JML_2015_prakash

Probing molecular interactions underlying imidazolium and pyridinium
based ionic liquids
Prakash L. Verma, Soniya S. Rao, Shridhar P. Gejji ⁎
Department of Chemistry, Savitribai Phule Pune University, Pune 411 007, India
a b s t r a c ta r t i c l e i n f o
Article history:
Received 30 July 2015
Received in revised form 2 October 2015
Accepted 6 October 2015
Available online xxxx
Keywords:
Hydrogen bonding
Density functional
Vibrational frequencies
NCI plots
Electronic structures of 1-hexyl- and 1-octyl- substituted 3-methylimidazolium (mim), -pyridinium (py), 4-
methylpyridinium (mpy) cations and halide (Cl−
or Br−
) ion-pairs have been obtained from the M06-2x/6-
31++G(d,p) density functional theory. The imidazolium cation binds strongly to the anion than py or mpy. The-
oretical calculations demonstrated that Cl−
ion-pairs possess stronger and more directional hydrogen bonding
than those containing Br−
ions bringing about the distinct structural features in the ion-pairs which are explained
in terms of non-covalent interactions reduced density gradient analysis. Calculated vibrational frequencies have
shown that the methine C1–H1 vibration of the cation shifts to lower wavenumber (red shift) in the Cl−
ion-pairs
while the shift in the opposite direction was predicted for the Br−
ion-pairs The direction of frequency shift of
characteristics stretching vibration has been explained through the change in dipole moment of ion-pair with re-
spect to the stretching coordinate of the corresponding covalent bond. As opposed to Cl−
ion-pairs, the Br−
ion-
pairs reveal ∂μ
∂r
b0. Effect of hydration on the structure and energetics of the ion-pair has been simulated by sub-
sequent addition of one to three water molecules. Cation–anion binding and polarizabilities of solvated ion-pairs
increase steadily with the number of water molecules. A charge transfer from the anion in hydrated ion-pairs is
evident from the natural bond orbital analysis.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Ionic liquids (ILs) composed of organic cations and organic/inorganic
anions are molten salts with their melting point well below 100 °C
exhibiting liquidus behavior over a wide range of temperature. The phys-
icochemical properties of ILs can be tailored by combining different cat-
ions and anions or varying their proportion which renders their use for
task specific applications and hence, are regarded as ‘designer solvents’.
In particular, ILs composed of imidazolium (im), pyridinium (py) and
phosphonium cations have widely been studied in recent literature
owing to their ease of synthesis, low viscosity [1] and stability [2]. It
has been observed that the presence of solvent in ILs induce significant
deviation in physicochemical properties such as densities, refractive in-
dices, Kamlet–Taft parameters, polarizabilities and dynamic viscosities
[3–5]. In particular, imidazolium and pyridinium based ILs absorb atmo-
spheric water influencing their transport behavior. Yaghini et al. [6] stud-
ied transport properties of imidazolium based ILs composed of TFSI−
or
TfO−
anions employing the pulsed field gradient nuclear magnetic reso-
nance spectroscopy and conductivity measurements. These experiments
revealed that self-diffusion coefficients of ion increases upon addition of
water with rapid diffusion of the cation than anion. Stark and co-workers
[7] further showed that room-temperature ionic liquids (RTILs)
composed of imidazolium cation and Cl−
, NO3, BF4, PF6 anions absorb
water from atmospheric air at ambient temperature. It was further dem-
onstrated that the presence of chloride ions increase their viscosities.
Pursuance to this, Garcia and co-workers [8] carried out conductometry,
spectrofluorimetry and NMR experiments for a series of long-chain
alkylated imidazolium and pyridinium based ILs in aqueous solution.
The RTILs exhibit surface activity and those containing alkyl chains
with more than eight carbon atoms demonstrated antimicrobial
activities.
Despite of above experimental investigations the theoretical studies
focusing on unraveling the underlying non-covalent interactions of
RTILs are rather limited. The molecular insights accompanying the
formation of binary systems should prove useful in modeling of their
physicochemical properties. On theoretical front the molecular dynam-
ics simulations were performed to understand the structure of molecu-
lar aggregates/ion-pairs accompanying these ILs. Schröder et al. [9] have
shown that collective network of ILs and water mixtures possess cross-
linked ionic species with dipolar water. MD simulations by Hanke and
co-workers [10] on dialkyl imidazolium and PF6 ions underlined hydro-
gen bonding interactions for varying concentrations of water in these
ILs. The effect of alkyl chain and the presence of water in imidazolium
based ILs containing bromide ion revealed increasing cationic tail aggre-
gation which influence their structural and dynamical properties
[11–13]. MD simulations on hydrated imidazolium-based ILs [14] con-
taining Cl−
showed that the association decreases with an increase of
Journal of Molecular Liquids 212 (2015) 885–899
⁎ Corresponding author.
E-mail address: spgejji@chem.unipune.ac.in (S.P. Gejji).
http://dx.doi.org/10.1016/j.molliq.2015.10.012
0167-7322/© 2015 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq

alkyl chain on the cation which differs significantly from those contain-
ing NTf2 or ethyl sulfate anion. The microscopic insights for the cation–
anion binding accompanying ILs can be probed by employing the densi-
ty functional theory (DFT).
The density functional calculations combined with infrared
spectroscopy experiments demonstrated that electrostatic interactions
dictate the local structure of water governing the organization of ionic
species [15]. Molecular interactions between methanol and methyl
imidazolium analyzed by Zhu et al. [16] showed that the hydrogen
bonding interactions contribute largely to render stability to binary
systems. Further investigations on imidazolium and halide anion
established that the miscibilities of ILs arise due to anion–water interac-
tions [17]. To this direction Cha et al. [18] investigated the bulk struc-
tures of [Bmim]X−
, (X = Cl, Br, I, BF4) using the infrared and NMR
spectroscopy experiments combined with DFT calculations. A large fre-
quency downshift for the C(2)–H stretching was observed in the
infrared spectra with decreasing size of the halide anion. Secondly, the
downfield signal of C(2)–H proton in 1
H NMR spectrum confirmed the
presence of strong hydrogen bonding interactions in these ILs.
From the above discussion, it is clear that the reliable structures of
ion-pairs or aggregates in RTILs can be derived with the use of DFT in-
corporating dispersion corrections which account for underlying non-
covalent interactions. The present endeavor precisely focuses on
unraveling such interactions in the ILs based on imidazolium,
pyridinium and methyl substituted pyridinium cations and their binary
mixtures with water using the M06-2x based density functional theory.
The cation–anion binding and their interactions with the solvent
(water) forming binary mixtures are pivotal for modeling of physico-
chemical properties. The present work deals with how these interac-
tions manifest in binding, charge distribution and spectral
characteristics of 1-alkyl-3-methylimidazolium halide [Cnmim], N-
alkylpyridinium halide [Cnpy], 1-alkyl-4-methylpyridinium halide
[Cnmpy] (n = 6 and 8) which are considered as proto type examples.
The influence of solvent has further been modeled through explicit
Fig. 1. Optimized structures of [C6mim][Cl], [C6py][Cl], [C6mpy][Cl], [C8mim][Cl], [C8py][Cl] and [C8mpy][Cl]. Atomic numbering scheme has been shown.
886 P.L. Verma et al. / Journal of Molecular Liquids 212 (2015) 885–899

