Characterization of tricoordinate boron chemical shift tensors: Definitivehigh-field solid-state NMR evidence for anisotropic boron shielding

Despite the large known chemical shift (CS) range for boron and the large n umber of B-11 NMR studies of glasses, no boron CS tensors have been charact erized to date. We report the application of solid-state NMR techniques at moderate (9.4 T) and high (17.63 T) applied magnetic field strengths to the characterization of the boron CS tensors in trimesitylborane (BMes(3)) and triphenyl berate (B(OPh)(3)). The boron CS tenser of the former compound e xhibits a remarkably large span, Omega = 121 +/- 1 ppm, which encompasses t he known range of isotropic chemical shifts for tricoordinate boron compoun ds. Conversely, the effect of the boron CS tenser on the B-11 NMR spectra o f B(OPh)(3) is difficult to observe and quantify even at field strengths as high as 17.63 T; we find Omega less than or equal to 10 ppm. This marked d ifference in the boron nuclear magnetic shielding tensors is reproduced acc urately by a series of ab initio and DFT calculations with a range of basis sets. The difference is rationalized in the context of Ramsey's theory of nuclear magnetic shielding by considering contributions to the paramagnetic shielding in the tricoordinate boron plane. Differences in the in-plane sh ielding tenser components for the molecules considered are a result of vari ations in the effectiveness of the mixing of occupied a orbitals with virtu al pi orbitals under the influence of an applied magnetic field. A similar explanation has been invoked to rationalize C-13 isotropic chemical shifts in classical and nonclassical carbocations. We also report experimental and calculated boron nuclear quadrupolar coupling constants and asymmetry para meters for BMes(3) and B(OPh)(3). A combination of experimental and theoret ical results provides the orientation of the CS and electric field gradient tensors in the molecular framework.