Spin-Spin Coupling between Quadrupolar Nuclei in Solids: 11B-75As Spin Pairs in Lewis Acid-Base Adducts

Solid-state B-11 NMR measurements of Lewis acid base adducts of the form R3AsBR3' (R = Me, Et, Ph; R' = H, Ph, C6F5) were carried out at several magnetic field strengths (e.g., B-0 = 21.14, 11.75, and 7.05 T). The B-11 NMR spectra of these adducts exhibit residual dipolar coupling under MAS conditions, allowing for the determination of effective dipolar coupling constants, R-eff(As-75, B-11) as well as the sign of the As-75 nuclear quadrupolar coupling constants. Values of R-eff(As-75, B-11) range from 500 to 700 Hz. Small isotropic I-couplings are resolved in some cases, and the sign of (1)J(As-75, B-11) is determined. Values of C-Q(As-75) measured at B-0 = 21.14 T for these triarylborane Lewis acid base adducts range from -82 +/- 2 MHz for Et3AsB(C6F5)(3) to -146 +/- 1 MHz for Ph3AsBPh3. For Ph3AsBH3, two crystallographically nonequivalent sites are identified with C-Q(As-75) values of -153 and -151 +/- 1 MHz. For the uncoordinated Lewis base, Ph3As, four As-75 sites with C-Q(As-75) values ranging from 193.5 to -194.4 +/- 2 MHz are identified. At these applied magnetic field strengths, the As-75 quadrupolar interaction does not satisfy high-field approximation criteria, and thus, an exact treatment was used to describe this interaction in B-11 and As-75 NMR spectral simulations. NMR parameters calculated using the ADF and CASTEP program packages support the experimentally derived parameters in both magnitude and sign. These experiments add to the limited body of literature on solid-state As-75 NMR spectroscopy and serve as examples of spin spin-coupled quadrupolar spin pairs, which are also rarely treated in the literature.