solvation of ion-pairs with subsequent addition of water molecules. Cat-
ion–anion interactions along with their solvated aggregates engender
electron density redistribution and emerge with signature in corre-
sponding vibrational spectra. The strength of these interactions can be
correlated to the frequency shift of the characteristic normal vibrations.
In the present work, such frequency shifts have been analyzed in terms
of dipole moment partial derivatives and natural bond orbital (NBO)
analysis. An approach based on the difference molecular electron densi-
ty (MED) plots combined with the quantum theory of atoms in mole-
cules (QTAIM) has further been utilized to explain the direction of
frequency shifts of normal vibrations in their calculated infrared spectra.
The computational method has been outlined below.
2. Computational method
Optimizations of [C6mim]⁺[X]−
, [C6py]⁺[X]−
, [C6mpy]⁺[X]−
,
[C8mim]⁺[X]−
, [C8py]⁺[X]−
and [C8mpy]⁺[X]−
(X = Cl or Br) ion-pairs
and individual ions have been carried out using DFT with Gaussian 09
program [19]. To account for hydrogen bonding and non-covalent inter-
actions underlying the ILs, we employ the hybrid meta-GGA (general-
ized gradient approximation) exchange correlation Minnesota M06-2x
functional [20–23] in the DFT. The 6-31++G(d,p) basis set was used.
These functionals account well for dispersive interactions [24,25] and
have accomplished remarkable success for predicting reliable structure
and binding energies. Stationary point structures thus obtained were
confirmed to be local minima on the multivariate potential energy sur-
face from the vibrational frequency calculations, since all the normal vi-
bration frequencies in these conformers turned out to be real. The
potential energy distributions (PED) were computed and the normal vi-
brations were visualized through the displacement of atoms around
their equilibrium (mean) positions through GAUSSVIEW-5 program
[26]. The frequency shifts in the calculated vibrational spectra have fur-
ther been explained by computing the dipole moment derivatives with
respect to the stretching coordinate. Binding energies were obtained by
subtracting the sum of zero point corrected SCF energy of individual
ions/molecules from that of the individual ion-pair/hydrated ion-pair.
To delve into underlying molecular interactions and their manifestation
in the infrared spectral characteristics quantum theory of atoms in mol-
ecules (QTAIM) [27,28] approach was used. The bond critical points
(bcp) in the MED topography for different hydrogen bonds were identi-
fied. Effect of solvation on the ion-pair structures and energies were
simulated by subsequent addition of water molecules (1 to 3) explic-
itly. The ‘charge transfer’ accompanying the solvation were analyzed
through the NBO [29] analysis. The non-covalent interactions were
envisaged using the reduced gradient density plots which were
visualized through the VMD program [30]. The energy decomposi-
tion analysis (EDA) further was performed [31]. Lastly, the global
electrophilicity (ω), chemical potential (μ), chemical hardness (η)
[32–34] and regional (electrophilic) Fukui functions [35,36] have
been obtained.
3. Results and discussion
3.1. Structure and binding energies
Conformers of Cnmim-, Cnpy- and Cnmpy- (n = 6 and 8) with X−
(X = Cl, Br) ion-pairs were derived from the B3LYP/6-31++G(d,p)
theory shown in Figs. S1–S12 of the supporting information. Relative
stabilization energies in kcal mol−1
are given in parentheses. The lowest
energy ion-pairs were further subjected to the M06-2x/6-31++G(d,p)
optimizations. The stationary point geometries (local minima) thus
obtained are depicted in Fig. 1. The most acidic proton from the cation
facilitating C1–H1⋯Cl interactions renders stability to the ion-pair
structure. As shown in Table 1, the interaction with Cl−
in the ion-pair
follows the order: C6mim N C6py N C6mpy. Selected bond distance pa-
rameters from the present calculations for [Cnmim]-, [Cnpy]- and
[Cnmpy]-Cl−
(n = 6 and 8) systems are shown in Table 2. An increase
of alkyl chain from C6mim to C8mim does not influence the energetics
of these ion-pairs, in consonance with the results by Garaga et al. [37].
The Br−
deviates largely from the plane containing imidazolium
or pyridinium ring in the ion-pairs which is evident from the
Br⋯H1C1N1 dihedral angles reported in Table S1. The hierarchy in
binding energies of pyridinium ion-pairs with Br−
turns out to be
qualitatively similar. On the other hand, an increase of alkyl chain
from hexyl to octyl shows a reversal of trend engendering stronger
binding for methyl substituted pyridinium. The binding of C8mpy with
Br−
can be attributed to the orientation of octyl group with respect to
the anion being significantly different from that in its chloride analog.
Thus the Br⋯H16 interactions have been inferred in the [C8mpy][Br]
(cf. Fig. 2). Selected bond-distances and -angles in Br−
ion-pairs are
given Table S2 of supporting information.
Table 2
Selected bond distances (in Å) and angles (in °) of isolated C6mim, C6py, C6mpy, C8mim, C8py, C8mpy cations and their respective ion-pairs with Cl−
anion.
C6mim C6py C6mpy C8mim C8py C8mpy [C6mim][Cl] [C6py][Cl] [C6mpy][Cl] [C8mim][Cl] [C8py][Cl] [C8mpy][Cl]
C1–H1 1.080 1.084 1.084 1.080 1.084 1.084 1.115 1.111 1.110 1.117 1.111 1.111
C1–N1 1.331 1.347 1.349 1.331 1.348 1.347 1.333 1.347 1.346 1.333 1.348 1.346
C1–N2 1.334 1.333 1.336 1.336
C4–N2 1.466 1.466 1.459 1.459
N1–C5 1.476 1.476 1.475 1.475
N1–C6 1.489 1.486 1.489 1.486 1.489 1.488 1.489 1.488
C2–H2 1.079 1.079 1.078 1.078
C3–H3 1.079 1.079 1.078 1.078
H1–C1–N1 125 116 116 125 116 116 122 115 115 121 115 115
H1–C1–N2 125 125 130 130
C5–N1–C1 125 125 123 123
C6–N1–C1 119 119 119 119 118 119 118 119
C4–N2–C1 125 125 124 124
H4–C4–N2 108 108 108 108
H8–C5–N1 106 106 106 106
H6–C6–N1 106 106 106 106 106 106 106 106
∠C1–H1⋯Cl 152 162 162 153 161 163
Table 1
Binding energies (in kcal mol−1
) for ion-pairs.
Cl ion-pair Br ion-pair
C6mim 93.3 98.6
C8mim 93.3 98.5
C6py 92.0 96.5
C8py 91.9 96.8
C6mpy 89.7 95.5
C8mpy 90.0 97.3
887P.L. Verma et al. / Journal of Molecular Liquids 212 (2015) 885–899

