Boron-11 nuclear magnetic resonance spectroscopy (boron NMR or 11B NMR) is an analytical technique and form of nuclear magnetic resonance spectroscopy used to detect and identify boron atoms in boron-containing molecules. Although 10B is also an NMR-active nucleus, 11B NMR is more widely used. Since using normal borosilicate glass NMR tubes contain boron and can heavily interfere with the 11B NMR signal, alternatives are used, such as quartz glass.

Nucleus Receptivity 11B NMR has relatively high sensitivity, since the 11B isotope has high natural abundance. The gyromagnetic ratio of the nucleus is greater than that of 10B and also slightly greater than that of 13C. Additionally, the electric quadrupole moment (Q) of 11B is lower than that of 10B. Due to these reasons, 11B is more favorable for NMR than 10B, the other major boron isotope. Property 10B 11BAbundance19.9%80.1%Spin3Gyromagnetic ratio (rad T-1 s-1)2.874⋅1078.584⋅107 Gyromagnetic ratio (MHz T-1)4.574733913.6616080Electric quadrupole moment (mb)84.540.59

The T1 (relaxation time) for 11B is 10-2–10-3 seconds, much shorter than nuclei such as 1H or 13C. Quadrupolar effects

11B is a quadrupolar nucleus—meaning it has a non-spherical charge distribution—and predominantly undergoes the quadrupolar relaxation mechanism, resulting in broad peaks of over 10 Hz, which can obscure fine J-coupling.

Chemical shifts

Typically, the reference sample defined as 0 ppm in 11B NMR is BF3·OEt2, with an absolute frequency of 32.083974 MHz with respect to 100.00 MHz. Boric acid, H3BO3, may also be used as a reference in solid-state 11B NMR, with a peak at 14.9 ppm. The solvent effect on 11B NMR chemical shifts are small enough in magnitude that they can typically be neglected, although noticeable solvent shifts of up to 4 ppm have been observed, namely for polyhedral boranes and related compounds.

The chemical shift range for 11B NMR is typically +100 to -120 ppm. Specifically, typical chemical shifts for different boron chemical environments are roughly: Trigonal planar 11B: +100 to 0 ppm,

Tetrahedral 11B: 0 ppm to -120 ppm. However, factors such as line shapes and quadrupolar coupling constants must be accounted for in order to assign structures, and chemical shifts alone are not always able to determine structures. For example, some tetrahedral boron complexes may show positive chemical shifts, depending on the environment.

Coupling In solution-state, several 11B-11B coupling constants (J) have been measured to range from 9-151 Hz. In solid-state, the coupling constants range from 98-136 Hz. The reported coupling constant for terminal 11B-1H bonds range from 100-190 Hz, while that of a hydrogen bridging bond is less than 80 Hz. Fine coupling may be obscured by broad peaks caused by the aforementioned quadrupolar effects.

Applications 11B NMR is used for identifying the environment of boron in boron-containing molecules such as diboron compounds, boronic esters, boronic acids, borates, and boron-containing Lewis acid-base adducts. It has been used to study reactions such as hydroboration.

11B also has biomedical applications, such as Boron Neutron Capture Therapy.

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