Determination of NH proton chemical shift anisotropy with (14)N-(1)H heteronuclear decoupling using ultrafast magic angle spinning solid-state NMR.

The extraction of chemical shift anisotropy (CSA) tensors of protons either directly bonded to (14)N nuclei (I=1) or lying in their vicinity using rotor-synchronous recoupling pulse sequence is always fraught with difficulty due to simultaneous recoupling of (14)N-(1)H heteronuclear dipolar couplings and the lack of methods to efficiently decouple these interactions. This difficulty mainly arises from the presence of large (14)N quadrupolar interactions in comparison to the rf field that can practically be achieved. In the present work it is demonstrated that the application of on-resonance (14)N-(1)H decoupling with rf field strength ∼30 times weaker than the (14)N quadrupolar coupling during (1)H CSA recoupling under ultrafast MAS (90kHz) results in CSA lineshapes that are free from any distortions from recoupled (14)N-(1)H interactions. With the use of extensive numerical simulations we have shown the applicability of our proposed method on a naturally abundant l-Histidine HCl·H2O sample.