Investigation of H atom and free radical behavior in gas hydrates
Mina Mozafari, Jean-Claude Brodovitch, Lalangi Chandrasena, Paul Percival
Simon Fraser University, Department of Chemistry, 8888 University Drive, Burnaby, BC V5A 1S6
Gas Hydrates
MuSR technique
Natural gas hydrates are crystalline solids composed of water and gas.
The gas molecules (guests) are trapped in water cavities (host) that are
composed of hydrogen-bonded water molecules. All common natural
gas hydrates belong to the three crystal structures, cubic structure I (sI),
cubic structure II (sII), or hexagonal structure H (sH). Structure I is
formed with guest molecules having diameters between 4.2 and 6 Å,
such as methane, ethane and carbon dioxide. Larger (6 Å < d < 7 Å)
guest molecules such as propane form structure II. Still larger molecules
(typically 7 Å < d < 9 Å) form structure H when accompanied by smaller
molecules such as methane.1
Transverse Field µSR
For Muoniated free radicals at
high magnetic fields:
    12 A
Thus, Aµ is determined from
the difference in radical
precession frequencies.
Figure 2a: Schematic of a TF-µSR set up3
Muon Level-Crossing Resonance
Coupling constants (hfcs) of
nuclei other than the muon
can be obtained from levelcrossing resonance fields.
Figure 1: Common clathrate hydrate
.
2
2
1 A  Ak A  2MAk
B

2 (γ  γk )  e ( A  Ak )
structures2
Clathrates of cyclopentane, cyclopentene, tetrahydrofuran, 2,5dihydrofuran, furan, acetone, isobutane, propene oxide, 2,3-dimethyl-2butene, isobutene, cis-butene, benzene and a semi-clathrate hydrate
were prepared for the purpose of this study.
Figure 2b: Schematic of a LF-µSR set up3
Purpose
To determine the motion and reactions of H atoms and free radicals within and between the cages of clathrate hydrates, with the ultimate aim of inhibiting explosions.
a)
b)
Fourier Power
Results and Discussion
A+ - A-
Table 1: Hyperfine coupling constants determined from the LCR spectrum
BLCR/kG
DM
hfc /MHz
A+ - A-
2,5-dihydrofuran hydrate results:
Assignment
12.5
13.436
1
366
muon in CHMu
14.319
0
98
proton, b-H, axial
14.579
0
93
2 proton, b-H,
equatorial
22.830
0
-59
proton, a-H
13.0
13.5
14.0
14.5
15.0
15.5
21.9
Field (kG)
Figure 3: Muoniated radical
from 2,5-dihydrofuran
Mu in tetrahydrofuran hydrate:
22.4
22.9
23.4
0
100
200
300
400
Frequency /MHz
Field (kG)
Figure 4: LCR resonances of a) βHs and b) αH in the muoniated
radical formed from 2,5-dihydrofuran hydrate at -12ºC.
Figure 5: Fourier power TF-μSR
Spectrum of the muoniated 2,5dihydrofuran radical in hydrate
at -12ºC and 14.6 kG field.
Future work
a)
We plan to detect both Mu and a
muoniated radical simultaneously at
low temperature in the same sample
and follow the reaction of Mu as the
temperature is increased.
Fourier Power
Fourier Power
b)
Figure 7: Structure II propane + methane mixed gas hydrate.5
50
100
150
200
250
200
250
Frequency /MHz
300
350
400
Frequency /MHz
Figure 6: Fourier power TF-μSR spectra of muonium in THF hydrate at a) 100 G and b) 200 G and -10ºC.
Aµ= 4420 MHz for hydrate but Aµ= 4510 MHz for ice4
Acknowledgements
Financial support from Simon Fraser University and NSERC is gratefully acknowledged.
References
1. Sloan, E. D.; Koh, C. A., Hydrates of Natural Gases, 3rd ed.; CRC Press-Taylor and Francis group, 2008.
2. Strobel, T. A.; Hester, K. C.; Koh, C. A.; Sum, A. K.; Sloan, E. D. Chem. Phys. Lett. 2009, 478, 97-109.
3. Sonier, J. E.; Muon Spin Rotation/Relaxation/Resonance Brochure 2001.
4. Roduner, E.; Percival, P.W.; Han, P.; Bartels, D. M. J. Chem. Phys. 1995, 102, 5989-5997.
5. Sugahara, T.; Kobayashi, Y.; Tani, A.; Inoue, T.; Ohgaki, K. J. Phys. Chem. A 2012, 116, 2405-2408.