3.2. Normal vibration analysis
Normal vibrations in hitherto ion-pair systems were assigned
through potential energy distribution (cf. Section on Computational
Method) computed within the M06-2x based density functional theory.
The strength of hydrogen bonding interactions accompanying such ion-
pairs can be gauged from the frequencies of their characteristic normal
vibrations which shift to either lower (red shift) or higher wavenumber
(blue shift) in their infrared spectra. Hobza and coworkers [38–40] ini-
tially suggested that ‘blue shift’ results from improper hydrogen bond-
ing. Hermansson [41] has explained the shift of such characteristic
vibration in terms of partial derivative of change of dipole moment
along the direction of the stretching coordinate. An approach combining
the quantum theory of atoms in molecules with the molecular electron
density topography through mapping of the bond critical points in the
MED topography on different molecular density contour plots has
been utilized by Gejji et al. [42–44] to rationalize the frequency shift(s).
Alternatively, the strengthening and weakening of bonds in the local re-
gion engendering frequency shifts were envisaged using the NBO
analysis.
As pointed out in the preceding section the vibrational frequencies of
the ion-pairs and individual cations were derived from the hessian ma-
trix (whose elements refer to the second order partial derivatives) with-
in the M06-2x/6-31++G(d,p) framework of theory. Calculated
vibrational spectra portraying the molar absorption coefﬁcient (or,
molar absorptivity in units of 0.1 m2
mol−1
) versus the frequency (in
cm−1
) of C6mim (in black) C6py (shown as red) and C6mpy (portrayed
in blue) and Cl−
ion-pairs, in different regions namely, (a) 3400–
Fig. 2. Optimized structures of [C6mim][Br], [C6py][Br], [C6mpy][Br], [C8mim][Br], [C8py][Br] and [C8mpy][Br]. Atomic numbering scheme has been shown.

Fig. 3. IR spectra of [C6mim][Cl], [C6py][Cl] and [C6mpy][Cl] ion-pairs in (a) 3400–2600 cm−1
(b) 1800–900 cm−1
(c) 900–400 cm−1
regions. (For interpretation of the references to color
in this ﬁgure, the reader is referred to the web version of this article.)

Fig. 4. IR spectra of [C6mim][Br], [C6py][Br] and [C6mpy][Br] ion-pairs in (a) 3400–2900 cm−1
(b) 1800–800 cm−1
(c) 800–400 cm−1
regions.

2600 cm−1
(b) 1800–900 cm−1
(c) 900–400 cm−1
, are depicted in
Fig. 3. The region 3400–2600 cm−1
(cf. Fig. 3(a)) reveals the intense
C1–H1 stretching at the 3310 cm−1
of the isolated [C6mim]⁺ cation
shifts to the lower wavenumber 2745 cm−1
for the [C6mim][Cl] ion-
pair. The observed 2746 cm−1
band in the infrared spectra in [Bmim][Cl]
based ILs was assigned to C–H vibration in the earlier literature [18]. Ac-
cordingly, the corresponding vibration in C6py and C6mpy ion-pairs
occurs at the 2795 cm−1
and 2827 cm−1
, respectively. An increase of
alkyl chain from hexyl to octyl does not bring about any significant
change in the spectra depicted in Figs. S13 and S14 of the supporting in-
formation. These results concur with the earlier observations by Garaga
et al. [37] for the bis(trifluoromethanesulfonyl)imide based ILs. The
hierarchy for the downshift of the C1–H1 stretching emerges as
[Cnmim][Cl] N [Cnpy][Cl] N [Cnmpy][Cl], (n = 6, 8). Fig. 3(b) depicts the
1800–900 cm−1
region in which intense 1206 cm−1
vibration was
assigned to N1–C5 stretching in the [C6mim][Cl] ion-pair. Likewise, the
1013 cm−1
band in these ion-pairs stems from the C1–H1 wagging.
Fig. 4(a) displays infrared spectra of bromide ion-pairs. A blue shift of
38 cm−1
was predicted for [C6py][Br] ion-pairs compared to that of
65 cm−1
for the methyl substituted ion-pair. The methine C1–H1
stretching can be used as a probe to distinguish the bonding features in
the chloride and bromide ion-pairs.
As compared to chloride ion-pairs, the methine C1–H1 vibration
appearing at the 3329 cm−1
in C6mim cation exhibits a shift in opposite
direction in the bromide ion-pairs. This vibration corresponds to
3296 cm−1
and 3322 cm−1
band in the C6py and C6mpy ion-pairs,
respectively. A concomitant increase of its intensity may as well be
noticed. The low intense vibrations of the hexyl chain of ion-pairs
containing Cl−
as well as Br−
anions emerge in the region 3096–
3038 cm−1
and are strongly coupled. Selected normal vibration
frequencies have been summarized in Table S3 through Table S6 of
the supporting information. Analysis of vibration spectra of [C6py][X]
and [C6mpy][X] (X = Cl or Br) ion-pairs or with extended alkyl chains
lead to the similar inferences. To get further insights for the molecular
interactions accompanying these ion-pairs and consequent shift in the
frequency of C1–H1 vibrations NBO and QTAIM analyses have been car-
ried out.
3.3. Natural bond orbital (NBO) analysis
Cation–anion interactions bring about the variation of strengths of
different bonds in the ion-pair and electron distribution consequent to
bond weakening (/or strengthening) which is transparent from the en-
hanced (/or depleted) electron density in the corresponding localized
antibonding orbital. Accordingly the electron density in the C1–H1 anti-
bonding orbital (σ*) in Cnmim, Cnpy and Cnmpy (n = 6 and 8) cations in
their isolated state and those in the ion-pairs are summarized in Table 3.
The residual atomic charges, occupancies of natural orbital and charge
transfer contributions (E(2)
) to the ion-pair have also been given. It
may thus be inferred that the ion-pairs reveal strong orbital interactions
between the antibonding orbital of the proton donor σ* C1–H1 and lone
pair(s) on the proton acceptor Cl(LP). Moreover, the charge transfer en-
ergy E(2)
contributions arising from the Cl(LP) → σ* C1–H1 interaction
follows the order: [Cnmim][Cl] N [Cnpy][Cl] N [Cnmpy][Cl], which points
to larger bond strength for the C1–H1⋯Cl hydrogen bond in the
[Cnmim][Cl] compared to [Cnpy][Cl] and [Cnmpy][Cl], (n = 6, 8).
Noteworthy enough, the strength of C1–H1⋯Cl hydrogen bonding
does not vary significantly with the increase of alkyl chain from hexyl
to octyl group. The measured vibrational spectra of 1-alkyl-3-
methylimidazolium bis(trifluoromethanesulfonyl)imide based ILs earli-
er [37] have shown that the normal vibration frequencies are nearly in-
sensitive to increased alkyl chain for more than six carbon atoms.
Similar inferences are borne out from the M06-2x based density func-
tional calculations on im, py and mpy based ILs studied in the present
work. This is also evident from the charge transfer contributions for
Cl(LP) → σ* C1-H1 interaction, which renders the atom center to be
more electron deficient. Thus the ion-pair formation enhances electron
density within corresponding antibonding orbital (σ*) further elongat-
ing the bond with consequent frequency down-shift of the correspond-
ing stretching in its infrared spectra. The normal vibrations are
compared in Table 4. On the other hand, the ion-pairs containing Br−
anion reveal charge transfer from Br(LP) to the cation. The enhanced
electron density within the π* C1–N1 orbitals further facilitate C1⋯Br
interactions. Here the delocalization contribution arising from the π*
C1–N1 is thus relatively large compared to σ* C1–H1 of the Br−
ion-
pair. Electron density redistribution brings about transfer of a larger
portion of electron density to non-interacting bonds within the cation
Table 4
Electron density in antibonding orbital σ* (in au), bond distances r (in Å), and frequency of
vibration ν (in cm−1
) for C1–H1 bond in C6mim, C6py, C6mpy, C8mim, C8py, C8mpy
cations and their respective ion-pairs with Cl−
and Br−
anion.
cation Cl ion-pair Br ion-pair
σ* r ν σ* r ν σ* r ν
C6mim 0.0107 1.080 3310 0.0952 1.115 2745 0.0115 1.077 3329
C6py 0.0106 1.084 3256 0.0826 1.111 2795 0.0112 1.078 3296
C6mpy 0.0105 1.084 3249 0.0816 1.110 2827 0.0132 1.077 3322
C8mim 0.0107 1.080 3291 0.0989 1.117 2720 0.0115 1.077 3324
C8py 0.0105 1.084 3253 0.0812 1.111 2793 0.0114 1.078 3292
C8mpy 0.0105 1.084 3249 0.0751 1.111 2796 0.0104 1.079 3296
Table 5
AIM parameters (in au) at the bond critical points of ion-pairs.
Ion-pairs ρbcp for
Cl⋯H1
∇2
ρ for
Cl⋯H1
Ion-pairs ρbcp for
Br⋯C1
∇2
ρ for
Br⋯C1
[C6mim][Cl] 0.0383 0.0769 [C6mim][Br] 0.0242 0.0633
[C6py][Cl] 0.0343 0.0728 [C6py][Br] 0.0352 0.0382
[C6mpy][Cl] 0.0344 0.0735 [C6mpy][Br] 0.0456 0.0703
[C8mim][Cl] 0.0395 0.0777 [C8mim][Br] 0.0244 0.0634
[C8py][Cl] 0.0338 0.0723 [C8py][Br] 0.0274 0.0709
[C8mpy][Cl] 0.0351 0.0742 [C8mpy][Br] 0.0371 0.0668
Table 3
Occupancies of antibonding C1-H1 natural orbitals (σ*), NBO charges on H1 and charge transfer contributions, E(2)
(kcal mol−1
) in cations and Cl−
ion-pairs.
System
σ*(C1–H1) Change E(2)
Cation ion-pair Cation ion-pair (Cl)LP → σ*(C1–H1)
C6mim 0.0107 0.0952 0.2757 0.3077 37.08
C6py 0.0106 0.0826 0.2751 0.3132 29.87
C6mpy 0.0105 0.0816 0.2735 0.3126 29.78
C8mim 0.0107 0.0989 0.2757 0.3065 39.03
C8py 0.0105 0.0812 0.2752 0.3137 29.17
C8mpy 0.0105 0.0751 0.2735 0.3152 25.23

