Tribromide Ionic Liquids: Probing the Charge Distribution of the Anion by XPS

Article abstract of DOI:10.1134/S0036024420050167

Abstract: We investigate a typical trihalide ionic liquid, 1-octyl-3-methylimidazolium tribromide, using X?ray photoelectron spectroscopy. The electronic environment of the anion is analyzed in detail. The Br 3d binding energies of the two components are both higher than that measured for 1-octyl-3-methylimidazolium bromide, due to the delocalization of the negative charge spreading over three atoms within the anion. The basicity of [Br₃]^– is estimated based upon N 1s binding energy. It is found similar to that of [BF₄]^–.

Full text of DOI:10.1134/S0036024420050167

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2020, Vol. 94, No. 5, pp. 1053–1056. © Pleiades Publishing, Ltd., 2020.
STRUCTURE OF MATTER
AND QUANTUM CHEMISTRY
Tribromide Ionic Liquids:
Probing the Charge Distribution of the Anion by XPS
Rui Mua, Aimin Dengb,*, and Shuang Menb,**
a School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang, 110159 China
b School of Material Science and Engineering, Shenyang Ligong University, Shenyang, 110159 China
* e-mail: dengaiminsy@126.com
**e-mail: menshuang@hotmail.com
Received August 28, 2019; revised August 28, 2019; accepted October 15, 2019
Abstract—We investigate a typical trihalide ionic liquid, 1-octyl-3-methylimidazolium tribromide, using
X-ray photoelectron spectroscopy. The electronic environment of the anion is analyzed in detail. The Br 3d
binding energies of the two components are both higher than that measured for 1-octyl-3-methylimidaz-
olium bromide, due to the delocalization of the negative charge spreading over three atoms within the anion.
The basicity of [Br3]– is estimated based upon N 1s binding energy. It is found similar to that of [BF4]–.
Keywords: tribromide ionic liquids, charge distribution, binding energy
DOI: 10.1134/S0036024420050167
INTRODUCTION
N 1s binding energy, are also made. It concludes that
[Br3]– is similar basic with [BF4]–.
Trihalide ionic liquids are usually of less viscosity
and hydrophobic nature. They have found specific
applications in electrochemistry [1] and halogenations
[2]. Among all trihalide anions, tribromide has been
widely employed for the bromination of organic sub-
strates over the past decade, mainly because it is cum-
bersome handling liquid bromine due to its hazardous
nature. Since 2005, ionic liquids have been success-
fully investigated by X-ray photoelectron spectroscopy
(XPS), ascribing to their negligible volatility [3, 4].
Binding energy derived from XPS can effectively
reflect the cation–anion interactions in ionic liquids
[5, 6], and act as a sensitive probe of the electronic
environment of a certain element present in an ionic
liquid [7–9]. Recently, XPS study of trihalide ionic
liquids has been conducted. It has indicated that the
combination of a halide anion with elemental halogen
yields a much less basic trihalide anion, i.e., I– to [I3]–.
Furthermore, the stability of trihalide anions under beam
irradiation has also been demonstrated [10]. It inspires
the idea to investigate other stable and characteristic
trihalide anions, e.g., [Br3]– by XPS.
EXPERIMENTAL
Materials
All chemicals were obtained from Chinese Market
and were used as received, except for 1-methylimidaz-
ole, which was distilled over calcium hydride prior
to use.
Synthesis of [C8C1Im]Br: 1-methylimidazole was
placed in a two necked round-bottomed flask fitted
with a water condenser topped with a blue silica tube.
1-Bromooctane (0.9 molar equivalents) was added
dropwise into the flask with stirring at 70°C. The reac-
tion was allowed to proceed for 48–72 h. The unre-
acted 1-methylimidazole was then removed by wash-
ing the mixture with ethyl acetate three times. The
desired product, [C8C1Im]Br, was firstly dried using a
rotary evaporator and then under high vacuum at 60°C
for 12 h to yield a pale yellow liquid [11].
Synthesis of [C8C1Im][Br3]:[C8C1Im][Br3] was
prepared simply by mixing [C8C1Im]Br and liquid
bromine (1.1 molar equivalents) to allow complete
reaction at room temperature, and purified under high
vacuum for 12 h.
