Dipolar Heteronuclear Correlation Solid-State NMR Experiments Between Half-Integer Quadrupolar Nuclei: The Case of 11B-17O

Abstract

With 73 % of all NMR-active nuclei being quadrupolar, there is great interest in the development of NMR experiments that can probe the proximity between quadrupolar spins. Here, several pulse sequences for magic angle spinning (MAS) 11B-17O Resonance-Echo Saturation-Pulse DOuble-Resonance (RESPDOR) and dipolar Heteronuclear Multiple Quantum Correlation (D-HMQC) solid-state NMR experiments were investigated. In these pulse sequences, Rotational-Echo Double-Resonance (REDOR) recoupling with central transition (CT) selective 180 pulses were applied to either the 11B or 17O spins to recouple the 11B-17O dipolar interactions. 11B{17O} RESPDOR experiments on model 17O-enriched boronic acids showed that application of dipolar recoupling on the 11B channel yielded more signal dephasing than when recoupling is applied on the 17O channel; however, short effective 11B transverse relaxation time constants (T2’) hinders the acquisition of dephasing curves out to long recoupling durations. Application of REDOR recoupling to 17O spins was found to produce significant dephasing without compromising the 11B T2’. Comparison of experimental 11B{17O} RESPDOR curves to that of numerical simulations enabled the 17O isotopic enrichment to be estimated. 2D 11B{17O} D-HMQC spectra were recorded with either 11B or 17O REDOR recoupling at a variety of radio frequency field conditions. Lastly, 2D 11B{17O} and 23Na{17O} D-HMQC spectra of a 17O-enriched sodium borate glass were acquired to demonstrate the practical application of these heteronuclear correlation experiments to probe structural connectivity between two quadrupolar spins. Importantly, the high-field 2D 11B-17O D-HMQC NMR spectrum revealed two unique 17O sites correlating to 4-coordinate BO4 (B[4]), which were attributed to the expected B[3]-O-B[4] and unexpected B[4]-O-B[4] bridging O atoms. The heteronuclear correlation experiments outlined here should be applicable to a variety of quadrupolar spin pairs.