Choline salicylate ionic liquid by X-ray scattering, vibrational spectroscopy and molecular dynamics

Article abstract of DOI:10.1016/j.molliq.2016.02.020

We report here a combined experimental and theoretical study on the bio-compatible salicylate choline ionic liquid. The liquid structure has been investigated by X-ray diffraction and vibrational (IR and Raman) spectroscopy. Local structure has been obtained from ab initio calculations on static ion pairs and from dynamic simulations of a small portion of the liquid. The theoretical models indicate that salicylate is connected by hydrogen bonding to choline mainly through the carboxylate group and forms stable ion pairs. A strong intramolecular interaction hinders internal rotations of the OH group of salicylate and competes with the hydrogen bonding with choline. When the liquid has been simulated by classical force fields we found a good agreement with the X-ray experimental features, comparable to that obtained from AIMD simulations. Important insights on hydrogen bonding between carboxylate and choline have been also derived from the analysis of the CO stretching modes of carboxylate measured in the Raman and IR spectra and calculated from VDOS-Wannier centers procedures.

Full text of DOI:10.1016/j.molliq.2016.02.020

Journal of Molecular Liquids 218 (2016) 39–49
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Choline salicylate ionic liquid by X-ray scattering, vibrational
spectroscopy and molecular dynamics
a
a
a
a,
b
b,
Luana Tanzi , Michele Nardone , Paola Benassi , Fabio Ramondo ⁎, Ruggero Caminiti , Lorenzo Gontrani ⁎
a
Department of Physical and Chemical Sciences, University of L'Aquila, Via Vetoio, I-67100, L' Aquila, Italy
Department of Chemistry, University of Rome ‘La Sapienza’, P.le Aldo Moro 5, I-00185 Rome, Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here a combined experimental and theoretical study on the bio-compatible salicylate choline ionic liq-
uid. The liquid structure has been investigated by X-ray diffraction and vibrational (IR and Raman) spectroscopy.
Local structure has been obtained from ab initio calculations on static ion pairs and from dynamic simulations of a
small portion of the liquid. The theoretical models indicate that salicylate is connected by hydrogen bonding to
choline mainly through the carboxylate group and forms stable ion pairs. A strong intramolecular interaction hin-
ders internal rotations of the OH group of salicylate and competes with the hydrogen bonding with choline.
When the liquid has been simulated by classical force fields we found a good agreement with the X-ray experi-
mental features, comparable to that obtained from AIMD simulations. Important insights on hydrogen bonding
between carboxylate and choline have been also derived from the analysis of the CO stretching modes of carbox-
ylate measured in the Raman and IR spectra and calculated from VDOS-Wannier centers procedures.
Received 6 October 2015
Accepted 7 February 2016
Available online xxxx
2010 MSC:
0
9
0-01
9-00
Keywords:
X-ray diffraction
Ab initio calculations
Molecular dynamics
Vibrational spectroscopy
Ionic liquids
©
2016 Published by Elsevier B.V.
1
. Introduction
hydrogen bonding interaction. Recently infrared and Raman spectra
10] and X-ray studies [11] of three choline-based BioILs coupled with
[
Attention on ionic liquids (ILs) [1-3], molten salts at room tempera-
carboxylate anions have been reported and important structural fea-
tures have been derived from classical and ab initio molecular dynamics.
Furtherly, systems consisting of choline cation coupled with a series of
amino acid anions have been investigated by X-ray diffraction [12,13],
IR spectroscopy [12] and molecular dynamics [12-14]. All these studies
[10-14] show that hydrogen bonding is a crucial feature in establishing
the local geometric structure for these BIoILs.
Among choline-based BioILs, Yu et al. [15] synthesized choline
benzoate and choline salicylate. From their density data [15] it was
observed that they have molar volume lower than common ionic liquids
ture, is increasing day by day in science and technology. Among their
unique characteristics like conductivity, polarity, chemical and thermal
stability, nonflammability, the negligibly low vapor pressure is the
fundamental feature to consider them as green solvent. The toxicity of
a few ILs [4] lead to the development of ILs entirely composed of
biomaterials (BioILs). They combine some of the excellent properties
of traditional ILs with the advantage of bio-degradability, biocom-
patibility and non-toxicity. Their facile and green preparation, such
as simple exchange and/or acid/base reactions [5], and the use of
readily available and inexpensive starting materials, are further
advantages of BioIls over ILs. The BioILs based on cholinium cation
and consequently they can moderately solubilize species like CO
in this aim, a computational study has been carried out on these ionic
liquids in the pure state and after CO adsorption by classical and quan-
2
. With-
(
2-hydroxyethyl)trimethylammonium coupled with a large variety
2
of carboxylic acids of natural origin have found great interest [6]. Most
studies on BioILs deal with their synthesis, their physical chemical
properties [7-9] whereas few papers have studied their structure.
Their important properties stem from the complexity of the interactions
between molecular constituents, including long range Coulombic
terms, short range van der Waals contributions along with anisotropic
tum chemical methods [16]. In order to characterize the structure of this
class of choline-based ionic liquids by experiments, we present here a
study of one member of the series, choline salicylate [Ch][Sal], in
which we employ a combination of ab initio molecular dynamics
(AIMD), classical molecular dynamics (MD) and experimental (X-ray
diffraction, infrared and Raman spectroscopy) measurements with the
aim of interpreting geometrical features and dynamic aspects of its
structure. We have chosen a carboxylate anion, salicylate, able to form
both intramolecular and intermolecular hydrogen bonding and it is in-
teresting to compare its structure with that of systems where choline
is coupled with simpler alkylcarboxylate anions [10,11]. The diffraction
⁎
Corresponding authors.
