Determination of relative orientation between 1H CSA tensors from a 3D solid-state NMR experiment mediated through 1H/1H RFDR mixing under ultrafast MAS

To obtain piercing insights into inter and intrarnolecular H-bonding, and pi-electron interactions measurement of H-1 chemical shift anisotropy (CSA) tensors is gradually becoming an obvious choice. While the magnitude of CSA tensors provides unique information about the local electronic environment surrounding the nucleus, the relative orientation between these tensors can offer further insights into the spatial arrangement of interacting nuclei in their respective three-dimensional (3D) space. In this regard, we present a 3D anisotropic/anisotropic/isotropic proton chemical shift (CSA/CSA/CS) correlation experiment mediated through H-1/H-1 radio frequency-driven recoupling (RFDR) which enhances spin diffusion through recoupled H-1-H-1 dipolar couplings under ultralast magic angle spinning (MAS) frequency (70 kHz). Relative orientation between two interacting H-1 CSA tensors is obtained by fitting two-interacting H-1 CSA tensors by fitting two-dimensional (2D) H-1/H-1 CSA/CSA spectral slices through extensive numerical simulations. To recouple H-1 CSAs in the indirect frequency dimensions of a 3D experiment we have employed gamma-encodecl radio frequency (RE) pulse sequence based on R-symmetry (R18(8)(7)) with a series of phase-alternated 270(0)degrees-90(180)degrees composite-180 degrees pulses on citric acid sample. Due to robustness of applied H-1 CSA recoupling sequence towards the presence of RE field inhomogeneity, we have successfully achieved an excellent H-1/H-1 CSA/CSA cross-correlation efficiency between H-bonded sites of citric acid. (C) 2015 Elsevier Inc. All rights reserved.