Measurement of C-H distances in solids at natural abundance via proton-detected fast MAS NMR spectroscopy.

Measurement of heteronuclear distances from the radio-frequency (rf) pulse-based recoupling of NMR experiments is vital for structural refinement and dynamics studies at the atomic level. Despite advancements in the design and development of recoupling methods, the extraction of the directly-bonded heteronuclear XH distances in samples with XH moieties remains challenging. This is primarily due to the interference effects from the neighboring spins/local fields leading to distorted dipolar coupling powder lineshapes. In this regard, we present a proton-detected 3D C chemical shift (CS)/C-H dipolar coupling/H chemical shift (CS) correlation experiment under fast magic angle spinning (MAS), which has the potential to measure the directly-bonded CH distances in naturally abundant samples with CH groups. We have implemented the windowless ROCSA-based C-H dipolar interaction recoupling scheme under H evolution to achieve undistorted C-H dipolar coupling powder lineshapes in contrast to our previously reported windowless ROCSA-DIPSHIFT method under X-nuclei evolution, wherein extraction of the directly-bonded CH distances in samples with CH groups is shown to be sensitive to the presence of the local fields due to neighboring spins. C-H distances reported from the method presented in this work are also validated from the results emerging from the quantum chemical calculations.