Phonon-like Hydrogen-Bond Modes in Protic Ionic Liquids

Article abstract of DOI:10.1021/jacs.7b03036

Gigahertz- to terahertz-frequency infrared and Raman spectra contain a wealth of information concerning the structure, intermolecular forces, and dynamics of ionic liquids. However, these spectra generally have a large number of contributions ranging from slow diffusional modes to underdamped librations and intramolecular vibrational modes. This makes it difficult to isolate effects such as the role of Coulombic and hydrogen-bonding interactions. We have applied far-infrared and ultrafast optical Kerr effect spectroscopies on carefully selected ions with a greater or lesser degree of symmetry in order to isolate spectral signals of interest. This has allowed us to demonstrate the presence of longitudinal and transverse optical phonon modes and a great similarity of alkylammonium-based protic ionic liquids to liquid water. The data show that such phonon modes will be present in all ionic liquids, requiring a reinterpretation of their spectra.

Full text of DOI:10.1021/jacs.7b03036

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Communication
Phonon-like hydrogen-bond modes in protic ionic liquids
Judith Reichenbach, Stuart Ruddell, Mario González-Jiménez, Julio
Lemes, David Andrew Turton, David J. France, and Klaas Wynne
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 May 2017
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Phonon-like hydrogen-bond modes in protic ionic liquids
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Judith Reichenbach, Stuart A. Ruddell, Mario González-Jiménez, Julio Lemes, David A. Turton,
David J. France, and Klaas Wynne*
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School of Chemistry, WestCHEM, University of Glasgow, UK
Supporting Information Placeholder
studies have been carried out on ionic liquids in this
frequency domain using far-infrared and terahertz
time-domain spectroscopy5-8 (THz-TDS), as well as
Raman and optical Kerr-effect (OKE) spectroscopy.9-
ABSTRACT: Gigahertz to terahertz frequency infra-
red and Raman spectra contain a wealth of infor-
mation concerning the structure, intermolecular
forces, and dynamics of ionic liquids. However, these
spectra generally have a large number of contribu-
tions ranging from slow diffusional modes to under-
damped librations, and intramolecular vibrational
modes. This makes it difficult to isolate effects such
as the role of coulombic and hydrogen-bonding in-
teractions. We have applied far-infrared and ultrafast
optical Kerr-effect spectroscopies on carefully select-
ed ions with a greater or lesser degree of symmetry in
order to isolate spectral signals of interest. This has
allowed us to demonstrate the presence of longitudi-
nal and transverse optical (LO and TO) phonon
modes and a great similarity to liquid water of al-
kylammonium-based protic ionic liquids. The data
show that such phonon modes will be present in all
ionic liquids requiring a reinterpretation of their
spectra.
12
However, there are many other motions besides
those associated with hydrogen bonding that give
rise to spectral bands in this range: librations, low-
frequency intramolecular vibrations, as well as orien-
tational relaxation. Therefore, it is difficult to obtain
reliable information on hydrogen bonding.
In previous work on (non-ionic) liquids and solu-
tions, we have shown that highly symmetric mole-
cules or even atoms can be used to “switch off” sin-
gle-molecule related dynamics in order to isolate in-
termolecular dynamics.13-14 The far-infrared spectrum
is proportional to the dipole-dipole correlation func-
tion <ꢀ(0)ꢀ(t)> where ꢀ is the dipole-moment densi-
ty, while the OKE (or reduced depolarized Raman)
spectrum is proportional to the polarizability-
polarizability correlation function <
α(0)α(t)> where
α
is the collective anisotropic polarizability tensor.15-
16
In molecules or ions with a center of inversion or
tetrahedral symmetry, the molecular dipole moment
and anisotropic polarizability tensor are by defini-
tion zero and the spectra are solely determined by
intermolecular dynamics. These dynamics should be
the same whether measured using OKE or using far-
IR and should therefore give rise to corresponding
bands in the two spectra (although with different
relative amplitudes).
