Nuclear overhauser effect (NOE) enhancement of ¹¹B NMR spectra of borane adducts in the solid state

The observation of an enhancement or a reduction in the intensity of the resonance of a spin, I, upon saturation or inversion of the resonance of a second spin, S, that lies close in space (<0.5 nm) is a key step in the NMR structure determination of both small and large molecules in solution. In addition, the heteronuclear NOE is routinely used to enhance the spectra of insensitive nuclei, such as 13C, in solution. The NOE is generally considered to be an NMR phenomenon of spin I=1/2 nuclei in solution, but here we show that a strong 11B {1H} NOE can be observed in the solid-state 11B NMR spectra of borane adducts, yielding fractional enhancements, fI{S}=(I - I0)/I0, of the magic angle spinning (MAS) NMR signal of up to 155%. This is an interesting and unusual observation as 11B (spin I=3/2) is a quadrupolar nucleus and the corresponding NOE is absent in solutions of the same materials.