with an increase of the bond strength of σ* C1–H1 thereby shifting the
corresponding vibration to higher wavenumber. These inferences are
supported by the frequency data for the ion-pairs displayed in
Tables S3–S6 of supporting information. Coulombic ﬁeld strength
around the cation affects the polarization and energies of σ* C1–H1
and π* C1–N1 natural orbitals consequent to cation–anion binding. As
may readily be noticed the charge transfer energy contributions E(2)
ac-
companying Br−
ion-pair formation are nearly insensitive to the in-
crease of alkyl chain from hexyl to octyl as may readily be noticed in
case of Cl−
ion-pair reported in the Table S7 of the supporting
information.
3.4. Difference MED-, QTAIM-analysis and charge distributions in ion-pairs
We now discuss how the MED critical point topography can be
utilized to understand the direction of ‘frequency shifts’ as a result of
cation–anion interactions. Electron density values (ρbcp) in | e | /bohr3
at the bond critical point along with its Laplacian ∇2
ρ (in | e | /bohr5
)
for the hydrogen bond are reported in Table 5. The non-covalent hydro-
gen bonding interactions in a molecular system emerge with its signa-
ture in bcp of the corresponding bond in the MED topography; its
strength being proportional to the electron density at that site (ρbcp)
[45,46]. Thus, C1–H1⋯Cl hydrogen bonding yield the ρbcp parameters
in the range 0.0338 au to 0.0395 au for Cl−
based ion-pairs. NBO charges
in ion-pairs are displayed in Table 6 and Tables S8–S12 of the supporting
information. The difference electron density, Δρ, was calculated by
subtracting the sum of electron densities of the individual anion and
cation from the corresponding ion-pairs. Fig. 5 displays the difference
electron density contour maps of [C6mim][Cl] (a prototype example)
in the plane passing through Cl−
and C1–H1 bond from the cation. Cal-
culated bcp (shown in red and blue color) in the MED topography are
shown. Difference electron density contours clearly show strengthening
or weakening of the bond which further manifests in the frequency shift
of the corresponding stretching vibration. The Δρ contours in the range
of (±0.001 to ±0.0009 au) are displayed in Fig. 5 where the blue lines
represent positive valued contours and the negative valued contours
appear in red. The zero-valued contour is depicted as a green line. As
may readily be noticed, the difference electron density map of
[C6mim][Cl] reveal bcps for C1–H1⋯Cl interactions (with chlorine
interacting with the most acidic proton of the cation) falls in the region
(red) where the electron density is depleted. This explains the frequen-
cy downshift of the C1–H1 stretching vibrations of the ion-pair
compared to the corresponding vibration in the isolated cation. The
frequency shifts in the [C6py][Cl], [C6mpy][Cl], [C8mim][Cl], [C8py][Cl]
and [C8mpy][Cl] ion-pairs further can be explained on parallel lines.
On the other hand, the bcp corresponding to C1⋯Br interactions was
located in the (blue) region revealing the increased electron density;
an increase of C1–H1 bond strength thus is evident. Consequently, the
corresponding vibration accompanying such ion-pair formation reveals
the frequency shift to higher wavenumber. Electron density contour
maps are depicted in Figs. S15–S25 of the supporting information. The
decreased IR intensity accompanying such vibration arises from the
negative dipole moment derivative with respect to stretching coordi-
nate. In other words, the hydrogen bonding interactions render stability
to the ion-pair with an increase in dipole moment on the approach of
anion along this direction. These arguments have further been extended
to ion-pairs containing Cl−
and Br−
anions studied in this work. The
downshift in the frequency of C1–H1 vibration was predicted for the
Cl−
ion-pairs contrary to those containing bromide ions. The Br−
ion-
pairs engender ‘blue shift’ for the corresponding vibration and thus it
Fig. 5. Difference electron density maps in [C6mim][Cl] (contours in the range ±0.001 to ±0.0009 au) are shown. Filled circles in red and blue show bond critical points. See text for details.
(For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)
Table 6
Selected charges (in au) driven from the NBO analysis for [C6mim]+
cation and their ion-
pairs with Cl−
and Br−
anion.
Atoms [C6mim]+
[C6mim][Cl] [C6mim][Br]
C1 0.275 0.276 0.312
C2 −0.044 −0.075 −0.054
C3 −0.041 −0.056 −0.058
C4 −0.456 −0.451 −0.458
C5 −0.246 −0.254 −0.238
C6 −0.471 −0.475 −0.493
C7 −0.463 −0.465 −0.458
N1 −0.352 −0.367 −0.379
N2 −0.361 −0.383 −0.387
H1 0.276 0.308 0.290
H2 0.285 0.263 0.262
H3 0.284 0.265 0.265
H4 0.259 0.270 0.241
H5 0.265 0.245 0.287
H6 0.265 0.245 0.238
H7 0.266 0.236 0.242
H8 0.259 0.297 0.260
H9 0.243 0.221 0.222
H10 0.244 0.271 0.288
Cl −0.864
Br −0.866

