Ab initio study of hydrated potassium halides KX(H2O)1-6 (X=F,Cl,Br,I).

TLDR: The ionic dissociation of salts was examined with a theoretical study of KX hydrated by up to six water molecules KX(H2O)n and single point energy calculations using the coupled cluster theory with single, double, and perturbative triple excitations were performed on the MP2 optimized geometries.

Abstract: The ionic dissociation of salts was examined with a theoretical study of KX (X=F,Cl,Br,I) hydrated by up to six water molecules KX(H2O)n (n=1-6). Calculations were done using the density functional theory and second order Moller-Plesset (MP2) perturbational theory. To provide more conclusive results, single point energy calculations using the coupled cluster theory with single, double, and perturbative triple excitations were performed on the MP2 optimized geometries. The dissociation feature of the salts was examined in terms of K-X bond lengths and K-X stretch frequencies. In general, the successive incorporation of water molecules to the cluster lengthens the K-X distance, and consequently the corresponding frequency decreases. Near 0 K, the KX salt ion pairs can be partly separated by more than five water molecules. The pentahydrated KX salt is partly dissociated, though these partly dissociated structures are almost isoenergetic to the undissociated ones for KFKCl. For the hexahydrated complexes, KF is undissociated, KClKBr is partly dissociated, and KI is dissociated (though this dissociated structure is nearly isoenergetic to a partly dissociated one). On the other hand, at room temperature, the penta- and hexahydrated undissociated structures which have less hydrogen bonds are likely to be more stable than the partly dissociated ones because of the entropy effect. Therefore, the dissociation at room temperature could take place for higher clusters than the hexahydrated ones.

Figures

TABLE II. MP2/aVDZ+ B3LYP/6-311+ +G** sp interaction energies of KX H2O 1–6 clusters. Values in parentheses are the B3LYP/6-311+ +G** sp energies. Energies are in kcal mol−1. − Ee=− Ee N+ Ee B /2± BSSE/2 , where − Ee N and − Ee B are the binding energies without and with BSSE correction, respectively. Thus, the lower and upper values are − Ee N and − Ee B, respectively. Hydration energies with ZPE correction are denoted by − E0. H298 and G298 are the enthalpy change and Gibbs free energy change for hydration at 298 K. − G298 could be underestimated for n 4 because of low frequency terms though the magnitude would not be reliable, the relative stabilities would be useful information . The lowest energies for each cluster type and the structures corresponding to the lowest MP2 E0 are highlighted in bold characters.

FIG. 5. a CI S / /MP2/aVDZ+ polarity change of the potassium atom open symbols and halide atom filled symbols for the transition from the ground state to the first excited state for the lowest energy structures of KX H2O n=1–6 and b MP2/aVDZ+ electron transfer qK+−−X− in electron from KX to the water cluster: qKX=− qwater .

TABLE III. CCSD T / aVDZ+interaction energies of KX H2O 1–5. See the footnote of Table II. The values of nearly isoenergetic lowest energy conformers for 5R355 X=F , 5R344, 5R355 X=Cl , 5R444 X=Br , and 5R3344 X=I are not reported due to the convergence problem. The CCSD T values are consistent with the MP2 values. The lowest energy structures are marked in bold characters. The BSSEs were used with the MP2/aVDZ+values because the difference between CCSD T and MP2 BSSEs is usually very small, and the thermal energies were also used with the MP2 values.

FIG. 2. MP2/aVDZ+ interaction energies E0 a and successive interaction energy E0 n− E0 n−1 b of the lowest-energy structures of KX H2O n=1–6.

TABLE I. Calculated and experimental structural parameters for water, potassium, halides, and potassium halides. Bond lengths re are in angstrom; angle in degree; frequencies in cm−1; ionization potential IP , electron affinity EA , and dissociation energy D0 in eV; and polarizability in Å3. Only OH frequencies s, a scaled are scaled by a scale factor of 0.96, while other K–X frequencies are not scaled. The data for H2O and halide atoms are from Ref. 9 .

