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Thursday, April 3, 2008

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.

1 comment:

Egon Willighagen said...

Brilliant compound! I like boron compounds, adding some extra diversity to organic chemistry, and added many (simple) boron NMR spectra to the NMRShiftDB , but have not seen one of this connectivity :)