NMR Study of Anion Dynamics in Solid KAlH4

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• 作者：Eric G. Sorte ; Samuel B. Emery ; E. H. Majzoub ; Tim Ellis-Caleo ; Zayd L. Ma ; Blake A. Hammann ; Sophia E. Hayes ; Robert C. Bowman ; Jr. ; Mark S. Conradi
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：March 20, 2014
• 年：2014
• 卷：118
• 期：11
• 页码：5725-5732
• 全文大小：376K
• 年卷期：v.118,no.11(March 20, 2014)
• ISSN：1932-7455

文摘

¹H and ²⁷Al 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 掳C. Based on the onset of rotation-induced line narrowing, the ¹H T₁ minimum, and the low- temperature hydrogen T₁蟻 minimum associated with reorientations, the rotational activation energy E_rot,act = 0.28 eV is determined. Similarly, we use the onset temperature of translational motion-induced line narrowing and the high-temperature T_1蟻 minimum to determine the diffusion activation energy E_diff,act = 0.70 eV. Lack of sharp structure in the first-order quadrupole pattern and the absence of second-order quadrupole structure in the ²⁷Al NMR data suggest asymmetry (畏 鈮?0) and/or variations in the anion electric field gradients from structural disorder.