Thermal stability of quaternary ammonium hexafluorophosphates and halides

Article abstract of DOI:10.1134/S1070427213060062

Thermal decomposition of hexafluorophosphates of short-chain tetraalkylammonium salts of the general formula R₃R'NPF₆, where R₃ = R' = CH₃, C₂H₅, C ₄H₉; R₃ = C₂H₅, R' = CH₂C₆H₆ or CH₂CH=CH₂, was studied by thermal gravimetric analysis. Measurements were performed in air in the temperature interval 20-500°N. The thermal stability of halides with the same cations in the same temperature interval was studied for comparison. The effect of cation on the thermal stability of the halides and hexafluorophosphates was examined. The mechanism of thermal decomposition of quaternary ammonium hexafluorophosphates was suggested.

Full text of DOI:10.1134/S1070427213060062

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 6, pp. 824−830. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © O.E. Zhuravlev, V.M. Nikol’skii, L.I. Voronchikhina, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 6, pp. 881−887.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Thermal Stability of Quaternary Ammonium
Hexafluorophosphates and Halides
O. E. Zhuravlev, V. M. Nikol’skii, and L. I. Voronchikhina
Tver State University, Tver, Russia
e-mail: pifchem@mail.ru
Received March 4, 2013
Abstract—Thermal decomposition of hexafluorophosphates of short-chain tetraalkylammonium salts of the general
formula R3R'NPF6, where R3 = R' = CH3, C2H5, C4H9; R3 = C2H5, R' = CH2C6H6 or CH2CH=CH2, was studied
by thermal gravimetric analysis. Measurements were performed in air in the temperature interval 20–500°С. The
thermal stability of halides with the same cations in the same temperature interval was studied for comparison. The
effect of cation on the thermal stability of the halides and hexafluorophosphates was examined. The mechanism
of thermal decomposition of quaternary ammonium hexafluorophosphates was suggested.
DOI: 10.1134/S1070427213060062
Quaternary ammonium, dialkylimidazolium, and
alkylpyridinium hexafluorophosphates, which were
prepared in the early 1990s, belong to the class of hydro-
phobic low-temperature ionic liquids (ILs) [1].
phase-transfer catalysis, because they combine the proper-
ties of common ILs and liquid crystal solvents.
Quaternary ammonium hexafluorophosphates as ionic
liquids are known and studied to a lesser extent; mainly
these are salts of unsymmetrical cations with a long-chain
radical or polyalkylammonium cation [8]. Hexafluoro-
phosphates with short-chain tetraalkylammonium cation
as ILs are unknown, because in going to highly symmetri-
cal molecules the polarity increases considerably, and the
crystallinity increases also; as a result, the melting point
of such salts is beyond the temperature interval for ionic
liquids, 0–100°С. However, if the salts contain as coun-
terion acidic AlCl4– anion instead of neutral PF6– anion,
the salts acquire properties of typical ionic liquids and
are used as catalysts. For example, trimethylammonium
chloride is used in isomerization of n-alkanes [9]; tetra-
butylammonium bromide, in phase-transfer catalysis in
phosphorylation of alcohols [10]; tetrabutylammonium
chloride, in hydrodechlorination of CCl4 [11]. In some
cases, ionic liquids based on ammonium salts, e.g., trioc-
tylammonium chloride, are preferable as solvents even
over BMIMPF6 [12].
The stability and chemical inertness of these organic
compounds, along with their commercial availability,
largely favored the progress of studies concerning syn-
thesis, catalysis, and other aspects of their chemistry
[2, 3]. The hydrophobicity of hexafluorophosphates is
their major advantage over tetrafluoroborates BF4– or
aluminates AlCl4– in application as ionic liquids. For
example, 1-butyl-3-methylimidazolium hexafluorophos-
phate (BMIMPF6) in combination with organic reagents
in aqueous solutions is used for extraction of alkali and
alkaline-earth metals [4, 5]. The extraction of organic
compounds of various classes into hexafluorophosphate
ILs has been studied to a greater extent than their extrac-
tion into other ionic liquids [6]; as extractants of neutral
organic compounds, hexafluorophosphate ILs behave
similarly to esters and aromatic hydrocarbons.
