N.J. Hess et al. / Chemical Physics Letters 459 (2008) 85–88
87
values determined on BH ND using H NMR T [4] and 15N NMR T1
2
the borane end of the molecule is accurately represented by the
threefold jump rotation model. Following a similar procedure, the
3
3
1
analysis [8]. However, it is in poorer agreement with the value of
9.6 kJ/mol measured by Reynhardt and Hoon whose low value
may result from either difficulty in spectrally isolating the amine
and borane hydrogen dynamics without the benefit of deuteration
or approaching the amine quantum tunneling regime [7].
An Arrhenius plot, shown in Fig. 3, comparing the calculated
experimental energy barrier determinations with the stated errors
in Ea reveals that if spectral isolation is used, the experimental
determinations are equivalent within experimental error regard-
less of whether QENS or NMR is used to measure the dynamics.
1
1
3 3
QENS data for NH BH were analyzed. The contribution from
the BH end of the molecule was calculated using the results of
3
1
1
the ND
el to the NH
mol and a
Our earlier analysis of the NH
ergy barrier for NH rotations, namely 7.98 kJ/mol. The amine
3
dynamics measured on the NH BH sample above 170 K included
3
BH
3
QENS analysis. Fitting a threefold jump rotation mod-
3
data, resulted in an activation energy of 14.8 ± 0.4 kJ/
ꢁ3
s
1
of 5.2 ꢂ 10 ps.
11BH
data resulted in lower en-
3
3
3
11
3
contributions from the borane group that were not expected to
contribute to quasielastic scattering based on the available NMR
data at the time [7]. However, the measurements reported here
4
. Conclusion
1
1
with ND
3 3
BH sample has allowed the borane dynamics to be iso-
lated and its contribution subsequently removed from the total
The results reported here conclusively demonstrate that the
11
dynamics measured on the NH
barriers for the NH and independently determined barriers for
BH rotations are compared to determinations by H, H, and
NMR methods in Table 1.
An energy barrier of 23.6 kJ/mol measured for borane rotation
3 3
BH sample. The corrected energy
threefold jump model accurately represents the diffusive motion
of the amine and borane groups and that spectral isolation of the
amine and borane groups is necessary for the correct determina-
tion of their rotational energy barriers. Future work is planned to
determine the dynamics of motion of hydrogen in the tetragonal
phase of AB as well as the dynamics of AB in mesoporous scaffolds
3
1
2
15
3
N
1
1
in ND
3 3
BH as determined by QENS measurement is in excellent
1
agreement with the values determined by H NMR T
1
analysis on
and lineshape analysis on
.[6] Similarly, the energy barrier of 14.8 kJ/mol value for
[
28].
NH
NH
3
BH
BD
[7] as well as the 2H NMR T
3 1
3
3
Acknowledgments
11
amine rotation in NH
3 3
BH determined by QENS agrees well with
This work was supported by the Office of Basic Energy Sciences
Hydrogen Fuel Initiative, Chemical Sciences Division, of the US
Department of Energy. Pacific Northwest national Laboratory is
operated for the DOE by Battelle. We acknowledge the support of
the National Institute of Standards and Technology, US Department
of Commerce, in providing neutron research facilities. Additionally,
this work utilized HFBS which is supported in part by the NSF un-
der Agreement No. DMR-0454672. A portion of the research de-
scribed in this paper was performed in the Environmental
Molecular Sciences Laboratory, a national scientific user facility
sponsored by the DOE’s Office of Biological and Environmental Re-
search and located at PNNL.
Table 1
Comparison of rotational energy barriers and determined by QENS and NMR
s
1
E
a
(kJ/mol) s (s) Sample/Ref.
1
Borane
QENS
ꢁ
17
23.6 ± 1.0
25.0 ± 0.4
26.4 ± 1.4
24.3 ± 1.9
8.4 ꢂ 10
6.7 ꢂ 10
1.2 ꢂ 10
4.3 ꢂ 10
ND
3
BH
3
/TW
1
ꢁ18
ꢁ17
ꢁ17
H NMR T
1
1
NH
NH
NH
3
3
3
BH
BD
BD
3
/[7]
/[6]
/[6]
2
H NMR T
3
2
H NMR LS
3
Amine
QENS
ꢁ
15
14.8 ± 0.4
14.5 ± 0.4
9.6 ± 0.4
5.2 ꢂ 10
4.8 ꢂ 10
9.5 ꢂ 10
5.7 ꢂ 10
NH
3
BH
3
/TW
1
5N NMR T
ꢁ15
ꢁ14
ꢁ15
15
1
NH
BH
BH
3
BH
3
/[8]
1
2
H NMR T
H NMR T
1
1
NH
ND
3
3
/[7]
13.7 ± 0.9
3
3
/[6]
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.4
4.9
5.4
5.9
000/T (K )
6.4
6.9
7.4
-
1
1
[
(
Fig. 3. Arrhenius plot of the calculated rotational energy barriers and experimental
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W = this work, LS = lineshape.
[