4 years ago

LiBH4 in Aerogel: Ionic Motions by NMR

LiBH4 in Aerogel: Ionic Motions by NMR
Mark S. Conradi, Hongyang Zou, John J. Vajo, Samuel B. Emery, Anton Gradišek
LiBH4 in nanometer-size pores of carbon aerogel displays a motionally narrowed fraction of its hydrogen NMR spectrum. The fraction of mobile spins grows with decreasing pore size, indicating that the mobile spins are those nearest the aerogel walls. Here, we report selective inversion experiments to measure the rate of exchange between the mobile and immobile BH4 groups. We find the exchange time constant to be nearly temperature independent at ∼5 ms. This unexpected result is explained by a broad distribution of motion rates, all thermally activated. The net effect is to place the border between the spins contributing to the broad and narrow resonance peaks always at the same rate of motion, leading to nearly the same rate of exchange. In addition, exchange within the mobile fraction prevents the development of an extremely sharp resonance component from the fastest moving BH4. We also report measurement of hydrogen T1 at 298 K as a function of hydrogen frequency, obtained by fast field-cycling relaxometry. The variation is nearly T1–1 ∝ ω0–1, demonstrating a broad distribution of Li+ hopping rates, reflecting the disordered environment in the nanometer-size pores.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b04520

DOI: 10.1021/acs.jpcc.7b04520

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