Low-Temperature Rotational Tunneling of Tetrahydroborate Anions in Lithium Benzimidazolate-Borohydride Li2(blm)BH4

Published

August 7, 2019

Author(s)

Alexander V. Skripov, Mirjana NMN Dimitrievska, Olga A. Babanova, R. V. Skoryunov, Alexei V. Soloninin, Fabrice Morelle, Y. Filinchuk, Antonio Faraone, Hui Wu, Wei Zhou, Terrence J. Udovic

Abstract

To investigate the dynamical properties of the novel hybrid compound, lithium benzimidazolate-borohydride Li₂(bIm)BH₄ (where bIm denotes a benzimidazolate anion, C₇N₂H₅u-^), we have used a set of complementary techniques: neutron powder diffraction, ab initio} density functional theory calculations, neutron vibrational spectroscopy, nuclear magnetic resonance, neutron spin echo, and quasielastic neutron scattering. Our measurements performed over the temperature range from 1.5 K to 385 K have revealed the exceptionally fast low-temperature reorientational motion of BH₄- anions. This motion is facilitated by the unusual coordination of tetrahedral BH₄- anions in Li₂(bIm)BH₄: each anion has one of its H atoms anchored within a nearly square hollow formed by four coplanar Li⁺ cations, while the remaining -BH₃ fragments extends into a relatively open space, being only lossely coordinated to other atoms. As a result, the energy barriers for reorientations of this fragment around the anchored B-H bond axis are very small, and at low temperatures this motion can be described as rotational tunneling. The tunnel splitting derived from the neutron spin echo measurements at 3.6 K is 0.43(20 υeV. With increasing temperature, we have observed a gradual transition from the regime of low-temperature quantum dynamics to the regime of classical thermally-activated jump reorientations. The jump rate of the uniaxial three-fold reorientations reaches 5 x 10¹¹ s^-1 at 80 K. Nearer room temperature and above, both nuclear magnetic resonance and quasielastic neutron scattering measurements has revealed a second process of BH_4- reorientations characterized by the activation energy of 261 meV. This process is several orders of magnitude slower than the uniaxial three-fold reorientations; the corresponding jump rate reaches approximately equal}7x10⁸ s^-1 at 300 K.

Citation

Journal of Physical Chemistry C

Volume

123

Pub Type

Journals

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Keywords

lithium benzimidazolate borohydride, NMR, nuclear magnetic resonance, quantum tunneling, quasielastic neutron scattering, QENS, rotational potential, rotational tunneling, tetrahydroborate

Citation

Skripov, A. , Dimitrievska, M. , Babanova, O. , Skoryunov, R. , Soloninin, A. , Morelle, F. , Filinchuk, Y. , Faraone, A. , Wu, H. , Zhou, W. and Udovic, T. (2019), Low-Temperature Rotational Tunneling of Tetrahydroborate Anions in Lithium Benzimidazolate-Borohydride Li₂(blm)BH₄, Journal of Physical Chemistry C, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=927921 (Accessed April 25, 2024)