E-mail addresses: fabio.ramondo@univaq.it (F. Ramondo),
lorenzo.gontrani@uniroma1.it (L. Gontrani).
http://dx.doi.org/10.1016/j.molliq.2016.02.020
0167-7322/© 2016 Published by Elsevier B.V.

40
L. Tanzi et al. / Journal of Molecular Liquids 218 (2016) 39–49
patterns from measured X-ray and those from MD simulations can be
compared to provide a validation of the theoretical models here applied.
Alternatively to X-ray studies, vibrational spectroscopy can be
extremely useful for evaluating the nature of the cation–anion interac-
tions, identifying ion pair coupling and clarifying conformational iso-
mers [17-27]. For example, the presence and the force of hydrogen
bonding may be obtained from the vibrational frequencies of the OH
group of choline and the COO– group of carboxylate since directly in-
volved in such interaction. On the other hand non-empirical AIMD sim-
ulations have been recently and successfully applied to a wide number
of ionic liquids [28-32] to investigate their vibrational properties and as-
sign the observed absorptions.
(with nitrogen as sharpening atom) to enhance the resolution of the
curve at high q values and to decrease the truncation error in the calcu-
lation of the Fourier transform from the reciprocal space (q) to the direct
one (r). The expression for M(q) is
2
f ð0Þ
ꢀ
ꢁ
N
2
MðqÞ ¼
exp ꢀ0:01q
ð3Þ
2
f ðqÞ
N
Fourier transform of qI(q)M(q) led to radial distribution function
r.d.f.)
(
2r
π
qmax
0
2
DðrÞ ¼ 4πr ρ þ
∫
qIðqÞMðqÞ sinðrqÞdq
ð4Þ
0
2
. Experimental details
2
3
where ρ
0
(electrons /Å ) is the bulk number density. When the uniform
2
2
.1. Synthesis
distribution component is dropped (4πr ρ ), we obtain the differential
0
correlation function, Diff(r), which contains only the structural contri-
Reagents used in the synthesis were choline hydroxide solution
bution to the distribution function,
(
2
46 wt.% in H O, Aldrich) and salicylic acid(99%, Aldrich), both used
2
without further purification. Choline salicylate was synthesized by
dropwise addition of salicylic acid to the choline hydroxide solution in
ratio 1:1 (e.g. 0.12 mol:0.12 mol), stirring continuously at room temper-
ature and pressure for 12 h. Most of the water in the reaction mixture
was removed under reduced pressure, using a rotary evaporator at
Diff ðrÞ ¼ DðrÞ ꢀ 4πr ρ0
ð5Þ
2.3. Infrared and Raman spectra
7
0 °C for 4 h. The mixture was then dried in vacuo, heating at 70 °C
The Fourier transformed infrared (FTIR) spectrum was measured at
−
1
and stirring for 24 h. The purity of these BioILs was checked by 1H
NMR and 13C NMR spectroscopy, using a Bruker Avance III spectrome-
ter operating at 400 MHz and 100.6 MHz, respectively. The spectra have
confirmed the absence of any major impurities and the water final
content, evaluated by 1H NMR analysis, has been estimated below
room temperature from 4000 to 400 cm
after 100 scans using the
Perkin Elmer Spectrum two FT-IR spectrometer. Liquid samples were
placed as thin films between KBr plates. The Raman spectrum was
measured using a LABRAM confocal-microscope Raman spectrometer
by HORIBA Jobin Yvon using 5 mW at 632 nm excitation source and a
20× collection optics. The instrumental resolution is of the order of
0
.4 wt.%.
−
1
2
–3 cm . Background fluorescence has been fitted using a polynomial
2
.2. X-ray scattering
expression and subtracted from the data.
3. Computational details
The large angle X-ray scattering experiments were performed using
the non-commercial energy-scanning diffractometer built in the De-
partment of Chemistry at the University La Sapienza of Rome. (Patent
no. 01126484— 23 June 1993). Detailed description of instrument, tech-
nique and the experimental protocol (instrument geometry and scatter-
ing angles) of the data acquisition phase can be found in a series of
papers by our group [33-38]. The appropriate measuring time (i.e. num-
ber of counts) was chosen to obtain scattering variable (q) spectra with
high signal to noise ratio (500,000 counts on average).
3.1. Liquids by molecular dynamics
In order to describe the liquid structure we have followed two differ-
ent approaches. Local structural characterization with particular atten-
tion to the hydrogen bonding interaction occurring between ion pairs
was initially performed by ab initio methods.
Quantum-mechanical calculations on the choline–salicylate ion pair
were performed using the Gaussian 09 package [39]. Equilibrium
geometry and vibrational frequencies were obtained using Moller–
Plesset (MP2) and density functional theory (DFT) methods with the
B3LYP [40,41] exchange and correlation functional and employing the
The expression of q is:
q ¼ 4
π sinðθÞ
¼ 1:014E sinðλÞ
ð1Þ
λ
6
-311++G** basis set.