Imidazolium and pyridinium hexafluorophosphates
with long-chain radicals (С12H25, С14H29, С16H33,
С18H37), in contrast to other ionic liquids, are thermo-
tropic liquids forming liquid crystal phases on melting
[7]. Such salts are of particular interest as solvents for
Because of the fact that in some organic processes it
is necessary to use ionic liquids at elevated temperatures,
their stability and decomposition temperature become im-
824

THERMAL STABILITY OF QUATERNARY AMMONIUM HEXAFLUOROPHOSPHATES
825
portant factors. Ionic liquids will exhibit different thermal
stability depending on the kind of the cation and anion.
acetone was added to a solution of triethylamine (0.5 mol)
in 30 mL of dry acetone. The mixture was refluxed for
0.5–3 h. The precipitate of the quaternary salt was filtered
off, washed with dry cold ether, recrystallized from a 1 :
2 ethanol–ethyl acetate mixture, and dried in a vacuum.
Yield 72 and 94%, respectively. The products are white
crystalline substances soluble in water and polar solvents
and insoluble in nonpolar solvents (ether, hexane). The
physicochemical properties are given in Table 1.
The decomposition of ionic liquids (like that of other
quaternary salts) at high temperatures is determined by
the structure of the organic amine (at the same anion).
For example, alkylammonium salts are the least stable
and can undergo alkyl interchange and dealkylation on
heating to 150°С, whereas imidazolium and pyridinium
salts are considerably more stable [13]. In the series of
similar cations, the thermal stability of salts does not ap-
preciably depend on the chemical structure of the cation
but depends on the kind of the anion; bulky anions, as a
rule, enhance the thermal stability of salts [14].
Quaternary ammonium hexafluorophosphates. A
0.05-mol portion of quaternary ammonium chloride was
dissolved in 20 mL of water, 0.05 mol of 60% HPF6 was
added dropwise to the resulting solution with continuous
stirring, and 0.05 mol of NaOH in the form of a 0.1 M
solution was added to the mixture obtained. Quaternary
ammonium hexafluorophosphate formed as a white amor-
phous mass was filtered off, washed with water, recrys-
tallized from methanol, and vacuum-dried for 24 h. The
yield varied from 70 to 95% depending on the structure
of quaternary ammonium cation. The recrystallized reac-
tion products are crystals of regular shape (Fig. 1). The
synthesized quaternary ammonium hexafluorophosphates
are hydrophobic substances immiscible with water and
soluble in polar organic solvents. The composition and
structure of the products were confirmed by elemental
analysis and IR spectroscopy (Table 2). Quaternary
ammonium hexafluorophosphates were prepared by the
following scheme:
In this work we studied the thermal stability of quater-
nary ammonium hexafluorophosphates containing short-
chain alkyl radicals in the cationic moiety in comparison
with the related halides belonging to the class of ionic
liquids. Also, we examined the relationship between the
structure of the cation and the thermal stability of qua-
ternary salts containing hexafluorophosphate PF6– and
halide anions as counterions.
EXPERIMENTAL
The IR spectra were recorded with a Bruker Equinox
55 Fourier spectrometer; samples were prepared as KBr
(ultrapure grade) pellets. Differential thermal gravimetic
analysis was performed with a Netzsch STA 449 F3 de-
vice in the temperature interval 25–550°С at a heating
rate of 5 deg min–1 in air. Elemental analysis was per-
formed with an a Hewlett–Packard 185 C,H,N-Analyser.
Photomicrographs were taken with a JEOLJSM-6610LV
scanning electron microscope.
HPF6
[R3N+R']X–
[R3N+R']PF6– + NaX,
NaOH
where R3 = R' = C2H5, X = I (I); R3 = C2H5, R' = CH2–
CH=CH2, X = Cl (II); R3 = C2H5, R' = CH2C6H5, X =
Cl (III); R3 = R' = C4H9, X = Cl (IV); R3 = R' = CH3,
Allyl- and benzyltriethylammonium chlorides.Asolu-
tion of benzyl or allyl chloride (0.51 mol) in 20 mL of
μm
μm
Fig. 1. Photomicrographs of a crystal of tetraethylammonium hexafluorophosphate (Table 2, compound no. 1).