Ionic liquids are organic salts with melting points
closer to traditional solvents than purely inorganic
salts. They retain the useful physical properties asso-
ciated with molten salts, such as low vapor pressure
and a wide electrochemical window. This combina-
tion makes them candidates for applications in syn-
thesis, catalysis, electrochemistry, and battery devel-
opment.1-4 Protic ionic liquids, which are capable of
forming hydrogen bonds, are currently under intense
investigation as electrolytes in batteries and hydro-
gen-based fuel cells.1, 3 Hydrogen bonding is thought
to play a major role in determining the physical
properties of ionic liquids in general and protic ionic
liquids in particular.5-7
Here we will show that this effect can be used in
the study of hydrogen bonding in protic ionic liq-
uids. Ions were initially chosen to switch off all intra-
molecular signals through symmetry. The symmetry
was then broken gradually in order to switch on par-
ticular spectral signals one by one. This allows us to
isolate hydrogen-bonding modes in protic ionic liq-
uids and demonstrate their phonon-like character.
Vibrational and relaxational modes associated with
hydrogen bonding are expected to occur in the giga-
hertz to terahertz frequency range. Spectroscopic
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The ionic liquids were studied using OKE and far- show intermolecular dynamics in the spectra on ac-
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infrared Fourier-transform infrared spectroscopies
(see SI). OKE experiments were carried out with
femtosecond laser pulses using pump-probe delay
times of up to one nanosecond. The Fourier trans-
form of the time-domain signal results in a depolar-
ized reduced Raman spectrum with a good signal-to-
noise ratio between ~1 GHz (0.03 cm-1) and ~15 THz
(500 cm-1).17-18
count of their high symmetry (see Figure 1, torsional
modes are expected to be weak in the OKE and very
weak in the far-IR spectra). In both spectra, one
main band is visible below 150 cm-1 with less intense
vibrational bands at higher frequency. However,
while the main band appears symmetric in the OKE
spectrum it shows an obvious shoulder at lower fre-
quency in the far-IR spectrum. Additionally, the
OKE spectrum shows two vibrational bands while
the far-IR spectrum shows only one.
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Initial OKE and far-IR measurements were carried
out on the ionic liquid tetrabutylammonium chlo-
ride consisting of ions that should predominantly
Figure 1. Terahertz spectra and fits of liquid and crystalline tetrabutylammonium chloride. (a) Shown is the experi-
mental OKE spectrum of the liquid at 90 °C (black), the overall fit (red), and component functions of the fit (blue and green). (b)
The experimental far-IR spectrum of the liquid at 90 °C (black), the overall fit (red), the contribution to the absorption due to
reflectivity (orange), and component functions of the fit (blue and green). (inset) A comparison of the far-IR spectra of the liquid
(red) and the crystal (dark blue, 2:1 by volume mixture of polyethylene with tetrabutylammonium chloride) at room tempera-
ture.
The vibrational bands were fitted with Brownian-
oscillator functions while the main band in both the
OKE and far-IR spectrum was fitted with a common
set of Gaussians with identical spectral parameters
(frequency, damping but not amplitude; see the SI
for a more detailed discussion of the fitting proce-
dure). As described in the SI, bands that are common
to the far-IR and OKE spectra (such as the band la-
beled A in Figure 1), typically peak at a slightly differ-
ent frequency due to the different inherent frequency
dependence of the two techniques.16 A Debye func-
tion was used to account for diffusive modes below
10 cm-1, which were not clearly observable in the far-
IR spectra. The main band seen in the far-IR spec-
trum could be fit with two Gaussian functions of
similar intensities, A at 47 cm-1 and C at 89 cm-1 (see
Table SI1). A third Gaussian function, D, was needed
to fit the tail at higher frequencies in both spectra.
The inset of Figure 1(b) shows the far-IR spectrum of
crystalline tetrabutylammonium chloride mixed with
polyethylene, which shows the two bands A and C as
narrow and clearly separated peaks confirming the
validity of the fitting procedure used for the liquid.
Next the symmetry of the cation was reduced and
hydrogen bonding was introduced by replacing tet-
rabutylammonium with tributylammonium. The
OKE and far-IR spectra of liquid tributylammonium
chloride are shown in Figure 2. Comparing Figure
2(b) with Figure 1(b), it can be seen that the single
band is now split into two in tributylammonium
chloride. The split between the A and C bands is
114 cm-1 in tributylammonium chloride (compared to
42 cm-1 in tetrabutylammonium chloride) and per-
sists in the crystalline spectrum (see the inset of Fig-
ure 2(b)).