Dynamic effects were introduced at ab initio level by AIMD simula-
where E is expressed in keV and q in Å−1. The various angular data were
processed according to the procedure described in literature [33-38],
normalized to a stoichiometric unit of volume containing one ion pair
and combined to yield the total (static) “structure factor”, I(q),
tions performed on a model of 9 ion pairs using the Born–Oppenheimer
Molecular Dynamics (BOMD) method implemented in the CP2K code
[
42]. Potential energy calculations were carried out using BLYP
exchange-correlation functional [41,43] with the Grimme's dispersion
correction D3 [44] and the hybrid Gaussian and plane wave (GPW)
basis set [45]. Gaussian basis set was DZVP-MOLOPTSR-GTH and plane
waves expansion was developed in a periodic cubic system with unit
N
2
i
IðqÞ ¼ Ie:u:ðqÞ ꢀ ∑ xi f
ð2Þ
i¼1
3
where f
i
are the atomic scattering factors, x
i
are the number concentra-
cell edge of 14.53 Å and truncated at 320 Ry. Goedecker–Teter–Hutter
tions of the i-type atoms in the stoichiometric unit (i.e. the group of
(GTH) pseudopotentials [46,47] were employed to describe core elec-
trons. A pre-equilibration was performed employing classical molecular
dynamics within periodic boundary conditions using the two-body
Generalized Amber Force Field (GAFF) [48]. AIMD simulations started
from a snapshot of the classical MD simulation and the system was
equilibrated for 6.3 ps in the NVT ensemble at 300 K using the individual
thermostat for each degree of freedom for the first 3 ps and a global
Nosé–Hooverchain thermostat [49-51] for the remaining time.
Timestep was set to 0.4 fs. Trajectory was then collected for 20 ps in
particles used as reference for data normalization, the ion pair in this
2
case) and Ie.u. is the observed intensity in electron units (electrons ).
Such function depends on the scattering contributions of all the par-
ticles of the system, according to the pairwise distances between them
(
in the case of X-rays, the atoms scatter the radiation through the elec-
tron clouds surrounding them) and therefore, it gives a sort of mathe-
matical picture of the spatial disposition of the sample atoms. This
function is multiplied by q and q-dependent sharpening factor, M(q)

L. Tanzi et al. / Journal of Molecular Liquids 218 (2016) 39–49
41
Fig. 1. Salicylate anion (a) and choline cation (b).
the NVT ensemble, saving velocities and coordinates every steps. During
the production run, the Wannier centers [52] of the whole system have
been evaluated at every sixth step in order to obtain the IR spectra.
In order to provide a full characterization of the liquid structure
within the range of radial distances accessible to experiments we have
performed a series of simulations with the previously cited (GAFF)
using Gromacs v.4.6.2 package [53] as molecular dynamics engine. Sim-
ulations were carried out starting from partial atomic charges obtained
using the Restrained Electrostatic Potential (RESP) method [54,55] by
fitting the electrostatic potential for isolated cations and anions at the
equilibrium geometry calculated at the HF/6-31G* level. The use of
HF/6-31G* method has been demonstrated to lead to the implicit polar-
ization required in the additive FF model of condensed phase systems,
and complies with the rest of the GAFF parameter set, optimized to be
compatible with the Cornell et al. families of Amber force fields [56]
that implement this charge derivation scheme. Electrostatic interactions
were calculated using Particle Mesh Ewald (PME) under periodic
boundary conditions and Linear Constraint Solver (LINCS) algorithm
was applied to all bonds involving hydrogen atoms. Cutoff radii for
van der Waals and direct-space Ewald interactions were set to 10 Å.
Parallelization was carried out with domain decomposition strategy
and Message Passing Interface (MPI) paradigm.
where rmax is half the box edge. The partial structure factors and the the-
oretical total structure factor were calculated with an in-house written
code. Theoretical I(q) was then multiplied by q and the sharpening
factor, M(q), to obtain a theoretical qI(q)M(q) function comparable
with the experimental one. Theoretical D(r) and Diff(r) were calculated
as described in the previous section for the experimental curves.
Simulations have been performed on a cubic cell with edges of about
4
5 Å containing 300 ion pairs (CHSAL). Equilibration consisted in
gradually heating the systems at 550 K in the NPT ensemble and then
in cooling them to 300 K using Nosé–Hoover thermostat [49-51] and
Parrinello–Rahman barostat [57-59] set to 1 atm. Equilibration time
was about 1 ns and a relaxation period of 500 ps without barostat
followed. Subsequently, the systems were simulated in NVT ensemble
for 3.5 ns with integration time step of 2 fs and trajectories were
collected every 1000 steps.
3
.2. MD-derived structure factor
Molecular Dynamics allows us to calculate theoretical structure
factors from pair correlation functions gij(r). In fact, structure factor
can be expressed with the following equation [60]
N
N
IðqÞ ¼ ∑ ∑ xixj f ðqÞf ðqÞHijðqÞ
ð6Þ
i
j
i¼1 j¼1
in which Hij(q) are the partial structure factors, defined in terms of
the pair correlation functions by the Fourier integral
Fig. 2. (a) Scan of the dihedral angles θ and τ calculated for salicylate anion at the B3LYP/6–
311++G** level; energies are calculated as E(θ, τ) − E(θ=0 , τ=0 ). (b) Scan of the
h
i
∘
∘
r max
0
r2 gijðrÞ ꢀ 1
sinðqrÞ
HijðqÞ ¼ 4πρ0∫
dr
ð7Þ
dihedral angle τ calculated for salicylate choline ion pair at the B3LYP/6–311++G**
qr
∘
level; energies are calculated as E(τ) − E(τ=0 ).

42
L. Tanzi et al. / Journal of Molecular Liquids 218 (2016) 39–49
3
.3. Vibrational spectra and assignment of vibrational modes
The minimization of functional Ω(n) leads to an eigenvalue problem
providing effective normal modes. We carried out minimization setting
n = 2. For each effective normal mode, the power spectrum becomes lo-
calized in frequency [61,62].
