Nuclear magnetic relaxation studies of BH₄ in metal Borohydrides

Abstract

A different approach of nuclear magnetic resonance (NMR) relaxation of the rotational motion of BH₄ tetrahedron in light metal borohydrides was introduced. Previous NMR relaxation studies were mainly performed using an approximation which is based on a single exponential transition approach for the spin–lattice relaxation of the spin under quadrupolar interaction. In this new approach, a relaxation rate equation was derived by strictly taking into account both electric quadrupolar and magnetic dipolar contributions to the transitions between Zeeman energy states of quadrupolar ¹¹B nucleus that was coupled to many proton spins. Two exponential relaxation times derived from our theoretical treatment were applied to the analysis of ¹¹B spin–lattice relaxation of LiBH₄ and Ca(BH₄)₂. The activation energies and the correlation times of the reorientational motions of BH₄ were evaluated from the two experimental spin–lattice relaxation times. The results were compared to previous activation energies and correlation times.

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