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 86 No. 6 2013

826
ZHURAVLEV et al.
Table 1. Physicochemical properties of quaternary ammonium hexafluorophosphates of the general formula [R3NR']+PF6–
Found, %/Calculated, %
Comdю
no.
R
R'
M, g mol–1
336.7
Tm, °C
IR, ν, cm–1
C
H
1
C2H5
C2H5
C2H5
C2H5
334
(dec.)
35.1
34.9
7.5
7.3
νC–Hаl 2960, 2940, 2905;
νPF – 844
6
2
3
CH2–CH=CH2
C6H5CH2
370.8
142
185
251
37.8
37.7
7.1
6.9
νC–Hаl 2990, 2973; ν=C–H
3010; νC=C 1647; νPF – 846
6
337.0
46.4
46.3
6.7
6.5
νC–Hаl 2988; νC–Hар 3027;
νC–Сар 1589, 1479; νPF – 845
6
4
5
C4H9
CH3
C4H9
CH3
387.1
219.0
49.9
49.7
9.7
9.3
νC–Hаl 2968, 2949; νPF – 841
6
377
(dec.)
22.1
21.9
5.8
5.5
νC–Hаl 2930, 2905; νPF – 847
6
X = I (V).
(Table 2, compound no. 4), triethylbenzylammonium
(no. 3), and triethylallylammonium (no. 2) chlorides are
stable up to 180°С and rapidly decompose at 240°С in one
step. It should be noted that the thermal decomposition
of salts with benzyl and allyl radicals occurs similarly,
although they are significantly inferior in stability to the
symmetrical salts (C2H5)4NI and (C4H9)4NCl, which
may be due to weakening of Coulomb interactions of
the cation and anion as a result of distortion of the cation
symmetry upon introduction of benzyl or allyl group
instead of ethyl group.
As seen from Table 1, the hexafluorophosphates
obtained have relatively high melting points. This fact
can be attributed to relatively high “ionicity” of the
compounds, because the positive charge is virtually fully
localized on the nitrogen atom, without delocalization
to the periphery of the molecule. Furthermore, virtually
all the organic cations in the compounds obtained have
short-chain radicals and exhibit relatively high symmetry.
These are, apparently, the major factors responsible for
so high melting points.
Data on the thermal stability of the synthesized quater-
The thermal stability of the starting quaternary am-
monium halides and hexafluorophosphates was studied
by thermal gravimetric analysis. The results obtained for
tetraalkylammonium halides are shown in Fig. 2.
It follows from the TG and DTAcurves of quaternary
ammonium halides that the processes occurring on heat-
ing are similar in character. However, the compounds
significantly differ in the thermal stability. For example,
highly symmetrical salts (CH3)4NI and (C2H5)4NI, con-
taining iodide ion as counterion, are practically stable
up to 360°С and decompose without melting at 440°С;
the decomposition products burn out with an exothermic
effect observed at 450°С. The chlorides are less stable,
which is consistent with the general trend in the thermal
stability of halides: I– > Br– >> Cl– > F– [14]. Despite the
fact that the temperature of 150°С is considered as the
maximal working temperature for the majority of qua-
ternary ammonium chlorides [15], tetrabutylammonium
T, °C
Fig. 2. TG curves of quaternary ammonium halides. (Δm)
Weight loss and (Т) temperature; the same for Fig. 3. Numerals
at curves are compound nos. in Table 2.
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THERMAL STABILITY OF QUATERNARY AMMONIUM HEXAFLUOROPHOSPHATES
827
Table 2. Thermal stability of quaternary ammonium halides of the general formula [R3NR']+X–
*
Tm
T50
Weight loss, %, at indicated T, °C
Compd. no.
