Boron has two naturally occurring NMR active nuclei. Both nuclei have spins of greater than ½ and are quadrupolar. ¹¹B has a spin of 3/2 and ¹⁰B is spin 3. ¹¹B is the better nucleus in all respects, having the lower quadrupole moment and being more sensitive. Regular NMR tubes are made of borosilicate glass and therefore contain boron. As a result there is a broad signal in the spectrum arising from the tube. It is therefore preferable to use quartz tubes that do not contain boron although these are much more expensive and fragile than regular tubes. Both nuclei have the same wide chemical shift range. Each type of signal has a characteristic chemical shift range (fig. 1) which is the same for both nuclei.

Fig. 1. Chemical shift ranges for boron NMR

¹⁰Boron

¹⁰B-NMR (fig. 2) has a lower sensitivity and yields broader signals than ¹¹B. It is therefore preferable to use ¹¹B unless the sample is enriched in ¹⁰B as may be the case for samples intended for neutron capture applications.

Fig. 2. ¹⁰B-NMR spectrum of BF₃.OEt₂ in CDCl₃

Coupling to proton is not usually observed except in the smallest and most symmetric molecules such as BH₄^- (fig. 3).

Fig. 3. ¹⁰B-NMR spectrum of BH₄^- in D₂O

Properties of ¹⁰B

(Click here for explanation)

Property	Value
Spin	3
Natural abundance	19.9%
Chemical shift range	210 ppm, from -120 to 90
Frequency ratio (Ξ)	10.743658%
Reference compound	15% BF3.OEt2 in CDCl3
Linewidth of reference	9 Hz
T1 of reference	0.5 s
Receptivity rel. to 1H at natural abundance	3.96 × 10-3
Receptivity rel. to 1H when enriched	0.0199
Receptivity rel. to 13C at natural abundance	23.2
Receptivity rel. to 13C when enriched	117
Linewidth parameter	14 fm4

¹¹Boron

¹¹B-NMR (fig. 4) is more sensitive and yields sharper signals than ¹⁰B and is therefore usually the boron nuclide of choice. The coupling between boron and fluorine contain positive and negative components that cancel each other out so that a singlet is observed for BF₃ (Fig. 4). A broad signal in the spectrum arising from the boron in the NMR tube is apparent in the spectrum below.

Fig. 4. ¹¹B-NMR spectrum of BF₃.OEt₂ in CDCl₃

Coupling to proton is not usually observed except in the smallest and most symmetric molecules such as BH₄^- (fig. 5).

Fig. 5. ¹¹B-NMR spectrum of BH₄^- in D₂O

The proton spectrum of BH₄^- displays couplings to ¹¹B (quartet) and ¹⁰B (septet) (fig. 6).

Fig. 6. ¹H-NMR spectrum of BH₄^- in D₂O showing coupling to ¹⁰B and ¹¹B

Properties of ¹¹B

(Click here for explanation)

Property	Value
Spin	3/2
Natural abundance	80.1%
Chemical shift range	210 ppm, from -120 to 90
Frequency ratio (Ξ)	32.083974%
Reference compound	15% BF3.OEt2 in CDCl3
Linewidth of reference	5 Hz
T1 of reference	0.5 s
Receptivity rel. to 1H at natural abundance	0.165
Receptivity rel. to 1H when enriched	0.206
Receptivity rel. to 13C at natural abundance	777
Receptivity rel. to 13C when enriched	970
Linewidth parameter	22 fm4

Safety note

Some of the materials mentioned here are very dangerous. Ask a qualified chemist for advice before handling them. Qualified chemists should check the relevant safety literature before handling or giving advice about unfamiliar substances. NMR solvents are toxic and most are flammable. Specifically, BF₃.OEt₂ is toxic, corrosive and reacts violently with water and BF₄^- is toxic: wear protective gloves and work in a hood. Many other boron compounds are toxic.