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Figure 2. Terahertz frequency-domain spectra and fits of liquid and crystalline tributylammonium chloride showing a
hydrogen-bonding induced split. Shown are the OKE (a) and far-IR (b) spectra of the liquid at 80 °C and a comparison with
the pellet far-IR spectrum (inset) at room temperature. Color coding as in Figure 1.
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In order to switch off hydrogen bonding but main-
tain asymmetry in the cation, tributylmethylammo-
nium chloride was also studied. As can be seen in
Figure 3(a), the spectrum of liquid tributylme-
thylammonium chloride is similar to tetrabu-
tylammonium chloride in showing no split in the far-
IR spectrum. Therefore, this split is not associated
with the break in symmetry but rather with the for-
mation of hydrogen bonds. Figure 3(b) compares the
far-IR spectra of liquid tributylammonium chloride
and bromide, which leads to a shift of both the A and
C bands towards lower frequency. The average ratio
of the center frequencies is 0.64 for the tribu-
tylammonium halides. For the tetrabutylammonium
halides (Figure SI1(a)) and for the tributylme-
thylammonium halides (Figure SI1(b)) an average
ratio of 0.70 was observed.
Figure 3. Effect of changing the cation or the anion on the terahertz frequency-domain far-IR spectra. (a) Comparison
of the far-IR spectra of liquid tributylammonium and tributylmethylammonium chloride. (b) Comparison of the far-IR spectra of
liquid tributylammonium chloride and bromide.
Fitting the OKE and far-IR data shows that there is
a small number of inhomogeneously broadened un-
derdamped bands in the 0-200 cm-1 (0-7 THz) range.
Underdamped bands in the terahertz range are usu-
ally associated with librational modes of the mole-
cules in the liquid. However, for such librational
bands to appear in the OKE spectrum, the molecular
polarizability tensor must be anisotropic. Similarly,
for them to appear in the far-IR spectrum, the mole-
cules must have a dipole moment. Due to its tetra-
hedral symmetry, tetrabutylammonium has neither.
Of course, the tetrahedral symmetry of tetrabu-
tylammonium is likely to be distorted in the liquid
and therefore one may expect a weak OKE signal and
weak far-IR absorption. However, as can be seen in
Figure 1(b), the peak absorption is 2.2 105 m-1, which
is very strong and comparable to that of liquid
water.19 Similarly, one might expect torsional modes
in this region but these are also predicted to be weak.
This strongly suggests that the absorption is caused
by the motion of the relatively light and mobile chlo-
ride anion in a cage of tetrabutylammonium
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cations.20 Such a motion corresponds to a lattice
phonon mode and momentum conservation would
dictate that this should be the LO or TO phonon
band. This behavior is similar to that seen in the te-
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rahertz spectrum of liquid water that exhibits
~2 THz TA and ~5 THz LA phonon modes at the
zone edge.23 As can be seen in Figure 2, there is a
large drop in intensity of the LO band (relative to the
TO band) on going from the far-IR to the OKE spec-
trum, which is consistent with other examples in the
literature.24-25
near the zone center (
). The inset to Figure 1(b)
shows that the broad band in the liquid narrows to
two bands in the solid consistent with the assign-
ment as the LO and TO phonons.
Changing the anion from chloride to bromide in
the alkylammonium salts gives rise to a change of the
center frequency of phonon bands. The frequency
change for two different salts 1 and 2 due to a change
9
The band labeled A has previously been assigned
to librational motions of the cation combined with a
weak contribution of unspecific cation–anion inter-
action modes.5, 26 Of course, the spectra of ionic liq-
uids with less symmetric cations could have contri-
butions from cation librations. The calculated per-
manent dipole moments of tetraethylammonium,
triethylmethylammonium, and triethylammonium
ions are 0.7 D, 1 D, and 2.4-3.4 D (depending on the
isomer. See SI). Thus, despite the cations having
small dipole moments relative to their volume, the
far-IR absorption is very strong and strongest for the
cation with the smallest dipole moment, ruling out
an origin of the far-IR absorption in librational mo-
tion. Similarly, the calculated molecular polarizabil-
ity tensors for the three cations are very nearly iso-
tropic (see SI). As the OKE signal strength is large in
all the ionic liquids studied here, this again rules out
an origin of the OKE spectra in librational motion.
