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Friday, August 11, 2017

Boron Isotope Effects in Fluorine NMR Spectra

In previous posts on this BLOG, examples of  1H/2H and 12C/13C isotope effects were discussed.  The figure below shows an example of a 10B/11B isotope effect observed in the 19F NMR spectrum of tetrabutyl ammonium tetrafluorobarate.
The spectrum clearly shows two resonances separated by 0.05 ppm with an intensity ratio of approximately 20:80 corresponding to the natural abundances of  10B and 11B, respectively.  The low frequency resonance is due to 11BF4-.  Since  11B is a spin I = 3/2 nuclide we observe a 1:1:1:1 quartet with J = 1.25 Hz corresponding to the one bond 19F - 11B coupling.  The high frequency resonance is due to 10BF4-.  Since  10B is a spin I = 3 nuclide we observe a very poorly resolved 1:1:1:1:1:1:1 septet with J ~ 0.4 Hz corresponding to the one bond 19F - 10B coupling.