TABLE V. MP2/aVDZ+predicted O¯H X stretching frequencies in cm−1: scale factor of 0.96 and intensities in 10 km/mol of KX H2O 1–6.

Full text

Ab initio study of hydrated potassium halides K X ( H 2 O ) 1 – 6 ( X = F , Cl , Br , I )
Adriana C. Olleta, Han Myoung Lee, and Kwang S. Kim
Citation: The Journal of Chemical Physics 126, 144311 (2007); doi: 10.1063/1.2715565
View online: http://dx.doi.org/10.1063/1.2715565
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Ab initio study of hydrated potassium halides KX„H
2
O…
1–6
„X=F,Cl,Br,I…
Adriana C. Olleta, Han Myoung Lee, and Kwang S. Kim
a兲
Center for Superfunctional Materials, Department of Chemistry, Pohang University of Science
and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea
共Received 6 November 2006; accepted 15 February 2007; published online 13 April 2007兲
The ionic dissociation of salts was examined with a theoretical study of KX 共X=F,Cl,Br,I兲
hydrated by up to six water molecules KX共H
2
O兲
n
共n =1–6兲. Calculations were done using the
density functional theory and second order Møller-Plesset 共MP2兲 perturbational theory. To provide
more conclusive results, single point energy calculations using the coupled cluster theory with
single, double, and perturbative triple excitations were performed on the MP2 optimized geometries.
The dissociation feature of the salts was examined in terms of K–X bond lengths and K–X stretch
frequencies. In general, the successive incorporation of water molecules to the cluster lengthens the
K–X distance, and consequently the corresponding frequency decreases. Near 0 K, the KX salt ion
pairs can be partly separated by more than five water molecules. The pentahydrated KX salt is partly
dissociated, though these partly dissociated structures are almost isoenergetic to the undissociated
ones for KF/KCl. For the hexahydrated complexes, KF is undissociated, KCl/KBr is partly
dissociated, and KI is dissociated 共though this dissociated structure is nearly isoenergetic to a partly
dissociated one兲. On the other hand, at room temperature, the penta- and hexahydrated undissociated
structures which have less hydrogen bonds are likely to be more stable than the partly dissociated
ones because of the entropy effect. Therefore, the dissociation at room temperature could take place
for higher clusters than the hexahydrated ones. © 2007 American Institute of Physics.
关DOI: 10.1063/1.2715565兴
I. INTRODUCTION
The study of dissociation of a salt in water essentially
involves the separation of the associated ions that constitute
the salt. Despite its apparent simplicity, it is difficult to de-
scribe at the molecular level the mechanism whereby a salt
dissociates in water due to its experimental difficulty.
1–4
The
mechanism governing the salt dissociation is highly relevant
to diverse biological, environmental, and atmospheric chemi-
cal processes
5,6
as well as recognition by ionophores.
7
Ab initio calculations on the dissociation of alkali halides
in water clusters have scarcely been studied to date.
8–15
This
prompted us to undertake a theoretical study to clarify the
characteristics of the processes. Most theoretical studies per-
formed in this field have focused on the characterization of
hydrated ions. Halide ions
16–20
and alkali metal cations
21–26
hydrated by water clusters have been thoroughly studied
both theoretically and experimentally. In this regard, detailed
ab initio studies of the dissociation phenomena of metal ha-
lides by water clusters are particularly interesting.
8–10
How-
ever, the dissociation of potassium halides by water clusters,
despite its importance in biological, environmental, and at-
mospheric chemical processes, has been the subject of much
less study.
5
One way of approaching the structure and properties of
aqueous solution of salts involves using a quantum chemical
method to examine clusters of a salt formed by water mol-
ecules. Salt dissociation involves a cooperative process be-
tween salt and water molecules. As water molecules are suc-
cessively incorporated, the cation-anion distance in the salt
increases gradually, resulting in an ion pair separated struc-
ture.
