Boron–boronJcoupling constants are unique probes of electronic structure: a solid-state NMR and molecular orbital study

Abstract

J couplings measured between 11B spin pairs in solid diboron compounds provide insight into electronic structure and crystallographic symmetry.

Solid-State NMR at the University of Ottawa

“…Finally, this work demonstrated how the X-quadrupolar coupling constant can be taken as a measure of the strength of the halogen bond in structurally similar systems: the p-type NLMO centred on Cl − identified as dominating the value of the quadrupolar coupling constant is the same orbital that donates electron density to the -hole on iodine. 13 C CP/MAS NMR spectroscopy on related systems additionally shows how the carbon chemical shifts of C-I … Y-Z fragments are characteristic of the halogen bond. When the carbon-iodine bond lengthens, the 13 C chemical shift is observed to increase.…”

“…In this section, I will describe some of the recent work of graduate student Frédéric Perras on the measurement and interpretation of J couplings between quadrupolar nuclei. 10,11,12,13,14 Most metal nuclides are quadrupolar, and their NMR spectra are typically dominated by the quadrupolar interaction. This often results in fast relaxation in solution, effectively wiping out the resonances, or very broad powder patterns in solids.…”

“…13 Quantum chemical calculations revealed several useful trends, including an inverse correlation between J( 11 B, 11 B) and the B-B bond length. A further chemical application described in this paper includes probing of the reaction of bis(catecholato)diboron with an N-heteroyclic carbene to form an sp 2 -sp 3 diboron compound of importance in nucleophilic ␤-borylation reactions.…”

“…Polymorphism refers to the ability of a substance to crystallize in multiple crystalline forms. 13 C CPMAS NMR is often used in academia and in industry to study polymorphism in solids, including drugs. While X-ray diffraction is also useful, the amorphous nature of some pharmaceuticals means that solid-state NMR can provide key advantages over diffraction.…”

“…While X-ray diffraction is also useful, the amorphous nature of some pharmaceuticals means that solid-state NMR can provide key advantages over diffraction. We have been particularly interested in applying 23 Na solid-state NMR to the study of pharmaceutical polymorphs and hydrates, because many drugs are formulated as sodium salts and because 23 Na offers spectra with no background resonances due to excipients, unlike 13 C. Below, I describe two recent 23 Na solid-state NMR studies of sodium naproxen and of sodium valproate formulations. 24,25 Sodium naproxen (SN) is a nonsteroidal anti-inflammatory drug that is available commercially as Aleve.…”