NMR Study of Anion Dynamics in Solid KAlH4

作者：Sorte Eric G^*; Emery Samuel B; Majzoub E H; Ellis Caleo Tim; Ma Zayd L; Hammann Blake A; Hayes Sophia E; Bowman Robert C Jr; Conradi Mark S

来源：Journal of Physical Chemistry C, 2014, 118(11): 5725-5732.

DOI：10.1021/jp5001978

H-1 and Al-27 NMR is used to reveal the motions of AlH4 anions in KAlH4. Line-narrowing from rotations and from translational diffusion is observed in the NMR of both nuclei. Unlike the anions in NaAlH4 and LiAlH4 that are not rotating on the NMR time scale at room temperature, the KAlH4 anions are already rotating rapidly at 23 degrees C. Based on the onset of rotation-induced line narrowing, the H-1 T-1 minimum, and the low- temperature hydrogen T-1 rho minimum associated with reorientations, the rotational activation energy E-rot,E-act = 0.28 eV is determined. Similarly, we use the onset temperature of translational motion-induced line narrowing and the high-temperature T-1 rho minimum to determine the diffusion activation energy E-diff,E-act = 0.70 eV. Lack of sharp structure in the first-order quadrupole pattern and the absence of second-order quadrupole structure in the Al-27 NMR data suggest asymmetry (eta not equal 0) and/or variations in the anion electric field gradients from structural disorder.

• 出版日期2014-3-20