To accurately predict the relative stability of dissociated
and undissociated forms, it is essential to use reliable com-
putational methods to incorporate the electron correlation by
employing reasonably large basis sets including diffuse func-
tions. In this regard, we performed high-level ab initio cal-
culations as well as density functional theory calculations on
potassium-halide clusters hydrated by up to six water mol-
ecules 关KX共H
2
O兲
n=1–6
, where X=F, Cl, Br, and I兴.
II. COMPUTATIONAL METHODS
Firstly, the structures of KX共H
2
O兲
1–6
water clusters were
investigated by using Becke’s three parameters with Lee-
Yang-Parr functionals
27
共B3LYP兲 with the 6-311+ +G
**
共sp兲
basis set. These low-energy conformers were further opti-
mized at the level of second order Møller-Plesset 共MP2兲 per-
turbational theory with the aug-cc-pVDZ+共2s 2p/2s兲 basis
set 共abbreviated as aVDZ+兲. Here, 共sp兲 and 共2s2p /2s兲 are
highly diffuse basis sets.
28
In the case of potassium atom 共K兲,
we used the energy adjusted Stuttgart effective core poten-
tials and added a d exponent of 0.48 to the valence bases.
29
For X=Br and I, we used the diffuse basis set with the ef-
fective core potential of Lajohn et al.
30
For better reliability,
single point energy calculations based on the coupled cluster
theory with single, double, and perturbative triple excitations
关CCSD共T兲/aVDZ+兴 on the MP2/aVDZ+ optimized geom-
etries were performed. Both MP2 and CCSD共T兲 calculations
were carried out with frozen core.
31
a兲
Electronic mail: kim@postech.ac.kr
THE JOURNAL OF CHEMICAL PHYSICS 126, 144311 共2007兲
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The vibration analysis using the analytical second de-
rivative matrix was carried out to characterize the nature of
the stationary points. If an optimized structure, usually with
some symmetry constraints, has one or more imaginary fre-
quencies, we further optimized the structure along the imagi-
nary normal modes until we obtained the true local minimum
structure where all the frequencies are real.
The interaction energies 共−⌬E
e
兲 for hydration of KX are
reported with the median value of the energies with and
without basis set superposition error 共BSSE兲 correction,
while half BSSE is reported like an error bar.
28
Zero-point-
vibrational energy 共ZPE兲 and thermal energy corrections
based on the B3LYP/6-311+ +G
**
共sp兲 or MP2/aVDZ+ fre-
quencies were used to obtain the ZPE-corrected binding en-
ergies
共−⌬E
0
兲 and the binding enthalpy 共−⌬H
298 K
o
兲 and binding
Gibbs free energies 共−⌬G
298 K
o
兲 at 298 K and 1 atm.
Random phase approximation 共RPA兲 calculations at the
B3LYP/6-311+ +G
**
共sp兲 level and configuration interac-
tions with single excitations 关CI共S兲兴 at the MP2/aVDZ+
level of theory were carried out to obtain charge-transfer-to-
solvent 共CTTS兲 energies. The analysis of polarization effects
and charge transfer was carried out with charges fitted to the
electrostatic potential 共ESP charges兲. ESP charges were cal-
culated by the natural bond orbital 共NBO兲 population
analysis.
32
All the calculations were carried out with the
GAUSSIAN-03 programs.
33
III. RESULTS AND DISCUSSION
A. Energetic properties
Table I shows that the calculated binding energies and
frequencies of the salts in isolation are in reasonable agree-
ment with the experimental values, and the variations related
to the electron affinity and polarizability of the anions are
compared very well with the experimental trend.
34,35
Com
-
pared with the B3LYP results, the MP2/aVDZ+ results ap-
pear more consistent with the CCSD共T兲 /aVDZ+ results.
Thus, most of our discussion will be based on the
MP2/aVDZ+ structural parameters 共geometries and frequen-
cies兲 and on the CCSD共T兲/aVDZ+ energies. At the B3LYP
level the polarizability values were underestimated with re-
spect to the experimental ones, whereas at the MP2 level
they were overestimated. However, their overall results are
consistent.
Figure 1 shows the important low-lying energy struc-
tures of KX共H
2
O兲
n=1–6
. The number of water molecules and
some letters to distinguish among different structural ar-
rangements are employed to identify the structure of
clusters.
9
Each conformer is denoted as
nRn
p
n