may as well be conjectured that hydrogen bonding as well as electro-
static contributions differs significantly than those of Cl−
ion-pairs.
The frequency shifts have further been discussed employing the
approach presented in the earlier literature [41]. Accordingly, the dipole
moment of the ion-pair as a function of C1–H1 bond distance as a
coordinate are reported in Table 7. As may readily be noticed, for the
[C6mim][X] (X = Cl and Br) ion-pairs (as a test example) the gradient
of dipole moment as a function of C1–H1 separation parameter turns
out to be negative for the Br−
ion-pairs and the blue shift for the
corresponding stretching is thus evident. As opposed to this, the Cl−
ion-pairs with diminutive dipole moments on contraction of C1–H1
bond reveal ð∂μ
∂r
N0Þ and are accompanied by frequency downshift of
the corresponding stretching vibration. The frequency upshifts predict-
ed for the rest of the ion-pairs can also be explained on parallel lines.
These data are reported in Tables S13 and S14 of the supporting
information.
3.5. Effect of solvation
To understand the interactions of water with ILs and predicting the
activity coefficients of water therein COSMO-RS and FTIR measure-
ments on the binary systems containing water and methyl imidazolium
were carried out by Coutinho et al. [47]. Excess enthalpies and Gibbs
free energy estimates in these binary mixtures suggested the formation
of the complexes in 3:1 proportion of water and the ion-pair. Beyond
three water molecules, water–water cooperative interactions may as
well be observed defining the water percolation limit, which further
have been supported by MD simulations [10,48]. With this motif, the
energetics and structure of ion-pair hydration of imidazolium and
pyridinium based ion-pairs with sequential addition of water molecules
forming their binary mixtures have been further dealt with. Thus the
lowest energy [C6mim][Cl] hydrated ion-pairs obtained from the
present theory are displayed in Fig. 6. The order of binding energies
Table 7
Calculated bond distance, r (Å), dipole moment (Debye) and its derivative for [C6mim][Cl] and [C6mim][Br] ion-pairs. See text for details.
[C6mim][Cl] [C6mim][Br]
r r
1.1218 12.3088 0.23 1.0835 8.8804 –0.35
Bond elongation 1.1198 12.3084 0.25 0.24 1.0815 8.8811 –0.35 –0.35
1.1178 12.3079 0.25 1.0795 8.8818 –0.35
Equilibrium 1.1158 12.3074 0.00 1.0775 8.8825 0.00
1.1138 12.3068 0.30 1.0755 8.8831 –0.30
Bond contraction 1.1118 12.3062 0.30 0.30 1.0735 8.8838 –0.33 –0.32
1.1098 12.3056 0.30 1.0715 8.8845 –0.33
∂r
∂μμ
∂r
∂μμ
Fig. 6. Optimized structures of [C6mim][Cl](H2O), [C6mim][Cl](H2O)2 and [C6mim][Cl](H2O)3. Atomic numbering scheme has been shown.

for the solvated ion-pairs (cf. Tables S15–S16 of the supporting informa-
tion) are qualitatively same as that obtained for the gas phase
structures. It may as well be inferred that the ion-pair favors solvation
by water. Successive addition of 1 to 3 water molecules to the ion-pair
engenders longer H1⋯X bond distance. The solvated ion-pairs of
[Cnmim], [Cnpy], [Cnmpy] (n = 6, 8) are depicted in Figs. S26 through
S36 of the supporting information. As may be noticed the solvent
(water) molecules arrange around the anion of ion-pair facilitating
hydrogen bonding interactions at the expense of those from H1 in the
[Cnmim][X](H2O) system. Successive addition of water molecules
further facilitate charge transfer from the anion to water and thus
[Cnmim][X](H2O)m (m = 2 or 3) systems are void of hydrogen bonding
from the H1 proton. Redistribution of molecular electron density
consequent to hydrogen bonding interactions in the hydrated ion-pair
reflects in the ‘blue shift’ of C1–H1 stretching frequency. Successive
addition of water molecules lead to the weakening of cation–anion
Fig. 7. (a) Color-filled RDG isosurface plot: Non-covalent interaction (NCI) regions in [C6mim][Cl] ion-pair (green colored isosurface denotes weak H-bonds and the red colored isosurface
stands for steric effects). (b) NCI index plot: The plot of function 1 (sign(λ2) ρ values) on the X-axis vs. function 2, the reduced density gradient (RDG) on the Y-axis. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version of this article.)

binding affecting the physicochemical properties. The enhanced anion–
water interactions with the successive addition of water molecules
manifest in the vibrational spectra of hydrated ion-pairs (cf. Figs. S37–
S48 of the supporting information).
3.6. Non-covalent interactions (NCIs) analysis
Non-covalent interactions underlying the hitherto ILs have been an-
alyzed employing the method proposed by Johnson et al. [49,50] based
on electron density and its components (ρ(r) and its Laplacian, ∇2
ρ)
with their reduced gradient(s) being
RDG rð Þ ¼
1
2 3π2ð Þ
1
3
∇ρ rð Þj j
ρ rð Þ
4
3
:
The Laplacian is further decomposed into three eigenvalues as
∇2
ρ=λ1 +λ2 +λ3(λ1 ≤λ2 ≤λ3) derived from the Hessian matrix. The
second component, λ2 distinguishes the bonding (λ2 b 0) from the
Fig. 8. (a) Color-ﬁlled RDG isosurface plot: Non-covalent interaction (NCI) regions in [C6mim][Br] ion-pair (green colored isosurface denotes weak H-bonds and the red colored isosurface
stands for steric effects). (b) NCI index plot: The plot of function 1 (sign(λ2) ρ values) on the X-axis vs. function 2, the reduced density gradient (RDG) on the Y-axis. (For interpretation of
the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 9. (a) Color-ﬁlled RDG isosurface plot: Non-covalent interaction (NCI) regions in [C6mim][Cl](H2O)m (m = 1–3) ion-pairs (green colored isosurface denotes weak H-bonds and the red colored isosurface stands for steric effects). (b) NCI index
plot: The plot of function 1 (sign(λ2) ρ values) on the X-axis vs. function 2, the reduced density gradient (RDG) on the Y-axis. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)
896P.L.Vermaetal./JournalofMolecularLiquids212(2015)885–899

