The effect of methyl group rotation on 1H-1H solid-state NMR spin-diffusion spectra

Fast magic-angle spinning (MAS) NMR experiments open the way for proton-detected NMR studies and have been explored in the past years for a broad range of materials, comprising biomolecules and pharmaceuticals. Proton-spin diffusion (SD) is a versatile polarization-transfer mechanism and plays an important role in resonance assignment and structure determination. Recently, the occurrence of negative cross peaks in 2D 1H-1H SD-based spectra has been reported associated with higher-order SD effects, in which the chemical shifts of the involved quadruple of nuclei need to compensate each other. We herein report negative cross peaks in SD-based spectra observed for a variety of small organic molecules involving methyl groups. We combine experimental observations with numerical and analytical simulations to demonstrate that the methyl groups can give rise to coherent (SD) as well as incoherent (Nuclear Overhauser Enhancement, NOE) effects, both in principle manifesting themselves as negative cross peaks in the 2D spectra. The simulations however reveal that higher order coherent contributions dominate the experimentally observed negative peaks. Methyl groups are prone to the observation of such higher order coherent effects. Due to their low-frequency shifted 1H resonances, the chemical-shift separation relative to for instance aromatic protons in spatial proximity is substantial (> 4.7 ppm in the studied examples) preventing any sizeable second-order spin-diffusion processes, which would superimpose the negative contribution to the peaks.