p
⬘
¯ /m
q
m
q
⬘
¯. Here, n is the number of water mol-
ecules, R stands for a ring structure, 兵n
p
n
p
⬘
¯其 is a set of
numbers of heavy atoms 共excluding H atoms兲 present in the
rings including both K and X atoms, and 兵m
q
m
q
⬘
¯其 is a set
of numbers of the atoms in the rings where K and X atoms
are not present together. 兵n
p
n
p
⬘
¯其 and 兵 m
q
m
q
⬘
¯其 are given
in ascending order. To provide the information of coordina-
tion for each structure, 共k/l兲 is used to denote the coordina-
tion number for the cation/anion. We also used additional
notations: L for linear shape and “Cube” for cubical shape. In
short notation, nRn
p
n
p
⬘
¯ /m
q
m
q
⬘
¯ will be simply denoted
as nRn
p
n
p
⬘
¯ except for Fig. 1.
In order to obtain the lowest energy structures, we inves-
tigated various hydration structures of NaX共H
2
O兲
1–6
,
9
which
included the hydration structures of salts reported in previous
papers,
8–15
and also considered the hydration structures of
alkali metal cations,
21–26
halide anions,
20
bases, and acids.
10
We also considered the possible structures based on the to-
pological analysis, as we previously investigated for water
clusters and excess electron-containing water clusters.
36
This
topological analysis was done in consideration of various
structures depending on the coordination numbers of the al-
kali metal cation 共M
+
兲 and the halide anion 关cn共#
M+
,#
X−
兲,
where #
M+
or #
X−
was considered to be 2, 3, or 4兴. For
example, for n=5 共pentahydration兲,cn共3,3兲 can have two
types of H-bond relay pathways comprised of a set of differ-
ent numbers of water molecules between M
+
and X
−
. With-
out taking into account the cross-passing H bonds, the two
types of paths, path共2,2,1兲 and path共3,1,1兲, have the shapes
of 5R344 and 5R355 in Fig. 1, respectively. By taking into
account the cross-passing H bonds, path共2,2,1兲 generates
5R3444/34, 5R3444/44, 5R4444/444, etc. 共not shown兲.In
this way, we can consider quite many conformers for the
clusters. However, in most cases, it is clear that certain con-
formers do not need to be considered because of their highly
strained structures. When generated structures would be con-
sidered to be low-lying energy conformers, they were tested
for their relative stabilities at the B3LYP/6-31+G
*
level. In
this way, for each hydrated system of KX, more than 100
conformers were investigated for the first screening test, fol-
lowed by more accurate calculations 关B3LYP/6-311+
+G
**
共sp兲, MP2/aVDZ+兴 of ⬃50 low-energy conformers.
Then, for the four types of potassium halides, accurate
MP2/aVDZ+ calculations with geometry optimization and
frequency analysis were carried out on these ⬃200 different
conformers. Therefore, it is almost certain that the lowest
energy structures reported here would be the global mini-
mum energy structures at the given levels of theory, which
we cannot prove though. Furthermore, from our previous
study of NaX共H
2
O兲
1–6
clusters, we have obtained some use-
ful experiences for the structural changes from F to I. As the
ionic strengths and ionic radii of halide ions have large in-
fluence in the hydration,
20
the fluoride ion tends to favor
internal-bound states, while the chloride, bromide, and io-
dide ions have surface-bound states for isolated ionic hydra-
tion structures. However, in the binary cation-anion hydra-
tion systems, the difference in hydration structure depending
on F to I is rather small due to the strong ion-ion interaction.
Thus, the hydration structures of potassium-halide salts are
similar to those of sodium-halide salts, though there are
some differences. Since we reported a number of structures
in our previous study of NaX共H
2
O兲
1–6
, we here describe only
the important low-lying energy structures of KX共H
2
O兲
1–6
in
Fig. 1.
Table II describes the binding energies evaluated at
the B3LYP/6-311+ +G
**
共sp兲 and MP2/aVDZ+ levels of
theory. The most stable conformers for KX共H
2
O兲
n
for n =6
共X=F, Cl, Br, and I兲 at the B3LYP/6-311+ +G
**
共sp兲 level
144311-2 Olleta, Lee, and Kim J. Chem. Phys. 126, 144311 共2007兲
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are 1R3, 2R33, 3R333, 4R334, 5R344, and 6R444, respec-
tively, except for the case of KI共H