From static calculations on the ion pairs, vibrational frequencies and
normal modes were computed by diagonalization of the Hessian matrix
of the Potential Energy Surface (PES) at the equilibrium geometries.
From the AIMD results, vibrational frequencies were obtained by ana-
lyzing the Vibrational Density of States (VDOS) [61,62]:
4. Results and discussion
4.1. Ab initio static ion pair
N
Salicylate anion, Fig. 1(a), choline cation, Fig. 1(b) and choline
VDOSðωÞ ¼ ∑ ∫ h r_ iðtÞ r_ ið0Þi expðiωtÞdt
ð8Þ
i¼1
salicylate ion pair were studied to investigate coupling geometry and
structural changes due to association. Geometry optimizations reveal
that the OH and COO– groups of salicylate interact through strong intra-
molecular hydrogen bonding both before and after association with
choline.
The O–H distance, very short in isolated salicylate (1.452 Å B3LYP
and 1.4295 Å MP2) suggests an easy intramolecular proton transfer.
Such distance increases upon association with choline (1.641 Å B3LYP
and 1.625 Å MP2) revealing that the choline coordination favors the
localization of the proton on the OH group.
Since VDOS is defined as sum of the Fourier transform of the atomic
velocity autocorrelation functions (or power spectra), frequencies
derived from such procedure include anharmonicity and temperature
effects [62]. VDOS spectra can be decomposed into sum of power spec-
tra of collective coordinates of the molecule q
harmonic oscillations [61,62]:
k
describing independent
VDOSðωÞ ¼ ∑ ∫ h q_ ðtÞ q_ ð0Þi expðiωtÞdt
ð9Þ
k
l
k;l
As for other carboxylate anions [10,11], interaction of choline with
salicylate involves preferentially the COO– group for the presence of
strong Coulombic interactions. Previous calculations [16] revealed that
alternative structures, for example that where choline interacts with
the OH group of salicylate, are remarkably less stable and therefore
they were not considered in our study. The flexibility of the OH group
of salicylate has been already tested through B3LYP/6–311++G** cal-
culations of torsional barrier height on the salicylate anion by Aparicio
[16]. Relaxed torsional scan has been here calculated again on the isolat-
ed anion by MP2/6–311++G** level confirming the high rotational
barrier (about 90 kJ/mol). Simultaneous rotations of the COO– and OH
groups have been further investigated at the B3LYP/6-311++G**
level and Fig. 2(a) shows how the potential energy changes with the
dihedral angles (θ and τ) which define the orientation of the substitu-
ents with respect to benzene ring. Results confirm that intramolecular
k
The collective coordinates q are determined minimizing the func-
tional Ω(
n)
:
"
#
ꢃ
ꢄ
2
β
π
ꢂ
ꢂ
ꢂ
ꢂ
β
2π
ðnÞ
2n
q
k
n
q
k
Ω
¼ ∑ ∫dω ω P ðωÞ−
∫dωjω jP ðωÞ
ð10Þ
k
q
k
where P
(ω) is the power spectrum of k-th collective coordinate q
k
and
n is a fixed parameter, and imposing the decorrelation of vibrational
normal modes and the equipartitioning of the energy as necessary
conditions:
1
∫
dω∫ h q_ ðtÞ q_ ð0Þi expðiωtÞdt ¼ h q_ ðtÞ q_ ðtÞi ¼ kBTδ
ð11Þ
k
l
k
l
kl
2π
Fig. 3. The two stable ab initio ion pairs of [Ch][Sal]; the interaction distances (Å) and the interaction energies were obtained at the MP2/6–311++G** level.

L. Tanzi et al. / Journal of Molecular Liquids 218 (2016) 39–49
43
structure with that of ion pairs where choline interacts with
alkylcarboxylate anions, systems able to form only intermolecular
interactions [10,11]. We notice that in salicylate hydrogen bonding
with choline seems to be less stable due to competition between two
hydroxyl groups: the hydrogen bond distance is about 1.7 Å whereas
that in choline alkylcarboxylate pairs is within 1.6–1.7 Å. Among the
alternative interaction geometries, we found a structure in which
choline forms hydrogen bond with the other oxygen atom of carboxyl-
ate (O1), Fig. 3(b). The two structures together with geometries and in-
teraction energies are shown in Fig. 3. Their stabilities, calculated at
B3LYP and MP2, are indeed very similar (within 1 kJ/mol) suggesting
that the potential around the carboxylate group is almost symmetrical
and choline may interact with the oxygen atoms of the anion without
any preference. Such structures were furtherly studied using the same
plane wave basis set and the same functional, BLYP/GPW, employed
for AIMD simulations. The hydrogen bond distances appear shorter
(
1.55 Å) than B3LYP and MP2 values (1.69 Å) however the binding
energies of the two ion pairs are again very similar. We found only a
slight preference for the structure O1–OH probably not significant. As
already observed in choline alkylcarboxylate ion pairs [10,11], the coor-
dination structure is also affected by secondary interactions CH–O as
witnessed by some contacts between methyl hydrogen atoms and O1
or O2 atoms (see Fig. 3).