nonbonding characteristics (λ2 N 0). Thus the NCIs can be categorized as
attractive, moderately strong hydrogen bonds, repulsive steric interac-
tions and weak dispersion type interactions. The elegance of this meth-
od lies in elucidating the interactions in real-space as a graphical
representation distinguishing the non-covalent interactions. The posi-
tion, strength and type of an interaction shown as RDG isosurface are
represented on a blue–green–red (BGR) scale according to the values
and sign of (λ2)ρ ranging from −0.05 to +0.05 au. For the regions
with positive (λ2)ρ, the area is portrayed as red indicating strong repul-
sive non-bonded overlap with the attractive interactions displayed in
blue and the green region referring to very weak interactions. The NCI
method has been applied to the Cnmim-, Cnpy- and Cnmpy- (n = 6
and 8) with X−
(X = Cl, Br) ion-pair systems.
RDG isosurface in [C6mim][Cl] ion-pair is depicted in Fig. 7(a). As ev-
ident , the chloride and bromide ion-pairs exhibit distinct bonding fea-
tures. Thus the dark blue isosurface in the [C6mim][Cl] ion pair imply the
presence of relatively strong Cl⋯H1 hydrogen bonding interactions.
The green color isosurface indicate weak attractive interactions be-
tween chloride ion and H8, H10, and H11 protons from the alkyl chain
of the cation. On the other hand the greenish blue or cyan isosurface
dispersed between the cation and anion of [C6mim][Br] ion-pair (cf.
Fig. 8(a)). Thus, the bromide anion facilitates attractive interactions
with alkyl protons (H8, H10, H11 and H4) and the C1 center of the
imidazolium ring. It may, further be conjectured that the hydrogen
bonding interactions in bromide ion-pairs are weak and the polarizable
Br−
anion renders Coulombic C1⋯Br interactions with imidazolium
ring. The negative value of energy density (Hb = −0.0008 au) at the
bond critical point (BCP) obtained from the AIM approach suggest
that the C1⋯Br interaction possess certain degree of covalent character
which further supports the above inferences.
Fig. 7(b) represent NCI plots of [C6mim][Cl] ion-pair. The existence of
spike around −0.038 au imply the presence of strong (C1⋯H1) hydro-
gen bonding in the [C6mim][Cl] ion-pair. The spikes with negative
values near zero arise from the relatively weak H8⋯Cl, H10⋯Cl,
H11⋯Cl, interaction. On the other hand [C6mim][Br] displays a spike
around −0.024 au which imply attractive Coulombic C1⋯Br
interaction (cf. Fig. 8(b)). The emergence of spikes near zero regions
of the bromide ion-pair arise from weak hydrogen bonding interactions
with the alkyl chain as pointed out earlier. The [C6mim][Cl] ion-pair
reveals ρbcp values in the range of typical hydrogen bonded systems
(ρ ≈ 0.03 au), showing more electrostatic attraction, whereas the
[C6mim][Br] ion-pair exhibit characteristics of a more dispersive
interactions (ρ ≈ 0.01 au) [51].
To envisage the molecular interactions accompanying the formation
of binary systems of ILs with water, in particular, how the presence of
solvent affect such interactions NCI analysis was used. RDG isosurfaces
in the [C6mim][Cl](H2O)n(n = 1–3) are shown in Fig. 9(a). It is
noteworthy that the blue isosurface between Cl−
anion and the acidic
proton (H1) of the cation observed in the (free) ion-pair vanishes on
addition of water; the blue surface emerges between the water
molecule and the anion instead. In addition to this, attractive interac-
tions between the water molecule and cation protons may as well be in-
ferred from the surfaces shown as green in Fig. 9(a). Subsequent
addition of water to the anion facilitates stronger and more number of
hydrogen bonding interactions (shown as blue regions) between the
anion and water. Furthermore the green region extends between
anion–water aggregates and the cation suggests increased dispersive
interactions for the solvated [C6mim][Cl](H2O)n binary systems. NCI
plot reveals characteristics ‘spikes’ that become deeper on addition of
1 to 3 water molecules to the [C6mim][Cl] ion-pair. Thus stronger
interaction be it hydrogen bonding or dispersive ones, are inferred for
the hydrated ion-pairs. The spikes observed in lower electron density
region indicate a dispersion-type of interaction (cf. Fig. 9(b)).
An increase in alkyl chain from hexyl to octyl led to qualitatively sim-
ilar inferences from the NCI plots portrayed in the Figs. S49–52 of the
supporting information. A comparison of RDG isosurfaces of Cnpy- and
Cnmpy- (n = 6 and 8) with X−
(X = Cl, Br) ion-pairs shown in
Table 8
Frontier orbitals (HOMO and LUMO), chemical potential (μ), hardness (η) and electrophilicity (ω) parameters in Cl−
ion-pairs and their hydrated complex.
[C6mim][Cl] [C6py][Cl] [C6mpy][Cl] [C6mim][Cl] [C6py][Cl] [C6mpy][Cl]
1H2O 2H2O 3H2O 1H2O 2H2O 3H2O 1H2O 2H2O 3H2O
ΔE (in eV) 6.09 4.61 4.76 6.52 7.16 7.59 5.11 6.02 6.35 5.27 5.91 6.50
μ (in eV) 3.734 4.403 4.257 3.859 3.939 4.142 4.511 4.518 4.676 4.362 4.382 4.541
η 3.044 2.307 2.381 3.260 3.578 3.793 2.556 3.009 3.173 2.636 2.953 3.248
ω 2.290 4.201 3.804 2.284 2.169 2.262 3.981 3.392 3.446 3.609 3.252 3.175
Fig. 10. A plot of binding energy, B.E. (in kcal mol−1
) as a function of LogP (in au) for
[C6mim][Cl] and [C6mim][Cl](H2O)m (m = 1–3) ion-pairs.
Table 9
Polarizability (au), hydrophobicity, LogP (au), Fukui function, fCl
−
and softness (eV) param-
eter in Cl−
ion-pairs and their hydrated complex.
System Polarizability LogP fCl
−
Softness
[C6mim][Cl] 23.3 1.8 0.7669 243.3
[C6mim][Cl](H2O) 24.7 1.6 227.2
[C6mim][Cl](H2O)2 26.1 1.4 207.1
[C6mim][Cl](H2O)3 27.5 1.2 195.1
[C6py][Cl] 23.3 2.6 0.7487 320.8
[C6py][Cl](H2O) 24.7 2.4 289.5
[C6py][Cl](H2O)2 26.1 2.2 246.0
[C6py][Cl](H2O)3 27.5 2.0 233.5
[C6mpy][Cl] 25.2 2.9 0.7256 311.0
[C6mpy][Cl](H2O) 26.6 2.6 280.8
[C6mpy][Cl](H2O)2 28.0 2.4 250.6
[C6mpy][Cl](H2O)3 29.4 2.2 228.0
[C8mim][Cl] 27.0 2.7 0.7653 242.5
[C8mim][Cl](H2O) 28.4 2.5 222.6
[C8mim][Cl](H2O)2 29.8 2.3 207.1
[C8mim][Cl](H2O)3 31.2 2.1 195.7
[C8py][Cl] 27.0 3.5 0.7501 321.4
[C8py][Cl](H2O) 28.4 3.3 289.5
[C8py][Cl](H2O)2 29.8 3.1 247.6
[C8py][Cl](H2O)3 31.2 2.9 232.1
[C8mpy][Cl] 28.8 3.8 0.7237 309.9
[C8mpy][Cl](H2O) 30.2 3.6 281.6
[C8mpy][Cl](H2O)2 31.6 3.4 241.4
[C8mpy][Cl](H2O)3 33.1 3.1 229.1