Hence, the data show that by far the major contribu-
tion in this spectral range comes from translational
motion of the anion relative to the cations, which we
have identified as the LO and TO phonons.
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in masses only, can be calculated using
=
/
at the zone center
at the zone edge
(k = 0) and
(k = 2π), where mx is the mass of the lighter ion (the
anion in this case).21 The average ratio calculated
from our fits is 0.64 for the PILs (tributylammonium
chloride and bromide, 0.62 for the A and 0.69 for the
C band) and 0.70 for the non-protic ILs (tetrabu-
tylammonium chloride/bromide and tributylme-
thylammonium chloride/bromide). The expected
shift is 0.67 at the zone edge and 0.72 to 0.73 at the
zone center. Thus, the experimental values are closer
to those predicted for the zone edge, which can be
understood as a localization effect caused by the dis-
order in the liquid reducing the phonon dispersion.
It is immediately obvious by comparing Figure 1(b)
with Figure 2(b) and Figure 3 that the peak absorp-
tion in tetrabutylammonium chloride is an order of
magnitude higher than in the other ionic liquids.
The absorption is a frequency dependent quantity
(see Eq. SI6), however, the amplitudes used in fitting
and listed in Table SI1 are not frequency dependent
and scale with the (transition) dipole moment
squared.16 By comparing amplitudes of band A,
which is the band with the highest amplitude, it can
be seen that the ratio is > 9× for the chlorides and >
18× for the bromides. This shows that the anions are
less mobile in the tributylammonium and tributyl-
methylammonium ionic liquids. The simplest expla-
nation is that the anions and cations are more
strongly paired in these liquids due to the easier ac-
cessibility of the positive charge centered approxi-
mately on the nitrogen atom and therefore greater
coulombic interaction.
It is found that the OKE signal strength of all the
alkylammonium ionic liquids studied here are ap-
proximately the same and only a factor of 2-3 weaker
than that of typical pyridinium and imidazolium-
based ionic liquids. The far-IR absorption strength in
the few-terahertz region is also similar between al-
kylammonium24 and imidazolium-based10 ionic liq-
uids. This then implies that TO and LO bands are
likely to be significant in the terahertz far-IR and
OKE spectra of all ionic liquids irrespective of
whether they have significant hydrogen-bonding be-
tween ions or not. The masses of typical anions used
-
in ionic liquids (e.g., BF4 , dicyanamide, methyl-
sulfonate) are all rather similar to chloride and bro-
mide and therefore one expects TO and LO bands at
terahertz frequencies in the spectra of all ionic liq-
uids. In ionic liquids with less symmetric and strong-
ly anisotropically polarizable cations such as pyri-
dinium and imidazolium, it will be more difficult to
observe the TO and LO bands but their contribution
is predicted to be significant. This will require a rein-
terpretation of ionic liquid spectra and will provide
In the tributylammonium salts there is an addi-
tional splitting of the LO and TO phonons due to
additional hydrogen bonding interactions. It is gen-
erally expected that the LO phonon has a higher fre-
quency than the TO phonon.22 Hence the A band is
identified as the TO (hydrogen-bond bend) and the
C band as the LO (hydrogen-bond stretch) phonon
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se liquids.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge
on the ACS Publications website.
Materials, experimental details, additional data, data
analysis, curve fitting procedures, fit parameters, calcu-
lation of dipole moments and molecular polarizabili-
ties, melting points, and references (PDF).
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AUTHOR INFORMATION
Corresponding Author
*klaas.wynne@glasgow.ac.uk
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the Engineering and Physical Sciences Re-
search Council (EPSRC) for support through grants
EP/J009733/1, EP/J00975X/1, EP/K034995/1, and
EP/N508792/1.
Data availability
The FTIR data and the OKE data that support the
findings of this study are available in Enlighten: Re-
search Data Repository (University of Glasgow) with
the
identifier:
http://dx.doi.org/10.5525/gla.researchdata.404.
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