2
O兲
6
which has the struc-
ture of 6R4444. At the MP2/aVDZ+ level, the most stable
conformers for KX共H
2
O兲
1–4
共X=F, Cl, Br, and I兲 are 1R3,
2R33, 3R333, and 4R334, which are consistent with the
B3LYP results. However, in the case of KX共H
2
O兲
4
, 4R334 is
nearly isoenergetic to 4R344. Furthermore, we note that
K-water interactions are anisotropic due to the presence of
unoccupied d orbitals in the potassium atom, and this feature
appears as the halogen atom increases its polarizability.
Thus, though the most stable conformers for KX共H
2
O兲
5
for
X=F/Cl are 5R344, similar to the case of the NaX-water
clusters, those for X=Br/I are 5R355. However, both 5R344
and 5R355 are nearly isoenergetic for X=F/Cl/Br, while for
X=I, 5R355 is nearly isoenergetic to 5R3344. In the case of
n=6, the lowest energy structure for X=Cl/Br is 6R445,
whereas that for X=I is 6R335. Here, 6R4444 is nearly
isoenergetic to 6R444 and 6R3345 for X=F and X=Cl/I,
respectively. Table III confirms that the CCSD共T兲 /aVDZ+
interaction energies are consistent with and very close to the
MP2/aVDZ+ interaction energies. Therefore, we facilitate
our discussion based on the MP2 results because CCSD共T兲
values for n=6 were not obtained due to the convergence
problem.
As water molecules are incorporated, the cluster binding
energy increases because of the increased strength of the
interactions and the increased number of hydrogen bonds, as
shown in Fig. 2共a兲. However, in Fig. 2共b兲 the successive
binding energy 共i.e., energy change resulting from the addi-
tion of a new water molecule; ⌬E
0
n
-⌬E
0
n+1
兲 increases up to
TABLE I. Calculated and experimental structural parameters for water, potassium, halides, and potassium halides. 共Bond lengths 共r
e
兲 are in angstrom; angle
共⬍兲 in degree; frequencies 共
␻
兲 in cm
−1
; ionization potential 共IP兲, electron affinity 共EA兲, and dissociation energy 共D
0
兲 in eV; and polarizability 共
␣
兲 in Å
3
. Only
OH frequencies 关
␻
s
,
␻
a
共scaled兲兴 are scaled by a scale factor of 0.96, while other K–X frequencies are not scaled. The data for H
2
O and halide atoms are from
Ref. 9兲.
B3LYP/
6-311+ +G
**
关sp兴
MP2/
aVDZ+
MP2/
aVTZ+
CCSD共T兲/
aVDZ+
CCSD共T兲/
aVTZ+ Expt. Ref.
H
2
O r共OH兲 0.962 0.966 0.961 0.967 0.962 0.957 34
⬔HOH 105.0 103.9 104.1 104.0 104.2 104.5 34
␻
s
3817 3803 3822 3787 3812 3657 35 and 37
␻
a
3922 3937 3948 3905 3921 3756 35 and 37
␻
s共scaled兲
3664 3651 3669 3635 3660
␻
a共scaled兲
3765 3780 3790 3749 3764
K IP 4.497 4.210 4.210 4.220 4.220 4.34 38
␣
共K兲 52.77 56.65 56.74 ¯¯43.4 39
␣
共K
+
兲 0.88 0.85 0.86 ¯¯
F EA 3.548 3.547 3.596 3.230 3.312 3.401 40
␣
0.35 0.59 0.68 ¯¯0.557 41
␣
共F
−
兲 2.61 1.79 1.78 ¯¯
KF r
e
2.235 2.239 2.223 2.236 2.219 2.171 42
␻
e
398 403 414 406 418 426 42 and 43
D
0
4.804 5.047 5.226 4.745 4.920 5.28 44
Cl EA 3.729 3.554 3.578 3.431 3.506 3.614 45
␣
1.44 2.62 2.86 ¯¯2.18 41
␣
共Cl
−
兲 3.89 4.94 5.56 ¯¯
KCl r
e
2.692 2.739 2.714 2.746 2.719 2.667 46
␻
e
269 265 274 264 273 281 46 and 47
D
0
4.265 4.203 4.367 4.058 4.223 4.39 48
Br EA 3.594 3.315 3.440 3.270 3.270 3.364 49
␣
2.37 4.07 4.07 ¯¯3.05 39 and 41
␣
共Br
−
兲 5.46 9.04 9.04 ¯¯
KBr r
e
2.850 2.903 2.903 2.913 2.913 2.821 49 and 50
␻
e
212 202 202 200 200 213 46 and 51
D
0
3.913 3.626 3.690 3.678 3.678 3.96 52 and 53
I EA 3.279 3.118 3.088 3.071 3.071 3.059 54 and 55
␣
4.97 6.44 6.44 ¯¯5.35 41 and 56
␣
共I
−
兲 9.36 13.68 13.68 ¯¯
KI r
e
3.153 3.142 3.142 3.152 3.152 3.048 49 and 53
␻
e
170 173 173 172 172 186.5 46
D
0
3.144 3.262 3.261 3.195 3.195 3.31 56
144311-3 Hydrated potassium halides J. Chem. Phys. 126, 144311 共2007兲
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four water molecules but is saturated from n =4 because the
subsequent water molecule starts to build up a second solva-
tion shell. For n =3, each ion in 3R333 is tricoordinated by
three water molecules. The successive binding energies for
the first few water molecules are much larger in KF共H
2
O兲
n
than in KX共H
2
O兲
n
共X=Cl/Br/I兲.
B. Structural properties
Table IV lists the selected structural parameters calcu-
lated at the MP2/aVDZ+ level. The four salts 共X
=F/Cl/Br/I兲 exhibit similar lowest energy structures for n
艋4. In the case of the lowest energy clusters of