4.2. Classical molecular dynamics
Liquid structure was investigated through classical molecular
dynamics. Despite classical force fields not always reproducing all the
structural properties of protic ionic liquids [11,63], we decided to use
anyway the GAFF force field, a modified AMBER force field which shares
Fig. 4. RDFs of the oxygen–nitrogen distances (a) and RDFs of choline hydroxyl hydrogen
atom and salicylate oxygen atoms (b).
hydrogen bonding is the main cause of stability of the equilibrium struc-
ture of salicylate anion. The OH rotational barrier height is in fact very
high when carboxylate is coplanar to benzene (θ = 0°) and therefore
can form hydrogen bonding with OH group. A remarkable lowering of
barrier height is instead expected when rotation of OH group is carried
out by orienting the carboxylate in conformations which hinder
intramolecular interactions (for example θ = 90°). Consistently carbox-
ylate is nearly free to rotate when the OH group assumes orthogonal ori-
entation whereas it is rotationally hindered (70 kJ/mol) for coplanar
orientation of the OH group. The OH barrier height has been calculated
also for the salicylate choline ion pair and the results of Fig. 2(b) show
that the OH torsion in salicylate is clearly hindered also in the presence
of choline confirming that intramolecular hydrogen bonding is weakly
affected by the cationic coordination.
Several coordination geometries were studied in the previous work
[
16] and the structure in which carboxylate forms simultaneously intra-
and- inter-molecular hydrogen bonds through O atom (see Fig. 3a) was
found as the most stable ion pair. It's interesting to compare this
2
Fig. 5. Theoretical qI(q)M(q) and Diff(r) of CHSAL in comparison with experiment.

44
L. Tanzi et al. / Journal of Molecular Liquids 218 (2016) 39–49
with it the same functional form to keep generality and transferability
since it covers almost all the organic chemical space without need of
further parameterization.
The structure of the single ionic component was initially analyzed
through radial distribution functions of some intramolecular distances.
Concerning anion, our simulations confirm the rigidity of the salicylate
anion since no rotation occurs for both ring substituents. The distribution
distribution of the O–H distances, Fig. 4(b), very similar to that found
by Aparicio et al. [16], is a clear evidence that ions in liquid form very
stable ion pairs. In the present study we attempted to separate the O
from O atomic contribution in the O–H distance however both sites
showed similar probability to form hydrogen bonding with a very
small preference for O . The equivalence of the two hydrogen bonds is
1
2
1
consistent with the fact that the carboxylate group has a nearly
symmetric interaction potential in the classic force field here applied.
The hydrogen bond distance (1.61 Å) is higher than that in
alkylcarboxylate BioILs (1.55 Å) [11] and this is in agreement with ab
initio results on the ion pair. Another important difference from
alkylcarboxylate choline compounds is that the OH group of salicylate an-
ions could give additional hydrogen bonds with choline cations. However
the relative O3–H cho RDF shown in Fig. 4(b) reveals that this interaction
plays a marginal role as already predicted from ab initio calculations.
The occurrence of anion–anion interactions, through the possible
hydrogen bonding sites, as well as cation–cation interactions, through
O–HO interactions, can be discarded on the light of the respective radial
distribution functions and in agreement with the investigation of
Aparicio et al. [16].
2
of the O–H distance is very narrow around O and this is in agreement
with the high OH rotational barrier height found from ab initio calcula-
tions and discussed above. On the other hand, the radial distribution
function of Ncho–Ocho (Fig. 4a) allows us to investigate the distribution
of gauche and anti-conformations of choline cation in liquid. Any
variation of the NCCO dihedral angle changes the choline conformation
and it affects the Ncho–Ocho distance. The two narrow peaks are associ-
ated to anti (3.8 Å) and gauche (3.2 Å) conformations and Fig. 4a reveals
the presence of both the conformations with a small preference for the
anti-structure. This is not entirely consistent with the ab initio results
that predict an energetic preference for gauche conformations of isolated
and coupled choline [10]. Ab initio geometry optimizations were carried
out also in the present study for choline salicylate ion pairs starting
from anti-conformation of choline and the results confirm the higher sta-
bility (about 16 kJ/mol) for the gauche orientation. However introducing
a dielectric to simulate the bulk effect of the liquid is expected to lower
the gauch-anti relative stability [10]. In addition we note that the choline
conformation affects also the intermolecular hydrogen bonding since we
calculated a O–H distance for the anti-conformation (1.782 Å) higher
than that found for the gauche structure(1.694 Å).
1 2
Radial distribution functions of intermolecular N–O and N–O
distances are further shown in Fig. 4(a). It is interesting to observe
that the distribution of such intermolecular distances occurs at values
close to that of the N–O intramolecular distances of choline when it
assumes anti-conformations. In the same region of the radial function
3
another N–O distance is also expected, that due to O atom, whose dis-
tribution is centered to a higher value. Therefore in the region between
3.5 Å and 4.5 Å we expect to observe a quite complex pattern originated
by highly correlated intermolecular interactions (e.g. strong hydrogen
The structuring of liquid was then analyzed by monitoring the radial
distribution functions of some intermolecular distances. The narrow
Fig. 6. Radial distribution functions of the oxygen–nitrogen distances (a). Radial distribution functions calculated between salicylate hydroxyl hydrogen and oxygen atoms (b) and
between choline hydroxyl hydrogen and each salicylate oxygen atoms (c). AIMD Diff(r) in comparison with experiment (d).

L. Tanzi et al. / Journal of Molecular Liquids 218 (2016) 39–49
Table 1
45
bonds) along with intramolecular contacts between atoms with high
scattering factors (e.g. N–O of choline).
Selected vibrational frequencies (cm−1) of choline cation [Ch], salicylate anion [Sal] and
their ion pair [Ch][Sal] at different levels of calculation.