In this study, we investigate 1-octyl-3-methylimid-
azolium tribromide ([C8C1Im][Br3]) using XPS. The
electronic environment of the bromine centre is com-
pared to that of [C8C1Im]Br. The negative charge dis-
Synthesis of [C8C1Im][BF4]: [C8C1Im]Br was dis-
solved in deionized water in a round bottomed flask. A
tribution of [Br3]– is discussed in detail. Comparisons water condenser was set up and the solution was stirred
at 70°C. NaBF4 (1.2 molar equivalents) was dissolved
of the anion basicity, which can be correlated to the
1053

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RUI MU et al.
XPS Data Analysis
4
2
0
For data interpretation, a spline linear background
subtraction was used. Peaks were fitted using GL(30)
lineshapes; a combination of a Gaussian (70%) and
Lorentzian (30%) [14, 15].
Fitting procedure for C 1s region of [C8C1Im][X]
was conducted according to the established model
developed in our group [16]. XP spectra for all ionic
liquids were charge corrected by setting the measured
binding energy of the aliphatic C 1s component
(Caliphatic 1s) to 285.0 eV [6, 17, 18].
To aid visual interpretation of the XP spectra, all
spectra were normalized to the fitted area of N 1s peak
of [C8C1Im]Br. This peak was used for normalization
simply because the N atom is present in both ionic liq-
uids studied in this work in the same amount. It must
be noted that the experimental error associated with
the XPS measurement is of the order 0.1 eV.
Brterminal 3d3/2
Brcentral 3d5/2
Brcentral 3d3/2
Brterminal 3d5/2
Shake-up
76
72
68
64
Binding energy, eV
Fig. 1. Br 3d spectrum with fittings of [C C Im][Br ].
8
1
3
in deionized H2O and was added to the reaction drop
wise. After 72 h, the reaction mixture was allowed to
cool and the lower phase was washed with iced deion-
ized water three times to remove NaBr and unreacted
NaBF4. The desired [C8C1Im][BF4] was dried using a
rotary evaporator and then under high vacuum at 60°C
for 12 h to yield a colorless liquid [12].
RESULTS AND DISCUSSION
Comparison to Bromide-Based Analogue
A characteristic Br 3d high resolution spectrum is
composed of an unresolved doublet peak which origi-
nates from the 3d orbital with a spin-orbital coupling
energy difference of 1.05 eV [19], and the area ratio of
3d5/2 : 3d3/2 is 3 : 2. In this paper, unless otherwise
stated, the 3d5/2 component is selected to ensure valid
comparisons, simply because the intensity for this
component is larger.
Figure 1 shows the Br 3d XP spectrum with fittings
for [C8C1Im][Br3]. An unresolved triplet peak showing
binding energy between 73.0 and 67.0 eV is observed.
By carefully considering the chemical structure of
XPS Data Collection
All XP spectra were recorded using a PHI Quantera
II spectrometer employing a focused, monochro-
mated AlKα source (hν = 1486.6 eV), hybrid (mag-
netic/electrostatic) optics, hemispherical analyzer and
a multi-channel plate and delay line detector (DLD)
with a X-ray incident angle of 30° and a collection [Br3]–, it is apparent that bromine atoms are in two
angle, θ, of 0° (both relative to the surface normal).
electronic environment. Therefore, two components,
i.e., Brcentral and Brterminal, are used for the fitting of the
Br 3d spectrum. A satisfactory fitting can be achieved
by constraining the area ratio of 3d5/2 : 3d3/2 to 3 : 2
and the spin-orbital coupling energy gap to 1.05 eV, for
both bromine components. The FWHM value is
∼1.0 eV. Meanwhile, there is also a shake-up satellite
peak showing binding energy at 74.8 eV observed,
which is ascribed to the formation of the negative
The analyzed area was 100 μm × 100 μm.
