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

Abstract

To obtain piercing insights into inter and intramolecular H-bonding, and π-electron interactions measurement of 1H 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 1H/1H radio frequency-driven recoupling (RFDR) which enhances spin diffusion through recoupled 1H–1H dipolar couplings under ultrafast magic angle spinning (MAS) frequency (70kHz). Relative orientation between two interacting 1H CSA tensors is obtained by fitting two-interacting 1H CSA tensors by fitting two-dimensional (2D) 1H/1H CSA/CSA spectral slices through extensive numerical simulations. To recouple 1H CSAs in the indirect frequency dimensions of a 3D experiment we have employed γ-encoded radio frequency (RF) pulse sequence based on R-symmetry (R1887) with a series of phase-alternated 2700°–90180° composite-180° pulses on citric acid sample. Due to robustness of applied 1H CSA recoupling sequence towards the presence of RF field inhomogeneity, we have successfully achieved an excellent 1H/1H CSA/CSA cross-correlation efficiency between H-bonded sites of citric acid.