In order to provide a validation of the theoretical results here pre-
sented, we have compared the findings from the MD simulations to
the measured X-ray diffraction patterns. The structure function obtain-
ed from X-ray diffraction is reproduced in the upper panel of Fig. 5 and
the complementary Diff(r) function is presented in the lower panel. In
the same figure we present a comparison of the experimental curves
with the function calculated from the MD simulations. We have a
good agreement between the experimental and theoretical patterns
on all range for both the curves. Both structure factors and differential
correlation function follow experimental peak values and intensities in
both intra- and inter-molecular structural ranges. On the basis of this
agreement we can conclude that GAFF simulations provide a satisfacto-
ry description of choline salicylate liquid structure.
[Ch]
[Ch][Sal]
Fig. 3(b)
B3LYP
[Ch][Sal]
Fig. 3(a)
B3LYP
B3LYP
CP2K
CP2K
CP2K
Choline modes
νOH cho
δHOC cho
νCO cho
3831
1462
1101
314
3664
1415
1040
300
3228
1531
1114
918
2893
1531
1083
956
3237
1531
1113
931
2957
1523
1075
951
τCO cho
[
Sal]
Salicylate modes
νOH
δHOC
νCO sal
τCO
2357
1638
1262
1103
2048
1596
1304
1065
3168
1630/1668
1278
2829
1598
1226
950
3187
1627/1663
1283
2858
1587
1245
927
902
898
4
.3. Ab initio molecular dynamics
νCO as
1704
1340
815
1698
1331
820
1604
1368
812
1545
1314
797
1598
1371
809
1540
1327
795
νCO
s
A small portion of liquid was further studied by AIMD in order to val-
δOCO
γCOO
idate the results of classical MD description. Although the ab initio MD ap-
proach is much more computationally demanding than classical MD and
therefore it is often applied to small portion of liquids, it has the advan-
tage to include, at least in principle, the exact interaction energy up to
all the many body terms. The most important result here obtained is
that some structural features emerging from the classical description
are confirmed by ab initio simulations. For example the ring substituents
in the salicylate anions are still conformationally rigid whereas in the cho-
line cations a very small preference for anti-conformation is again found,
Fig. 6(a). Other structural aspects can be better described by an ab initio
approach. In particular intramolecular hydrogen bond, whose geometry
in GAFF simulation was fixed by some empirical parameters of the force
filed, can now be object of some considerations. Even if hydrogen con-
tinues to be mainly covalently bonded to O
during dynamics we can observe proton transfer from O
distribution function between O and Hsal has in fact the main peak at
values typical of very strong intramolecular hydrogen bonds (1.54 Å)
along with an additional low intensity peak at the covalent bond distance
of 1.06 Å, Fig. 6(b). The only remarkable difference between the two
796
747
711
696
710
692
ΔνOH cho
ΔνCO
603
236
771
231
594
227
707
213
364
367
As for alkylcarboxylate anions, the vibrational modes of carboxylate
group can be classified as CO symmetric and CO asymmetric stretching
vibrations and their frequencies differ each other, ΔνCO, by about
−
1
364 cm a value not far from those observed for the group in other car-
-
1
boxylate anions (324–309 cm ) [10]. In the benzoate anion, where the
carboxylate group is rigorously symmetric, such difference is calculated
at 338 cm -1. As for alkylcarboxylate choline ion pairs [10], hydrogen
bonding between cation and anion is expected to change the CO vibra-
tional coordinates and consequently their frequencies. In addition such
changes, expressed for example again as difference between the sym-
metric and asymmetric stretching frequencies, ΔνCO, can be correlated
with the strength of the intermolecular interaction [10]. In our case the
frequencies of Table 1 indicate that CO–HO hydrogen bonding causes a
3
, as it just occurs in ion pair,
3
to O . Radial
2
2
-
1
marked decrease of ΔνCO from 365 cm (isolated anion) to about
models is in the intermolecular hydrogen bond geometry: O
near double) probability to bind choline with respect to O
by the radial distribution function in Fig. 6(c); in addition O forms hydro-
gen bonds (1.68 Å) slightly shorter than O (1.72 Å). Thus AIMD simula-
1
has a higher
230 cm -1 (coupled anion). In addition it's worth noting that ΔνCO of
-
1
(
2
, as attested
salicylate choline is similar to that of formate (250 cm ), but higher
than those of propanoate (175 cm -1) and butanoate (184 cm ) choline
suggesting a hydrogen bonding weaker than alkylcarboxylate choline
ion pairs [10]. The strength of ion pairing can be evaluated more easily
following the modes of the choline cation. Table 1 shows that the red
shift of the OH stretching frequency (ΔνOH) (about 600 cm ) and
the blue-shift of torsion frequency (ΔτCO) (about 600 cm 1) are smaller
than those of alkylcarboxylate choline, 750 and 650 cm - , respectively,
-1
1
2
tions seem to reveal small asymmetries in the hydrogen bonding
acceptor properties of two carboxylate oxygens not appreciable from
zero temperature studies of ion pairs and from classical MD.
-
1
-
However this small structural differences do not drastically affect
the theoretical differential correlation function, since it has a shape
very similar to the GAFF Diff(r). The comparable nitrogen–oxygen dis-
tance distributions is probably one of the factor that contributes to
this agreement, see Fig. 6(a). When compared with X-ray diffraction
Diff(r) in Fig. 6(d), the AIMD curve gives good predictions of the exper-
imental data in all the range considered as already observed for
alkylcarboxylate choline liquids [11]. Both classical force field and ab
initio models describe the choline salicylate as a liquid where cations
and anions interact through hydrogen bonding weaker and less direc-
tional than that between choline and alkylcarboxylate components. As
a consequence also a two-body potential as GAFF here applied, not par-
ticularly accurate for protic ionic liquids, reproduces satisfactorily the
structural features of choline salicylate liquids.