R
R'
X–
°С
100
0
200
0
300
30
1
2
3
4
5
C2H5
C2H5
C2H5
C4H9
CH3
C2H5
I
>300
52
260
217
200
210
415
CH2–CH=CH2
C6H5CH2
C4H9
Cl
Cl
Cl
I
0
0
3
0
12
47
27
0
100
100
100
0
96
76
CH3
>300
a (T50) Temperature of 50% decomposition of the sample; the same for Table 3.
nary ammonium hexafluorophosphates are given in Fig. 3
and Table 3.As can be seen, all the salts surpass halides in
the thermal stability. For example, tetramethylammonium
hexafluorophosphate (CH3)4NPF6 appeared to be the most
stable. It is stable up to 380°С and rapidly decomposes
without melting in one step with simultaneous combus-
tion of decomposition products at 500°С (exothermic
effect at 450°С).
(C2H5)4NPF6 is stable up to 330°С, and on further heat-
ing it decomposes in two steps. The first fast step is
observed in the interval 330–385°С with an exothermic
effect at 375°С (combustion of decomposition products),
and the second, slower step, in the interval 385–430°С.
The hexafluorophosphates with the benzyl and allyl
substituents in the cation (Table 3, compound nos. 2, 3)
decompose similarly: The salts are stable up to 280°С,
after which they rapidly decompose in one step with an
endothermic effect at 330°С, followed by combustion of
the decomposition products at 370°С.
The compounds (C2H5)4NPF6 and (C4H9)4NPF6 dif-
fer in the thermal behavior from all the other compounds
studied. The DTA curves are shown in Fig. 4. These
salts are stable up to 280–330°С and, in contrast to the
other salts, decompose without melting in two steps.
For (C4H9)4NPF6, the first step occurs at 280–370°С
(endothermic effect at 340°С) and the second step, at
370–410°С. Tetraethylammonium hexafluorophosphate
As follows from our results, replacement of the halide
ion by bulky octahedral PF6– anion leads to a change in the
physicochemical properties of tetraalkylammonium salts,
namely, to an increase in their melting point (which does
not allow them to be classed with low-temperature ionic
liquids) and in the decomposition temperature.
T, °C
T, °C
Fig. 4. DTAcurves of (1) tetrabutylammonium and (2) tetraeth-
ylammonium hexafluorophosphates. (T) Temperature.
Fig. 3. TG curves of quaternary ammonium hexafluorophos-
phates. Numerals at curves are compound nos. in Table 3.
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828
ZHURAVLEV et al.
Table 3. Thermal stability of quaternary ammonium hexafluorophosphates of the general formula [R3NR']+PF6–
Weight loss, %, at indicated T, °C
Compound no.
R
R'
T50, °С
200
0
300
0
400
63
1
2
3
4
5
C2H5
C2H5
C2H5
C4H9
CH3
C2H5
384
334
348
365
476
CH2–CH=CH2
C6H5CH2
C4H9
4.5
1
13
10
4
90
83
85
4
0
CH3
0
0
According to published data [16], at equal charge of
the anions, an increase in their size in most cases leads to a
decrease in the melting point of the salts. For example, the
melting points of EMIMBF4, EMIMNO3, and EMIMPF6
are 87, 38, and 6°С, respectively. This trend is typical
of salts with a planar cation (Im+, Py+) and is associated
with the possibility of hydrogen bonding of the cation
with the counterion. In the case of octahedral PF6– anion,
all the equatorial fluorine atoms can participate in hydro-
gen bonding with 1-ethyl-3-methylimidazolium cation,
whereas for the tetrahedral BF4– anion only three fluorine
atoms are involved in hydrogen bonding. With respect to
the H-bond strength, the anions can be ranked in the fol-
lowing order: CF3COO– > BF4– > PF6– > B(C6H5)4– [17].
data [14], the initial steps of the cation decomposition
in quaternary ammonium salts start with cleavage of
the alkyl–N+ bond and can occur along two pathways:
β-elimination (E2) and nucleophilic substitution at qua-
ternary N atom (SN) (Scheme 1)
Presumably, thermal degradation of the quaternary
salts studied occurs by the mechanism of intramolecu-
lar β-elimination via 5-centered transition state [18]
(Scheme 2).