KX共H
2
O兲
1–3
, the water molecules play a role in bridging
between a cation and an anion. These structures show a
water-shared ion pair. The addition of the fourth water mol-
ecule starts to build up a second solvation shell as water
molecules can no longer coordinate to both ions, and so
strong water-water interactions begin to develop. Therefore,
not only the coordination number of the halide atom but also
the hydrogen bonds between water molecules increase. The
structures of clusters containing five or more water mol-
ecules were examined with various conformers that yielded a
number of minima on the potential surface.
The ionic radius of fluoride ion is shorter than that of
oxygen, and so the K–F distances are shorter than the K–O
distances in mono- and dihydrated K–F systems. However,
in the trihydrated systems the K–F distances are longer than
the K–O distances. The K–O stretching frequencies are in the
range of 190–220 cm
−1
. The hydrated Na
+
and K
+
ions have
different coordination numbers, 4 and 4–6, respectively. The
K
+
ion is on the transition point from the internal-bound 共Li
+
and Na
+
兲 to partly surface-bound 共Rb
+
and Cs
+
兲 structure.
The hydrated halide anions have the coordination number of
4 within the hexahydrated systems. The hydrated K–X salts
have more water-water H bonds than the hydrated Na–X
salts.
9
The water-water H bonding slightly decreases the
K–X distances for the tri- to pentahydrated KF and KCl sys-
tems. For the hexahydrated KX systems, 6Cube has a com-
pletely dissociated structure, but it is much less stable than
6R4444. 6R444 is partly dissociated for X=Cl/Br/I, but it is
still undissociated for X=F. In the case of X=I, 6R3345
which is regarded as a dissociated structure is more stable
than 6R4444. Here, the terms of 共full兲 dissociation 共i.e., sol-
vent separated ion pair兲, partial dissociation 共half-
dissociation or partial contact ion pair兲, and undissociation
共i.e., contact ion pair兲 are based on our previous work of
NaX共H
2
O兲
n
, and the brief discussion is given in the footnote
in Table IV.
Figure 3 clarifies the above discussion. Figure 3共a兲
shows the change of K–X bond length with respect to the
water cluster size n. For KF共H
2
O兲
n
where n 艋3, the bond
length increases, but for n =4 and 5 the bond length slightly
decreases as the interactions between water molecules begin
to develop and the binding energies increase. However, for
n=6, the bond length increases again. A similar trend is also
observed for other hydrated salts 共KCl, KBr, and KI兲, while
the bond length for n=4 共and 5兲 is almost constant with
respect to the change of the cluster size n. For n =4/5/6, the
hydrated KX systems have two nearly isoenergetic conform-
ers except for KBr共H
2
O兲
5
. A more appropriate way of com-
paring the effect of water molecules on the salt geometry is
in terms of the relative distance increase from the isolated
molecule. The largest increase in K–X distance occurs in KI
clusters followed by KCl and KBr clusters. The elongation
effect is greater in the clusters of six water molecules. This
suggests that the ion pair formed upon the dissociation is
more efficiently stabilized in the clusters for n =6. The pres-
ence of a partial second shell of water molecules between the
two ions introduces significant stabilization in the ion pair as
their dipole moments oppose that from the formation of the
ion pair. In the clusters containing four water molecules,
however, the dipole of one water molecule does not directly
oppose the dipole established by the ion pair, which results in
less efficient stabilization of the ion pair.
In Fig. 3共b兲, the K–O distance remains constant regard-
less of the number of water molecules. This reveals that the
characteristics of the interaction are similar in all clusters and
hence the interaction is governed by the presence of interwa-
ter hydrogen bonds and water-halide ion hydrogen bonds.
FIG. 1. Various conformers of KX共H
2
O兲
1–6
共X=F/Cl/Br/I兲. The important
structures are underlined.
144311-4 Olleta, Lee, and Kim J. Chem. Phys. 126, 144311 共2007兲
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Frequently asked questions

