As you can see, the spectrum consists of a large 1:1:1:1 quartet with 1JB-H = 80.6 Hz and a 1:1:1:1:1:1:1 heptet with a 1JB-H = 27.0 Hz, both centred around −0.01 ppm. The splitting pattern observed in the spectrum is due to coupling of the proton signals in BH4− with the 11B and 10B nuclei, which have nuclear spins of 3/2 and 3, respectively. This is a common phenomenon for these quadrupolar nuclei. Using the “2In + 1” rule, where I is the nuclear spin and n is the number of nuclei, it is easily determined that the 10B nucleus will split the proton signal into a heptet while the 11B nucleus will give the quartet. Due to the inherently quantitative nature of the NMR experiment, comparing the sum of the areas of the heptet (10B) with the combined peak areas of the quartet (11B) will give the isotopic distribution of the two boron isotopes. This results in an isotopic distribution of 18.35% for 10B and 81.65% for 11B which compares very well with the known ratio of 19.90% : 80.10%.[2]
While it is unlikely that this method could be applied to any other nuclei, this experiment provides an excellent introduction to isotopic ratios, coupling to quadrupolar nuclei, and a novel application of the NMR experiment beyond structural elucidation.
References
[1] a) Walker, J. M.; Starks, R. J.; Gray, G. A.; Schoolery, J. N.
Appl. Spectrosc. 1981,
35, 607;
b) Zanger, M.; Moyna, G.
J. Chem. Ed. 2005,
82, 1390.
[2] Coursey, J. S.; Schwab, D. J.; Dragoset, R. A. NIST Atomic Weights and Isotopic Compositions Home Page http://physics.nist.gov/Comp
(accessed November 2021).