In this case, the PF6– anion acts as a nucleophile,
with simultaneous heterolytic cleavage of the Cβ–H and
Cα–N+ bonds occurring in nucleophilic β-elimination
reactions. In the case of hexafluorophosphate ions, the
formation of the transition state is hindered, because the
hexafluorophosphate ion is bulky (3.3 Å) and is difficultly
incorporated in a three-dimensional structure [13]. On the
contrary, the presence of Cl– ion as counterion (radius
1.81 Å) leads to a considerable decrease in the thermal
stability owing to easy formation of the transition state in
the course of β-elimination. If there are no H atoms in the
β-position and the elimination is impossible, the process
preferentially occurs as nucleophilic substitution SN(C).
However, for quaternary ammonium hexafluorophos-
phates, we observed an opposite trend: increase in the
melting and decomposition temperatures of the salts.
This fact may be associated with the tetrahedral structure
of ammonium cations and with the lack of any hydrogen
bonds with the anion, which leads to enhancement of
Coulomb interaction, formation of ordered structures
by the ions, and increase in the melting and decomposi-
tion temperatures. In addition, high thermal stability of
hexafluorophosphates can probably be attributed to the
fact that decomposition of the salts is mainly determined
by decomposition of the cation. According to published
Similar results were obtained in studying the thermal
stability of quaternary ammonium tetrafluoroborates
[19] and 1,3-disubstituted imidazolium tetrachlorofer-
Scheme 1.
H
H
β
β
CH
R
CH
R
Cl _
F5P
F _
α
α
CH2
CH2
N+
N+
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THERMAL STABILITY OF QUATERNARY AMMONIUM HEXAFLUOROPHOSPHATES
829
Scheme 2.
_
[R3N+
R'] PF6
R = R' = C4H9
R=R'=C4H9
R = R' = CH
R=R'=CH3
3
β
α
_
_
N+(CH3)3PF6
CH2CH2C2H5 PF6
H3C
(C4H9)3N+
o
o
T, ºC
T, ºC
T C
E
SN(C)
T C
E
2
2
(C H ) N + CH
(C H 4) N + CH
CH3
PF6 + N(CH3)3
CHC2H5 + HF + PF5
CONCLUSIONS
9 3
2
4
9 3
2
rates [20]. Deceleration of the thermal decomposition
of tetrafluoroborates and tetrachloroferrates is due to
destabilization of the transition state, caused by steric
repulsion of the bulkier anion.
(1) All the quaternary ammonium hexafluorophos-
phates studied surpass in thermal stability quaternary
ammonium halides and are stable up to 300°С.
Compound no. 2 containing the allyl substituent
exhibits the lowest thermal stability among quaternary
(2) Tetrabutyl- and tetraethylammonium hexafluoro-
ammonium hexafluorophosphates. This may be due to phosphates, in contrast to the other salts studied, decom-
the fact that one of the β-C atoms in this compound is in pose without melting. The process occurs in two steps
the state of sp2 hybridization, which increases its C–H with complete decomposition above 400°С.
acidity. Therefore, in the transition state arising in the
course of β-elimination the detachment of this proton
requires smaller energy than the proton detachment from
the sp3-hybridized C atom.
(3) The effect of cation on the thermal stability of
quaternary ammonium hexafluorophosphates was deter-
mined. With symmetrical cations (CH3)4N+, (C2H5)4N+,
and (C4H9)4N+, the Coulomb interactions with PF6– anions
For all the compounds, decomposition of the cation are enhanced, which leads to an increase in the crystal-
leads to a decrease in the strength of electrostatic interac- linity and in the decomposition temperature of the cor-
tion with the anion. The PF6– anion cannot exist as sepa- responding compounds.
rate species. This leads to elimination of F– in the form
(4) Quaternary ammonium hexafluorophosphates
of HF (with β-H atom of one of alkyl groups), which, in
turn, favors formation of stable alkenes, tertiary amines,
HF, and PF5. If there is no β-H atom (compound no. 5),
the decomposition follows the SN(C) pathway. It is also
known that thermal decomposition of quaternary salts at
lower temperatures (200–300°С) preferentially occurs
by the SN(C) mechanism, and at higher temperatures, by
the E2 mechanism [21]:
containing benzyl or allyl radical with Csp2 atoms in
the cationic moiety are less stable and decompose in
the interval 280–360°С, with the decomposition pattern
being similar.
(5) The mechanism of thermal decomposition
of tetraalkylammonium hexafluorophosphates was
suggested.Theprocesspresumablyoccursas β-elimination
(E2) or as nucleophilic substitution (SN) depending on the
cation.