Ionic liquid samples were firstly put into a prepara-
tion chamber as thin films (approx. thickness 0.5–
1 mm) on a stainless sample stub. Pumping of ionic
liquids was carried out to achieve residual pressure of
≈10–7 mbar. The samples were then transferred to the
main analytical vacuum chamber. The pressure in the
main chamber remained below 1 × 10–8 mbar during charge delocalized structure, [Br3]–. The Brcentral 3d5/2
all XPS measurements, suggesting that all volatile
impurities are removed, leading to high purity sam-
ples [13].
shows binding energy at 70.0 eV; whilst Brterminal 3d5/2
at 68.5 eV, as shown in detail in Table 1. These two
components are both much more electropositive than
Table 1. Binding energies (eV) for [C8C1Im][Br3] and [C8C1Im]Br
C2 1s
[C8C1Im][X] Caliphatic 1s
C
hetero 1s
Brcentral 3d5/2 Brterminal 3d5/2
N 1s
Br shake up
74.8
Br–
285.0
285.0
287.3
287.5
286.3
286.4
401.7
401.9
67.5
[Br3]–
70.0
68.5
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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TRIBROMIDE IONIC LIQUIDS
1055
4
2
0
(a)
4
(b)
[C8C1Im]Br
[C8C1Im][Br3]
Shake-up
2
76
72
68
64
404
403
402
401
400
Binding energy, eV
Binding energy, eV
Fig. 2. XP spectra of [C C Im][Br ] and [C C Im]Br for: (a) Br 3d and (b) N 1s.
8
1
3
8 1
that of Br–. It is because that the negative charge
within [Br3]– is spread over three atoms. As a result,
the charge density is diluted compared to the case of
Br–, where the negative charge is located solely on one
atom. It is found that these two components show 2.5
observed as there is no delocalized π-system present in
Br–. The N 1s binding energy for [C8C1Im][Br3] is
found at 401.9 eV, which is 0.2 eV higher than that of
[C8C1Im]Br, as illustrated in Fig. 2b. It suggests the
[Br3]– transfers less point negative charge to the cation
and 1.0 eV higher binding energies than that of Br–, than Br–.
respectively. The central bromine atom shows much
higher binding energy, which indicates that the nega-
tive charge density locating on the central bromine
Anion Basicity
The chemical shift value of the H-atom on the C2
position of the imidazolium cation obtained from 1H
NMR spectrum is considered as an indicator of the
anion basicity [20]. The measured binding energies
of N 1s peak of ionic liquids can be subsequently cor-
related linearly to the chemical shift. A brief descrip-
tion of the correlation between NMR chemical shift
values (obtained as solutions in CD2Cl2) and the N 1s
binding energies for [C8C1Im][Br3] and [C8C1Im]Br
is illustrated in Fig. 3. In order to give a comparison,
data for [C8C1Im][BF4] are also included. It must be
noted that XPS spectra for [C8C1Im][BF4] are not
measured in this study, but is available in the litera-
ture [6].
atom is less than that on the two terminal bromine
atoms. The area ratio of Brterminal : Brcentral is ∼1.7
(2.0 in nominal), meaning that there are more contri-
butions from the two terminal bromine atoms to the
shake-up satellite peak.
On the other hand, as demonstrated in Fig. 2a, it is
clear that there is only one Br 3d peak, at 67.5 eV with
a FWHM value of 0.9 eV for [C8C1Im]Br, concluding
that there is only one bromine electronic environment
for [C8C1Im]Br. Meanwhile, no shake-up feature is
10.4
10.0
As shown in Fig. 3, the measured N 1s binding
energies follow a linear trend with the 1H NMR chem-
ical shift values. It concludes that [Br3]– is similar
basic with [BF4]–. However, both [Br3]– and [BF4]–
Brꢁ
9.6
9.2
8.8
are less basic than Br–. This observation allows further
understanding of the physico-chemical properties of
[Br3]–-based ionic liquids.
[Br3]ꢁ
[BF4]ꢁ
CONCLUSIONS
401.7
401.8
401.9
402.0
We have successfully measured XP spectra of
[C8C1Im][Br3], together with [C8C1Im]Br. A satisfac-
E, eV
tory fitting of Br 3d spectrum for [Br3]– is carried out
+
Fig. 3. N 1s binding energies (E) for [C C Im] -based
ionic liquids compared to the H NMR chemical shift (δ)
of the H-atom on the C position (in CD Cl ).
by employing two components, Brcentral and Brterminal
.
8
1
1
The electronic environment of the anion is identified.
2
The measured Br 3d5/2 binding energies of the two
2
2
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1056
RUI MU et al.
