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Chemical exchange in NMR interconverts environments during the observation. Fast exchange relative to their frequency difference produces averaged shifts; slow exchange resolves separate sites. A single peak alone does not prove a single static geometry.

$B$ is a 1:1 nitrogen-bound adduct, with $\mathrm{N}\to \mathrm{AlB}{\mathrm{r}}_3$ at one ring nitrogen. Its lone pair is diverted from N-to-B pi donation into the new coordinate bond. That nitrogen becomes approximately tetrahedral and the ring can pucker. The two B–N bonds adjacent to it lose pi character and lengthen; the neighboring unbound nitrogen atoms can donate more strongly into those borons, shortening the next pair of B–N bonds. The remaining pair forms a third class. The second structural panel marks these three two-bond sets; it represents connectivity and bond classes, not a measured geometry.

In the slow-exchange structure, two boron atoms flank the bound nitrogen and are equivalent to one another, while the third boron is unique. This gives two ${}^{11}$B environments in a $2:1$ ratio. At room temperature, rapid exchange of the Lewis acid among the three equivalent nitrogen sites averages every boron over the same environments, giving one boron-11 nuclear magnetic resonance signal. Cooling slows exchange relative to the resonance-frequency difference and resolves the two environments.

This is Chemical exchange in NMR, not restoration of equal instantaneous B–N bonds. A spectrum alone cannot distinguish intramolecular migration from reversible dissociation and reassociation; concentration dependence and crossover experiments would test that distinction. Related N-bound borazine adducts have been characterized by diffraction and variable-temperature NMR, supporting the structural logic while not supplying the exact temperature dependence of this particular hexamethyl compound.^[ref]

At low temperature the four terminal methyl groups contain 12 equivalent protons and the two bridging methyls contain six, giving the $2:1$ pair of singlets. On warming, terminal and bridging positions interchange rapidly on the NMR timescale. A methyl group then samples both environments during the observation, producing a single population-weighted resonance, ${\delta }_{\mathrm{av}}=(2{\delta }_{\mathrm{term}}+{\delta }_{\mathrm{bridge}})/3$. This Chemical exchange in NMR is not merely faster rotation of each methyl about its own Al–C direction, which cannot turn a terminal site into a bridge.

The concentration independence argues against a rate-limiting collision between two intact dimers. An intramolecular bridge-opening and reclosing rearrangement can exchange which methyls are bridges; a unimolecular dissociation followed by rapid reassociation is another mechanism whose observed rate could be concentration independent. The spectra establish rapid site exchange, but not a unique elementary exchange pathway. A variable-temperature line-shape analysis or exchange experiment would be needed to distinguish detailed mechanisms. Primary organoaluminium studies describe the same low-temperature resolution and warm-temperature averaging, while considering dimer dissociation.^[ref]