1
4
4
.4. Vibrational spectra
.4.1. Ion pairs from quantum mechanical study
Vibrational spectra of choline cation, salicylate anion and choline sa-
licylate ion pair were calculated from ab initio methods and frequencies
of the groups involved in hydrogen bonding are reported in Table 1.
Fig. 7. Infrared spectrum of [Ch][Sal] and its components.

46
L. Tanzi et al. / Journal of Molecular Liquids 218 (2016) 39–49
Fig. 8. VDOS spectrum of [Ch][Sal].
confirming that intermolecular hydrogen bond is weaker in choline
salicylate.
Table 1 shows also the frequencies and their shifts calculated from
CP2K method. The values are quite different from the B3LYP/6–
In analogy to harmonic frequencies and normal mode analysis, vi-
brational frequencies and effective normal modes can be obtained
from a VDOS decomposition, as displayed by the works of Martinez
[61,62]. Using this technique, dynamical solvation, temperature and
anharmonicity effects are directly and explicitly taken into account in
the vibrational frequencies and normal modes (Fig. 8). The comparison
with frequencies of ion pairs calculated at the same theoretical level
(CP2K) reveals that finite temperature weakens intermolecular hydro-
gen bonding and strengthens the intramolecular one. For example the
OH group of salicylate is involved in very stable hydrogen bonding,
and probably its chemical environment does not change its conforma-
tional rigidity so much. For this reason, salicylate OH bending shows fre-
quency similar to ion pair. On the contrary, the OH bending of choline is
more affected by the intermolecular environment and its frequency
lowers from the ion pair to AIMD models. Regarding the CH and OH
stretching frequency range, while the former are rather well localized,
the OH vibrations are distributed on a very broad range and their
mean value cannot be calculated with high accuracy from VDOS
spectrum. This is a consequence of the wide distribution of hydrogen
bonding energies and conformations that is typical of protic liquids.
Despite the scarce localization, the power spectra still indicate that
two hydroxyl groups of [Ch][Sal] form hydrogen bonding of different
strength: power spectra for choline distribute around a mean value of
3240 cm -1, whereas for salicylate the mean value is 2655 cm , lower
than ion pairs.
3
11++G** ion pair and in particular the BLYP/GPW level used for
CP2K calculations enhances the frequency shifts of the hydroxyl
modes. ΔνCO is instead less affected by the calculation level. Interesting
is also the comparison between the two ion pairs in which salicylate
forms intermolecular hydrogen bond respectively through O
atoms. Notwithstanding the geometries of the two structures suggest
that when interaction involves O intra- and inter-molecular hydrogen
1 2
and O
1
bonding is stronger, the OH choline stretching undergoes shifts very
similar in agreement with their close stability. Only at the BLYP/GPW
level the ΔνOH values are slightly different and that for O1–HO interac-
tion is higher. We can also notice that in the both interaction models the
OH stretching coordinates of choline and salicylate are significantly
coupled.
4
.4.2. AIMD spectra
The computation of Wannier centers during AIMD simulation allows
to obtain a theoretical IR spectrum. An advantage of the simulated
spectrum is the possibility to split it into its cationic and anionic compo-
nents: Fig. 7 shows the total IR spectrum and the partial IR spectra.
At first sight, we note that the total IR spectrum is largely dominated
by the intense bands of salicylate: choline has bands with comparable
-1
-1
-1
intensities only in the low 500–1100 cm and high 2800–3400 cm
frequency ranges. Beyond these regions, IR spectrum can be easily
assigned since all bands belong exclusively to salicylate, on the contrary,
inside this range there are overlapping bands.
4.4.3. Experimental IR and Raman spectra
Vibrational properties of liquid [Ch][Sal] were experimentally
studied through IR and RAMAN spectra (Fig. 9).
Fig. 9. Infrared and Raman spectrum of [Ch][Sal]. Bands of choline are marked with a star.

L. Tanzi et al. / Journal of Molecular Liquids 218 (2016) 39–49
47
Table 2
predicted in the VDOS spectrum. The OH stretching vibration of choline
presents a broad IR absorption between 3200 and 3400 cm owing to
Experimental IR and Raman frequencies (cm -1) of choline salicylate liquid and theoretical
frequencies from AIMD and [Ch][Sal] ion pair (CP2K).
-1
the wide distribution of intermolecular hydrogen bonds and choline
conformations. On the contrary, the OH group of salicylate, involved in
a strong intramolecular hydrogen bonding and conformationally quite
rigid, is expected at lower frequency and it gives the sharp IR peak at
IR
Raman
AIMD
CP2K
Choline modes
νOH cho
νCH cho
νCH cho
δHOC cho
ρCH3
ρCH3
νCO cho
νCC
3400–3200
2957
833 cm -1
.
2983
2947
3147
3123–2931
1523
2
3038
1422
-1
The frequencies calculated at 1634, 1587 and 1540 cm are a mix-
ture between benzene ring stretching modes, the bending of the OH
group and the asymmetric vibration of the carboxylate group. From
1330
1140
1088
1005
955
1346
1150
1090
1275
1145–1138
1075
973
951
the analysis of normal modes the highest frequency, 1634 cm -1
,
1022
970
seems to be more characteristic of C_C stretching and it well compares
τCO cho
νCN
-1
with the IR absorption at 1630 cm . The other strong IR bands at
860
861–810
1
588 cm -1 can be assigned to the asymmetric carboxylate stretching.
