11B Nuclear Spin-Electron Spin Interactions in 11B MAS NMR Spectra of Paramagnetic Metal Borohydrides

Isotropic and anisotropic couplings between the B-11 nuclear spin and the electron spin of paramagnetic electrons are observed in B-11 MAS NMR spectra of alpha-Pr(BH4)(3) and LiCe(BH4)(3)Cl. The B-11 NMR spectra of the central and satellite transitions are strongly influenced by the anisotropic nuclear spin-paramagnetic electron spin dipole-dipole interaction, which affects the spectra in a similar manner as the B-11 chemical shift anisotropy. The B-11 MAS NMR spectra are analyzed numerically, considering the quadrupole interaction and the anisotropic paramagnetic electron-nuclear spin dipole coupling along with the relative orientation of the two tensorial interactions. The observed isotropic shifts (delta(obs)) show a linear dependency with the inverse temperature for both compounds, indicating that delta(obs) is dominated by the contribution from the isotropic hyperfine shift. A similar temperature dependency is observed for the paramagnetic shift anisotropy for alpha-Pr(BH4)(3). The isotropic and anisotropic paramagnetic interactions are approximately 5 and 4 times larger for alpha-Pr(BH4)(3) as compared to LiCe(BH4)(3)Cl. This reflects mainly the difference in electron spin for the Pr3+ and Ce3+ ions and that the B site in alpha-Pr(BH4)(3) has two Pr3+ ions within a 3 angstrom distance, whereas boron in LiCe(BH4)(3)Cl only has a single Ce3+ site within this distance.

Automated summary

Isotropic and anisotropic couplings between the B-11 nuclear spin and the electron spin of paramagnetic electrons were observed in B-11 MAS NMR spectra of alpha-Pr(BH4)(3) and LiCe(BH4)(3)Cl. The isotropic and anisotropic paramagnetic interactions were found to be approximately 5 and 4 times larger for alpha-Pr(BH4)(3) compared to LiCe(BH4)(3)Cl, which mainly reflects the difference in electron spin for the Pr3+ and Ce3+ ions and the relative distances between ions in the compounds.