Q1. What are the contributions mentioned in the paper "Ab initio study of hydrated potassium halides kx„h2o...1–6 „x=f,cl,br, i.." ?

In this paper, the ionic dissociation of the KF, KCl, KBr, and KI clusters hydrated by up to six water molecules was studied. 

Q2. How many conformers were tested for their relative stability?

When generated structures would be considered to be low-lying energy conformers, they were tested for their relative stabilities at the B3LYP/6-31+G* level. 

Q3. What is the effect of dissociation on the ion pair?

The presence of a partial second shell of water molecules between the two ions introduces significant stabilization in the ion pair as their dipole moments oppose that from the formation of the ion pair. 

Q4. What is the effect of the incorporation of the sixth water molecule?

The incorporation of the sixth water molecule causes an additional increase in the K–F stretching frequency for the undissociate structure, while it causes a significant decrease in the K–X frequencies X=Cl/Br/ The authorfor partially or fully dissociated structures. 

Q5. What is the role of water molecules in bridging between a cation and an?

In the case of the lowest energy clusters of KX H2O 1–3, the water molecules play a role in bridging between a cation and an anion. 

Q6. How many kcal/mol are the barriers between different conformers?

The barriers between different conformers would be around 1–2 kcal/mol for example, for KCl H2O 4, the barrier between 4R344 and 4R334 is 1.6 kcal/mol . 

Q7. Why did the authors facilitate their discussion based on the MP2 results?

the authors facilitate their discussion based on the MP2 results because CCSD T values for n=6 were not obtained due to the convergence problem. 

Q8. What is the simplest way to optimize the low-energy conformers?

These low-energy conformers were further optimized at the level of second order Møller-Plesset MP2 perturbational theory with the aug-cc-pVDZ+ 2s2p /2s basis set abbreviated as aVDZ+ . 

Q9. What is the total charge of the ion pair?

The total charge of the ion pair is given as qCT which is associated with the charge transferred from the ion pair to the water molecules.