In tetramethylammonium hexafluorophosphate
(Table 3, compound no. 5), there is no β-H atom, and the
elimination is impossible.As a result, the thermal decom-
position preferentially occurs as nucleophilic substitution.
ACKNOWLEDGMENTS
It is not improbable that thermal decomposition of
The study was performed with the equipment of the
hexafluorophosphates occurs by both mechanisms, E2 Center for Shared Use at the Tver State University and
and SN(C), simultaneously, but at lower temperatures was financially supported by the Federal Target Program
(200–300°С) the nucleophilic substitution mechanism is “Scientific and Scientific-Pedagogical Personnel of Inno-
more probable, whereas at elevated temperatures the E2 vative Russia” for the years 2009–2013 (project no. 14.
mechanism should be realized.
V37.21.0653).
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ZHURAVLEV et al.
Systems), Moscow: Khimiya, 1982, p. 75.
REFERENCES
11. Wu, X., Letuchi, Y.A., and Eyman, D.P., J. Catal., 1996,
1. Kustov, L.M., Vasina, T.V., and Ksenofontov, V.A., Ross.
Khim. Zh., 2004, vol. 48, no. 6, pp. 13–35.
vol. 161, pp. 164–169.
12. Wolfson,A. and Wuyts, S., Tetrahedron Lett., 2002, vol. 43,
2. Shen, H.-J. and Juolen, Z.M.A., Tetrahedron Lett., 2003,
pp. 8107–8110.
vol. 44, pp. 981–989.
13. Bonhote, P. and Dias, A., Inorg. Chem., 1996, vol. 35,
3. Deng, Y., Shi, F., and Beng, J., J. Mol. Catal. A: Chem.,
pp. 1168–1178.
2001, vol. 165, pp. 33–37.
14. Yamamoto, T., Yamada, H., and Nanda, Y., Yukagaku, 1963,
4. Pletnev, I.V., Smirnova, S.V., Khachatryan, K.S., and Zer-
vol. 12, no. 9, pp. 42–46.
nov, V.V., Ross. Khim. Zh., 2004, vol. 48, no. 6, pp. 51–58.
15. Mutch, M.J. and Wilkes, J.S., Proc. Electrochem. Soc.,
1998, vol. 98, pp. 254–260.
5. Visser, A.E. and Swatloski, R.P., Sep. Sci. Technol., 2001,
vol. 36, pp. 785–804.
16. Wasserscheid, P. and Kelm, W., Angew. Chem. Int. Ed.,
2000, vol. 39, pp. 3772–3776.
6. Huddleston, J.G., Visser, A.E., and Reichert, W.M., Green
Chem., 2001, vol. 3, pp. 156–164.
17. Dupont, J., J. Braz. Chem. Soc., 2004, vol. 15, no. 3,
pp. 341–350.
7. Gordon, Ch.M., Holbrey, J., Kennedy, A., and Seddon,
K.R., J. Mater. Chem., 1998, vol. 8, pp. 2627–2636.
18. Ingold, C.K., J. Chem. Soc., 1933, vol. 34, pp. 526–535.
8. Welton, T. and Wasserscheid, P., Ionic Liquids in Synthesis,
19. Ryzhkov, Yu.A. and Voronchikhina, L.I., Zh. Prikl. Khim.,
Germany: Wiley, 2002.
1994, vol. 67, no. 5, p. 821.
20. Zhuravlev, O.E., Verolainen, N.V., and Voronchikhina, L.I.,
9. Ksenofontov, V.A. and Kustov, L.M., Eur. J. Chem. Catal.,
Zh. Prikl. Khim., 2011, vol. 84, no. 7, pp. 1086–1092.
2003, vol. 1, no. 3, pp. 305–311.
21. Wei Xie, Rongcai Xie, Wei-Ping Pan, and Hunter, D.,
10. Yanovskaya, L.A. and Yufit, S.S., Organicheskii sintez v
dvukhfaznykh sistemakh (Organic Synthesis in Two-Phase
Chem. Mater., 2002, vol. 14, no. 11, pp. 4837–4843.
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