1H NMR with 11B Decoupling

Boron has two isotopes with spin. ¹¹B has a spin I = 3/2 with a natural abundance of 80.42% while ¹⁰B has a spin I = 3 with a natural abundance of 19.58%. The ¹H NMR signals of protonated boron compunds are often quite complicated due to resolved or partially resolved J coupling between the protons and both ¹¹B and ¹⁰B. A signal for a proton coupled to a single boron would be the sum of two multiplets (a 1:1:1:1 quartet from ¹¹B and a 1:1:1:1:1:1:1 septet from ¹⁰B) in a ~ 4:1 ratio. Proton NMR spectra exhibiting boron coupling can be simplified considerably with the application of ¹¹B decoupling. In such a spectrum, the signals for the protons coupled to ¹¹B collapse into singlets while the signals for the protons coupled to ¹⁰B remain as multiplets. However, since ¹⁰B is only 19.58% abundant and the intensity is split into a multiplet, these signals are a tolerable contribution at the base of the narrower ¹H signals. The figure below shows a standard ¹H NMR spectrum and a ¹H spectrum with ¹¹B decoupling for ortho-carborane (C₂B₁₀H₁₂). The protons attached to the carbon atoms account for the sharp signal at high frequency while the protons attached to boron account for the rest of the spectrum. One can see the dramatic simplification in the spectrum with ¹¹B decoupling. One can also see the small contribution from the protons coupled to ¹⁰B in the baseline. The ¹H [¹¹B] spectrum confirms that there are 4 chemical shift inequivalent types of boron bearing protons which allows for an easy analysis of ¹H - ¹¹B J coupling in the standard ¹H spectrum (color coded in the figure). This analysis would be difficult or impossible in the absence of the ¹H [¹¹B] data.

Thank you to Dominique Duguay for providing the sample for the figure.