Composite-180° pulse-based symmetry sequences to recouple proton chemical shift anisotropy tensors under ultrafast MAS solid-state NMR spectroscopy

There is considerable interest in the measurement of proton (H-1) chemical shift anisotropy (CSA) tensors to obtain deeper insights into H-bonding interactions which find numerous applications in chemical and biological systems. However, the presence of strong H-1/H-1 dipolar interaction makes it difficult to determine small size H-1 CSAs from the homogeneously broadened NMR spectra. Previously reported pulse sequences for H-1 CSA recoupling are prone to the effects of radio frequency field (B-1) inhomogeneity. In the present work we have carried out a systematic study using both numerical and experimental approaches to evaluate gamma-encoded radio frequency (RF) pulse sequences based on R-symmetries that recouple H-1 CSA in the indirect dimension of a 2D H-1/H-1 anisotropic/isotropic chemical shift correlation experiment under ultrafast magic angle spinning (MAS) frequencies. The spectral resolution and sensitivity can be significantly improved in both frequency dimensions of the 2D H-1/H-1 correlation spectrum without decoupling H-1/H-1 dipolar couplings but by using ultrafast MAS rates up to 70 kHz. We successfully demonstrate that with a reasonable RF field requirement (<200 kHz) a set of symmetry-based recoupiing sequences, with a series of phase-alternating 270 degrees(0)-90 degrees(180) composite-180 degrees pulses, are more robust in combating B-1 inhomogeneity effects. In addition, our results show that the new pulse sequences render remarkable H-1 CSA recoupling efficiency and undistorted CSA lineshapes. Experimental results on citric acid and malonic acid comparing the efficiencies of these newly developed pulse sequences with that of previously reported CSA recoupling pulse sequences are also reported under ultrafast MAS conditions. (C) 2014 Elsevier Inc. All rights reserved.