symmetry sequences in fully protonated samples at fast magic angle spinning.

Proton-detected solid-state NMR at fast Magic Angle Spinning (MAS) is becoming the norm to characterize molecules. Routinely H-H and H-X dipolar couplings are used to characterize the structure and dynamics of molecules. Selective proton recoupling techniques are emerging as a method for structural characterization via estimation of qualitative and quantitative distances. In the present study, we demonstrate through numerical simulations and experiments that the well-characterized CN sequences can also be tailored for selective recoupling of proton spins by employing C elements of the type (β)(4β)(3β). Herein, several CN sequences were examined through numerical simulations and experiments. C6 recoupling sequence with a modified POST-element ((β)(4β)(3β)) shows selective polarization transfer efficiencies on the order of 40-50% between various proton spin pairs in fully protonated samples at rf amplitudes ranging from 0.3 to 0.8 times the MAS frequency. These selective recoupling sequences have been labeled as frequency-selective-CN sequences. The extent of selectivity, polarization transfer efficiency and the feasibility of experimentally measuring proton-proton distances in fully protonated samples are explored here. The development of efficient and robust selective H-H recoupling experiments is required to structurally characterize molecules without artificial isotope enrichment or the need for diffracting crystals.