Identifying H–N proximities in solid-state NMR using 14N overtone irradiation under fast MAS

Abstract

Two NMR methods for the identification of ¹H–¹⁴N proximities in solid powder samples are considered. Both approaches involve the irradiation of the ¹⁴N overtone transition (Δm = 2) and observation of ¹H signals under fast MAS and at an intermediate magnetic field strength (11.7 T). First, the recently reported ¹H–¹⁴N^OT HMQC experiment is developed into a generally-applicable, broadband version using frequency-swept WURST pulses capable of exciting the full range of ¹⁴N^OT signals. We demonstrate the validity of this approach on various samples, also showing that standard pulses can be used to selectively excite individual nitrogen sites and provide more accurate indirect ¹⁴N^OT powder patterns from which ¹⁴N NMR parameters can be extracted if required. As a second approach, we present some preliminary results demonstrating the feasibility of ¹H–¹⁴N^OT overtone rotary recoupling (ORR) under fast MAS. Dephasing fractions of up to 0.3 have been obtained using a basic TRAPDOR-style pulse sequence, a simple approach that shows much potential for optimisation. WURST pulses are also shown to provide improvements in the recoupling bandwidth. Crucially, for both of these methods our experimental results and exact numerical simulations confirm that the +2ω_r overtone sideband should be irradiated for optimal experimental efficiency.