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Title: Nuclear Magnetic Resonance Study of Anion and Cation Reorientational Dynamics in (NH4)2B12H12

Journal Article · · Journal of Physical Chemistry. C
ORCiD logo [1];  [1];  [1];  [1];  [2];  [3]
  1. Ural Division of the Russian Academy of Sciences, Ekaterinburg (Russia)
  2. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
  3. National Inst. of Standards and Technology (NIST), Gaithersburg, MD (United States)
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Here, diammonium dodecahydro-closo-dodecaborate (NH4)2B12H12 is the ionic compound combining NH4+ cations and [B12H12]2– anions, both of which can exhibit high reorientational mobility. To study the dynamical properties of this material, we measured the proton NMR spectra and spin–lattice relaxation rates in (NH4)2B12H12 over the temperature range of 6–475 K. Two reorientational processes occurring at different frequency scales have been revealed. In the temperature range of 200–475 K, the proton spin–lattice relaxation data are governed by thermally activated reorientations of the icosahedral [B12H12]2– anions. This motional process is characterized by the activation energy of 486(8) meV, and the corresponding reorientational jump rate reaches ~108 s–1 near 410 K. Below 100 K, the relaxation data are governed by the extremely fast process of NH4+ reorientations which are not “frozen out” at the NMR frequency scale down to 6 K. The experimental results in this range are described in terms of a gradual transition from the regime of low-temperature quantum dynamics (rotational tunneling of NH4 groups) to the regime of classical jump reorientations of NH4 groups with an activation energy of 26.5 meV. Our study offers physical insights into the rich dynamical behavior of (NH4)2B12H12 on an atomic level, providing a link between the microscopic and thermodynamic properties of this compound.

Research Organization:
Sandia National Lab. (SNL-CA), Livermore, CA (United States)
Sponsoring Organization:
USDOE National Nuclear Security Administration (NNSA)
Grant/Contract Number:
AC04-94AL85000; NA0003525; NA-0003525
OSTI ID:
1485459
Alternate ID(s):
OSTI ID: 1570262; OSTI ID: 1570263
Report Number(s):
SAND-2018-12085J; SAND2019-11024J; SAND2019-11023J; 668981
Journal Information:
Journal of Physical Chemistry. C, Vol. 122, Issue 6; ISSN 1932-7447
Publisher:
American Chemical SocietyCopyright Statement
Country of Publication:
United States
Language:
English

Figures / Tables (8)

Table 1(p. 10)table Table 1
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Figure 2(p. 21)figure Figure 2
Figure 3(p. 22)figure Figure 3
Figure 4(p. 23)figure Figure 4
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Figure 6(p. 25)figure Figure 6
Figure 7(p. 26)figure Figure 7

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Complex hydride
Reorientation
Rotational tunneling
Nuclear spin relaxation