Figs. S53–S64 of the supporting information suggests that the binding
characteristics underlying the pyridinium based ion-pairs are not affect-
ed significantly by methyl substitution on the cation. The NCI plots
shown in the figure further reaffirm the inferences drawn above.
These inferences are further supported by the EDA analysis summarized
in the Table S17 of the supporting information. The orbital interactions,
steric and dispersive contributions toward the total interaction energy
are given. As may readily be noticed the dispersive interactions contrib-
ute largely in the [C6mim][Br] than its Cl−
analog. Thus, the present
work demonstrates that Cl−
or Br−
monoatomic anions with relatively
strong charge localization interact differently toward the Imidazolium
or pyridinium cation.
3.7. Hydrophobicity and softness parameters
The hydrophobicities of molecular systems are partly governed
by electronic charge distribution and further can qualitatively be
correlated to hardness or softness parameters [52] defined in terms
of the energy separation for Frontier orbitals (HOMO and LUMO).
As shown in Table 8, for [C6mim][Cl], [C6py][Cl] and [C6mpy][Cl]
ion-pair, a steady increase of separation of the HOMO-LUMO separa-
tion energies (ΔE) with successive addition of water molecules was
observed. Similar inferences are borne out for the rest of the ion-
pairs as reported in Tables S18–S20 of supporting information. The
ion-pair formation is facilitated through interactions having signifi-
cant hydrophobic contributions. The logarithm of the 1-octanol/
water partition coefficient (LogP) is yet another parameter which
provides measure of molecular hydrophobicity (lipophilicity) [53–
55]. Calculated LogP parameters for the ion-pairs studied in this
work follow the order: [Cnmpy][X] N [Cnpy][X] N [Cnmim][X], (X =
Cl or Br) (n = 6, 8), which is reverse to that of binding energies
discussed earlier. A plot of binding energy as a function of LogP for
[C6mim][Cl] ion-pair and its hydrated complex displayed in Fig. 10
turns out to be linear with the correlation coefficient of 0.99. Binding
energies and LogP plots for the rest of the hydrated ion-pairs are
displayed in Fig. S65 of the supporting information. The molecular
insights for the hydration of ion-pairs have further been derived
through Fukui function parameters. Fukui function (fCl
−
) and static
polarizability (α) parameters of the isolated and solvated chloride
ion-pairs are compared in Table 9. As may be noticed, the [Cnmim][X]
(X = Cl or Br) reveal larger f−
implying greater tendency to accom-
modate water molecules which is also evident from the LogP param-
eters reported in Table S21 of the supporting information. The
conclusions are summarized in the following.
4. Conclusions
M06-2x/6-31++G(d,p) density functional theory has been
employed to systematically unravel molecular interactions underlying
the 1-hexyl- and 1-octyl-substituted 3-methylimidazolium (C6mim
and C8mim), -pyridinium (py) and -4-methylpyridinium (mpy) cations
and halide anion (Cl−
or Br−
) ion-pairs. The present calculations predict
stronger binding for the imidazolium cation than pyridinium and 4-
methylpyridinium ones. Calculated binding energies of the ion pairs
follows the order: [C6mim][Cl] ~ [C8mim][Cl] N [C6py][Cl] ~ [C8py]
[Cl] N [C6mpy][Cl] ~ [C6mpy][Cl]. Calculated infrared spectra reveal
that methine C1–H1 vibration can be used as probe to distinguish the
interactions from the Cl−
or Br−
anions. A frequency upshift for the
methine C1–H1 stretching was noticed in Br−
containing ion-pairs
while the Cl−
engenders the frequency shift in the opposite direction.
The direction of frequency shift has further been rationalized through
the molecular electron density topography combined with difference
electron density contour maps and further explained in terms of dipole
moment derivatives relative to the stretching coordinate in the ion-pair.
Reduced gradient density plots based on the NCI method clearly brings
about the distinct bonding features for the binding of Cl−
and Br−
anions to the same cation, which has been explained through the dom-
inance of electrostatic component in the bromide containing systems.
With successive addition of the water molecules (n = 1–3) to the ion-
pair a steady increase of binding energies was noticed. A linear correla-
tion of binding energies with LogP has further been established. The
hydrophobicity in the ion-pair decreases steadily with the addition of
water.
Acknowledgments
PLV and SSR are grateful to Savitribai Phule Pune University for the
award of research fellowships under the ‘University Potential for
Excellence’ Research Scheme. SPG is indebted to the Board of Research
in Nuclear Sciences, India for a research grant through the project
(37(2)/14/11/2015-BRNS). Authors thank the Center for Development
of Advanced Computing (CDAC), Pune for providing National Param
Supercomputing Facility.
Appendix A. Supplementary data
Optimized geometries, NBO parameters, reduced density gradient
NCI plots, vibrational frequencies, difference MED plots of ion-pairs
containing C8mim, Cnpy and Cnmpy (n = 6, 8) cation and halide (Cl−
and Br−
) anion. Supplementary data associated with this article can
be found in the online version, at http://dx.doi.org/10.1016/j.molliq.
2015.10.012.
References
[1] C. Wang, H. Luo, X. Luo, H. Li, S. Dai, Green Chem. 12 (2010) 2019–2023.
[2] D. Weingarth, I. Czekaj, Z. Fei, A. Foelske-Schmitz, P.J. Dyson, A. Wokaun, R. Kötz, J.
Electrochem. Soc. 159 (2012) H611–H615.
[3] M.E. Kandil, K.N. Marsh, A.R.H. Goodwin, J. Chem. Eng. Data 52 (2007) 2382–2387.
[4] E. Gomez, B. Gonzales, N. Calvar, E. Tojo, A. Dominguez, J. Chem. Eng. Data 51 (2006)
2096–2102.
[5] J.M. Lee, S. Ruckes, J.M. Prausnitz, J. Phys. Chem. B 112 (2008) 1473–1476.
[6] N. Yaghini, L. Nordstierna, A. Martinelli, Phys. Chem. Chem. Phys. 16 (2014)
9266–9275.
[7] K.R. Seddon, A. Stark, M.-J. Torres, Pure Appl. Chem. 72 (2000) 2275–2287.
[8] A. Cornellas, L. Perez, F. Comelles, I. Ribosa, A. Manresa, M.T. Garcia, J. Colloid Inter-
face Sci. 355 (2011) 164–171.
[9] C. Schröder, T. Rudas, G. Neumayr, S. Benkner, O. Steinhauser, J. Chem. Phys. 127
(2007) 234503.
[10] C.G. Hanke, N.A. Atamas, R.M. Lynden-Bell, Green Chem. 4 (2002) 107–111.
[11] B.L. Bhargava, M.L. Klein, Soft Matter 5 (2009) 3475–3480.
[12] B.L. Bhargava, M.L. Klein, J. Phys. Chem. A 113 (2009) 1898–1904.
[13] B.L. Bhargava, Y. Yasaka, M.L. Klein, Chem. Commun. 47 (2011) 6228–6241.
[14] P. Yee, J.K. Shah, E.J. Maginn, J. Phys. Chem. B 117 (2013) 12556–12566.
[15] Y. Danten, M.I. Cabaco, M. Besnard, J. Phys. Chem. A 113 (2009) 2873–2889.
[16] X. Zhu, H. Sun, D. Zhang, C. Liu, J. Mol. Model. 17 (2011) 1997–2004.
[17] Y. Wang, H. Li, S. Han, J. Phys. Chem. B 110 (2006) 24646–24651.
[18] S. Cha, M. Ao, W. Sung, B. Moon, B. Ahlström, P. Johansson, Y. Ouchid, D. Kim, Phys.
Chem. Chem. Phys. 16 (2014) 9591–9601.
[19] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A.
Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J.
Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H.
Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,
J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O.
Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma,
G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels,
M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V.
Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.
Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C.
Gonzalez, J.A. Pople, Gaussian 03, Revision E.01, Gaussian, Inc., Wallingford CT, 2004.
[20] Y. Zhao, D.G. Truhlar, Acc. Chem. Res. 41 (2008) 157–167 (PubMed: 18186612).
[21] Y. Zhao, D.G. Truhlar, J. Chem. Theory Comput. 1 (2005) 415–432.
[22] Y. Zhao, N.E. Schultz, D.G. Truhlar, J. Chem. Theory Comput. 2 (2006) 364–382.
[23] Y. Zhao, D.G. Truhlar, J. Chem. Phys. 125 (2006) 194101.
[24] T.H. Dunning Jr., J. Chem. Phys. 90 (1989) 1007–1023.
[25] R.A. Kendall, T.H. Dunning, R.J. Harrison, J. Chem. Phys. 96 (1992) 6796–6806.
[26] R. Dennington, T. Keith, J. Milliam, GaussView, Version 5, Semichem Inc., Shawnee
Mission KS, 2009.
[27] R.F.W. Bader, Atoms in Molecules: A Quantum Theory, Oxford University Press, Ox-
ford, UK, 1990.

