Distance measurements between boron and carbon at natural abundance using magic angle spinning REAPDOR NMR and a universal curve

Abstract

The rotational echo adiabatic passage double resonance (REAPDOR) magic angle spinning NMR experiment efficiently recouples the dipolar interaction between a spin-1/2 and a spin >1/2, enabling accurate and efficient measurement of their inter-nuclear distance. We demonstrate that under adiabatic conditions a universal curve that depends only on the inter-nuclear distance fits the REAPDOR recoupling curve for a ¹³C (spin-1/2)–¹¹B (spin-3/2) spin pair. In 4-(hydroxymethyl)phenylboronic acid MIDA ester the inter-nuclear distance between the methyl carbon and boron was determined to be 2.76 ± 0.14 Å, in agreement with the distance of 2.68 Å determined by X-ray crystallography. Similar experiments performed at two different spinning speeds and fit simultaneously to the same curve give a distance of 2.73 Å and distances to other carbons in the molecule are also determined. The low abundance of carbon-13 at natural abundance (1.1%) and the reduced abundance of boron-11 (80.1%) render the experiment insensitive to ¹³C–¹³C homonuclear couplings and significantly reduce the effect of a second boron nucleus at a longer distance. This approach can be extended to any spin-3/2 coupled to a spin-1/2.