Salicylate modes
νCH
νCH
In this region the Raman spectrum shows a strong and quite large
3072
3045
3196–3185
3144
-1
band at about 1600 cm . The other important vibration, the OH salicy-
3120
-
1
late bending, is predicted at 1587 cm , from ion pair model and at
1598 cm -1, from AIMD simulations, in a zone where broad absorptions
are measured in the IR and Raman spectra. The OH group of choline cat-
νOH
2833
1630
1588
2858
1634
1540
1587
1488
1461
1327
1245
νC_C
νCO as
δHOC
νC_C
νC_C
1600
1600
1550
1532
1598
1480
1460
1293
1240
1200
1022
837
-1
ions is expected to have a bending mode at lower frequency, 1523 cm
(
1487
1458
1387
1255
1208
1028
810
1490
1458
1395
1264
1232
1035
820
ion-pair) and 1422 cm -1 (AIMD), not easily observable in the
νCO
s
measured spectra. In this region we observe two IR intense peaks at
1487 cm -1 and 1458 cm , typical of phenol ring stretching modes,
less pronounced in the Raman spectrum and well reproduced by
AIMD simulations.
-1
νCO sal
δCH
δCH + νCC
δOCO
γCH
γCOO
δCCC
δCCC
1203–1213
1030
795
At lower frequency, the strong IR absorption at 1387 cm -1 can be
assigned to the carboxylate symmetric stretching and the correspond-
ing Raman band is found at 1395 cm -1. The C–O stretching modes of
two hydroxyl groups, νCO cho and νCO sal, are identified with the strong
766
770
783
742
707
668
567
540
685/691
669
612
698
564
538
γCCC
531
-1
absorptions at 1088 (IR)/1090(Raman) cm (choline) and 1255 (IR)/
1
264(Raman) cm -1 (salicylate). This mode seems to be not strongly
-1
Assignment of the main peaks is reported in Table 2 and it is based
on the theoretical results discussed above and on the experimental
spectra of the two components.
Choline bands were already measured in alkylcarboxylate choline
liquids [10] or in systems where choline is weakly coupled with anions
affected by chemical environment since the peak at 1088 cm is ob-
served in all choline based ILs spectra [10]. Some of the bands measured
at the lower frequency have been assigned as reported in Table 2.
In Table 3 the values of νCO measured for some ionic liquids are
compared with those of the same anions measured in different phases,
from gas-phase [67,68], sodium salts [69,70] to water solution [66,69,
71]. ΔνCO for gas-phase carboxylate anions, well reproduced by our
DFT calculations, decrease in condensed phase, in agreement with our
results. In addition ΔνCO for BioILs are comparable to those in water,
suggesting that hydrogen bonding between ions in these liquids is fun-
damental as hydrogen bonding between carboxylate anions and water.
[64] and we find them substantially unchanged in our liquid (see Fig. 9).
On the other hand IR and Raman spectra of salicylate complexes in
water solutions are very handful to assign the anionic bands [65,66].
As we have seen from the AIMD IR spectrum of choline salicylate liquid,
the region over 1200 cm -1 is mainly dominated by salicylate bands al-
though the assignment of all the observed peak is not easy. Regarding
the higher frequency range (N2000 cm -1), the Raman spectrum
-1
shows the band of CH stretching of choline cation (2983–2947 cm
and salicylate anion (3072–3045 cm ), well localized and satisfactorily
)
5. Conclusions
-1
We have reported in this work energy dispersive X-ray diffraction
data and IR and Raman spectra for a bio-compatible ionic liquid formed
by choline cations and salicylate anions. Different theoretical models
have been proposed to describe the liquid and the results have been
compared with the experiments. Quantum mechanical approaches
have been followed to describe the liquid structure: single ion pairs
and small portions of liquid have been modeled by ab initio calculations
and the results indicate that the ions are connected by strong hydrogen
bonds. Salicylate has different hydrogen bonding acceptor groups how-
ever the theoretical models indicate that salicylate is connected by cho-
line mainly through the carboxylate group. The X-ray pattern has been
successfully reproduced by ab initio MD simulations within the obvious
spatial and temporal limits of the employed model. Despite classical
force fields do not reproduce always all the observed structural features
of protic ionic liquids, in the present case the MD simulations obtained
with the GAFF potential give structure function and the complementary
Diff(r) one in good agreement with the X-ray patterns. Both AIMD and
MD simulations give the choline proton sharply localized around the
oxygen atoms of carboxylate and the acceptor–donor distance slightly
higher than in alkylcarboxylate choline liquids. An important difference
with respect to alkylcarboxylate choline liquid is that in salicylate based
Table 3
ΔνCO (cm -1) of some carboxylate anions observed in condensed and gas phases and cal-
culated from DFT theory.
Gas
Phase
Condensed
Phase
Exp
B3LYP
324a
Water
Na
Ionic
Ion pair AIMD
salts Liquids (B3LYP)
229b
201
162
136
173
143
c
255a
251a
263a
Formate
Acetate
Propionate 295
Butanoate
Benzoate
Salicylate
d
d
b
c
c
c
g
e
285
303
305
309
338
135
182
175
184
d
a
132b
a
a
a
161
140
236
a
b
a
a
a
135
155
207
201
315f
e
g
e
e
197
227
364e
h
201e
239e
a
Ref. [10].
Ref. [71].
Ref. [69].
Ref. [67].
This work.
Ref. [68].
Ref. [70].
Ref. [66].
b
c
d
e
f
g
h

48
L. Tanzi et al. / Journal of Molecular Liquids 218 (2016) 39–49
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