6. I. J. Villar-Garcia, E. F. Smith, A. W. Taylor, F. Qiu,
K. R. J. Lovelock, R. G. Jones, and P. Licence, Phys.
Chem. Chem. Phys. 13, 2797 (2011).
components for [C8C1Im][Br3] are found 2.5 and
1.0 eV higher than that of [C8C1Im]Br respectively,
due to the delocalization of the negative charge
spreading over three atoms. Brcentral component shows
much higher binding energy than Brterminal compo-
nent, indicating that the negative charge density locat-
ing on Brcentral component is less than that on the two
terminal bromine atoms. Furthermore, the area ratio
of Brterminal : Brcentral is ∼1.7, meaning that there are
more contributions from the two terminal bromine
atoms to the shake-up satellite peak. Comparisons of
the N 1s binding energy against the chemical shift
value of the H-atom bonded on C2 position of the cat-
ion with varied anions are carried out. It concludes
that [Br3]– is similar basic with [BF4]–.
7. J. S. Zhu, Y. F. Liu, and Q. L. Shen, Angew. Chem.,
Int. Ed. 55, 9050 (2016).
8. P. J. Carvalho, S. P. M. Ventura, M. L. S. Batista,
B. Schroder, F. Goncalves, J. Esperanca, F. Mutelet,
and J. A. P. Coutinho, J. Chem. Phys. 140, 64505
(2014).
9. D. Gerhard, S. C. Alpaslan, H. J. Gores, M. Uerding-
en, and P. Wasserscheid, Chem. Commun., No. 40,
5080 (2005).
10. S. Men, K. R. J. Lovelock, and P. Licence, Chem.
Phys. Lett. 679, 207 (2017).
11. J. G. Huddleston, A. E. Visser, W. M. Reichert,
H. D. Willauer, G. A. Broker, and R. D. Rogers, Green
Chem. 3, 156 (2001).
12. P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. De-
ACKNOWLEDGMENTS
Souza, and J. Dupont, Polyhedron 15, 1217 (1996).
We thank Liaoning Provincial Foundation of Science
and Technology (20180550482) for financial support. SM
acknowledges Beijing Key Laboratory of Quality Evaluation
Technology for Hygiene and Safety of Plastics (Beijing
Technology and Business University, Beijing 100048,
China) for the award of an Open Fund (QETHSP2019004).
13. A. W. Taylor, K. R. J. Lovelock, A. Deyko, P. Licence,
and R. G. Jones, Phys. Chem. Chem. Phys. 12, 1772
(2010).
14. C. D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor,
R. H. Raymond, and L. H. Gale, Surf. Interface Anal.
3, 211 (1981).
15. D. Briggs and G. Beamson, Anal. Chem. 64, 1729
(1992).
REFERENCES
16. S. Men, J. Jiang, and P. Licence, Chem. Phys. Lett.
674, 86 (2017).
1. A. Ejigu, K. R. J. Lovelock, P. Licence, and
D. A. Walsh, Electrochim. Acta 56, 10313 (2011).
17. S. Men, K. R. J. Lovelock, and P. Licence, RSC Adv. 5,
35958 (2015).
2. M. P. Kaushik and V. Polshettiwar, Indian J. Chem.,
Sect. B 45, 2542 (2006).
18. P. A. Frey and O. T. Magnusson, Chem. Rev. 103, 2129
(2003).
3. E. F. Smith, I. J. Villar-Garcia, D. Briggs, and P. Li-
cence, Chem. Commun., No. 45, 5633 (2005).
19. J. F. Moulder, W. F. Stickle, P. E. Sobol and
K. D. Bomben, Handbook of X-ray Photoelectron Spec-
troscopy: A Reference Book of Standard Spectra for Iden-
tification and Interpretation of XPS Data (Physical Elec-
tronics, Chanhassen, 1995).
4. S. Caporali, U. Bardi, and A. Lavacchi, J. Electron
Spectrosc. Relat. Phenom. 151, 4 (2006).
5. T. Cremer, C. Kolbeck, K. R. J. Lovelock, N. Paape,
R. Wölfel, P. S. Schulz, P. Wasserscheid, H. Weber,
J. Thar, B. Kirchner, F. Maier, and H.-P. Steinrück, 20. S. Spange, R. Lungwitz, and A. Schade, J. Mol. Liq.
Chem.-Eur. J. 16, 9018 (2010).
192, 137 (2014).
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