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

With 73% of all NMR-active nuclei being quadrupolar, there is great interest in the development of NMR experiments that can probe the proximity of quadrupolar spins. Here, pulse sequences for magic-angle spinning (MAS) B-11-O-17 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 was used with central transition (CT)-selective pi-pulses applied to either the B-17 or O-17 spins to recouple B-11-O-17 dipolar interactions. B-11{O-17} RESPDOR experiments on O-17-enriched boric acid and benzene diboronic acid showed that application of dipolar recoupling on the B-11 channel yielded more signal dephasing than when recoupling is applied on the O-17 channel; however, short effective B-17 transverse relaxation time constants (T2') hinder 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 those of numerical simulations enabled the 17O isotopic enrichment to be estimated. 2D 11B{17O} D-HMQC spectra were recorded with either 11B or O-17 REDOR recoupling under a variety of radio frequency field conditions. Lastly, 2D 11B{17O} and 23Na{17O} D-HMQC spectra of an 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 B-11-O-17 D-HMQC NMR spectrum revealed two unique 17O sites correlating to 4-coordinate BO4 ([4]B), which were attributed to the [3]B-O-[4]B and [4]B-O-[4]B bridging O atoms. The heteronuclear correlation experiments outlined here should be applicable to a variety of quadrupolar spin pairs.