[28] C.F. Matta, R.J. Boyd, The quantum theory of atoms in molecules, in: C.F. Matta, R.J.
Boyd (Eds.), An Introduction to the Quantum Theory of Atoms in Molecules,
Wiley-VCH, Weinheim, 2007.
[29] J.P. Foster, F. Weinhold, J. Am. Chem. Soc. 102 (1980) 7211–7218.
[30] W. Humphrey, A. Dalke, K. Schulten, J. Mol. Graph. 14 (1996) 33–38.
[31] T. Lu, F. Chen, J. Comput. Chem. 33 (2012) 580–592.
[32] R.G. Parr, L.V. Szentpaly, S.J. Liu, J. Am. Chem. Soc. 121 (1999) 1922–1924.
[33] R.G. Parr, D.A. Donnelly, M. Levy, W.E. Palke, J. Chem. Phys. 68 (1978) 3801–3807.
[34] R.G. Pearson, J. Am. Chem. Soc. 85 (1963) 3533–3539.
[35] R. Contreras, P. Fuentealba, M. Galvan, P. Perez, Chem. Phys. Lett. 304 (1999)
405–413.
[36] P. Fuentealba, P. Perez, R.J. Contreras, J. Chem. Phys. 113 (2000) 2544–2551.
[37] M.N. Garaga, M. Nayeri, A. Martinelli, J. Mol. Liq. 210 (2015) 169–177.
[38] P. Hobza, V. Spirko, H.L. Selzle, E.W. Schlag, J. Phys. Chem. A 102 (1998) 2501–2504.
[39] P. Hobza, Z. Havlas, Chem. Phys. Lett. 303 (1999) 447–452.
[40] P. Hobza, Z. Havlas, Chem. Rev. 100 (2000) 4253–4264.
[41] K. Hermansson, J. Phys. Chem. A 106 (2002) 4695–4702.
[42] R.V. Pinjari, K.A. Joshi, S.P. Gejji, J. Phys. Chem. A 111 (2007) 13583–13589.
[43] T.V. Kaulgad, N.R. Dhumal, S.P. Gejji, J. Phys. Chem. A 110 (2006) 9231–9239.
[44] N.R. Dhumal, S.P. Gejji, J. Phys. Chem. A 110 (2006) 219–227.
[45] U. Koch, P.L.A. Popelier, J. Phys. Chem. 99 (1995) 9747–9754.
[46] P.L.A. Popelier, J. Phys. Chem. A 102 (1998) 1873–1878.
[47] I. Khan, M. Taha, P. Ribeiro-Claro, S.P. Pinho, J.A.P. Coutinho, J. Phys. Chem. B 118
(2014) 10503–10514.
[48] C.G. Hanke, R.M. Lynden-Bell, J. Phys. Chem. B 107 (2003) 10873–10878.
[49] E.R. Johnson, S. Keinan, P. Mori-Sa'nchez, J. Contreras-Garcı'a, A.J. Cohen, W. Yang, J.
Am. Chem. Soc. 132 (2010) 6498–6506.
[50] J. Contreras-Garcı'a, E.R. Johnson, S. Keinan, R. Chaudret, J.-P. Piquemal, D.N. Beratan,
W. Yang, J. Chem. Theory Comput. 7 (2011) 625–632.
[51] K.E. Riley, P. Hobza, J. Chem. Theory Comput. 4 (2008) 232–242.
[52] R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules, Oxford Uni-
versity Press, New York, 1989.
[53] C. Hansch, A.J. Leo, Substituent Constants for Correlation Analysis in Chemistry,
Wiley, New York, 1979.
[54] V. Pliska, B. Testa, H. Waterbeemd, Lipophilicity in drug action and toxicology, Eds.,
VCH Publishers, New York, 1996.
[55] J. Sangster, Octanol-Water Partition Coefﬁcients: Fundamentals and Physical Chem-
istry, vol. 2, John Wiley and Sons, Chichester, 1997.

JML_2015_prakash

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (11)

Similar to JML_2015_prakash

Similar to JML_2015_prakash (20)

JML_2015_prakash