Molecular Dynamics Study of an

Aqueous Potassium Nitrate Solution

CHRISTOPH EBNER,1 ROLAND SANSONE,1

SUNANTHA HENGRASMEE,2 MICHAEL PROBST 1
1
Institute of General and Inorganic Chemistry, Innsbruck University, Innrain 52a,
A-6020 Innsbruck, Austria
2
Department of Chemistry, Faculty of Science, Khon Kaen University, 40000 Khon Kaen, Thailand

Received 28 February 1999; accepted 28 May 1999

ABSTRACT: We report the results of a molecular dynamics simulation on the system

KNO3rH 2 O. For the interaction between nitrate anion, water molecule, and potassium
cation, newly developed analytical potentials were employed whereas the water᎐water
interactions were described with the MCYL potential. Water and nitrate anion were
treated as flexible molecules. The periodic box contained 341 water molecules, 1 nitrate
anion, and 1 potassium cation. The structure of the hydration shell is investigated in
terms of radial and angular distribution functions. The vibrational density of states is
calculated to derive information about the frequency shifts of the anion upon hydration
and of the surrounding water molecules. We compare the findings of our simulations
with other molecular dynamics simulations involving nitrate anion and with quantum
chemical reaction field calculations. Our simulations show a well-defined but
orientationally very flexible hydration shell and small gas-to-liquid frequency shifts for
both nitrate anion and hydration shell water molecules. 䊚 1999 John Wiley & Sons, Inc. Int
J Quant Chem 75: 805᎐814, 1999

properties of aqueous nitrate solutions and some

Introduction reviews are given in w 1᎐3x . In contrast, rather few
theoretical studies dealing with the properties of
NOy w x
3 were performed 4᎐7 . The hydration shell

B ecause of the great importance of nitrate

anion in biology and chemistry, many exper-
imental studies have focused on elucidating the
structure and hydrogen bonding between solvent
molecules and nitrate anion is of special interest,
since it is assumed to lower the symmetry of the
Correspondence to: M. Probst.
nitrate anion and thus causes special features in
Contract grant sponsor: Austrian FWF. the vibrational spectrum of aqueous nitrate solu-
Contract grant number: P10106-MOB. tions.

International Journal of Quantum Chemistry, Vol. 75, 805᎐814 (1999)

䊚 1999 John Wiley & Sons, Inc. CCC 0020-7608 / 99 / 040805-10
EBNER ET AL.

There seem to be only a few computer simula- Similar potential functions were constructed for
tions on systems containing NOy 3 , and the ones we the Kq᎐H 2 O and Kq᎐NOy 3 interactions. For the
know of made use of relatively simple standard former, second-order Møller᎐Plesset ŽMP2. calcu-
potentials w 8᎐11x . A computer simulation involv- lations with the valence-double-zeta basis set aug-
ing an aqueous silver nitrate solution was per- cc-pVDZ w 18x for water and the compact effective
formed by Laaksonen and Kovacs w 8x . They used a core potential ŽCEP. double-zeta basis set w 19x for
rigid model with a Lennard-Jones force field for Kq were performed, and 135 configurations were
the nitrate and the rigid simple point charge ŽSPC. fitted to equation Ž2.:
water molecule w 12x . Another molecular dynamics
simulation was performed by Kataoka w 9x using qk qi Ak i Bk i Ck i Dk i
Vfitp ᎐ w s Ý q q q q .
the Carravetta᎐Clementi water model w 13x and an k, i rk i r k3i r k4i r k5i r k7i
empirical potential for the nitrate interactions. The Ž2.
findings of these two works are discussed together
with our results. For the K᎐On and K᎐Nn interaction, Hartree᎐Fock
calculations using the MIDI basis set w 20x aug-
mented with additional polarization w 21x and
Model Potentials diffuse w 20x functions were performed. A total of
104 configurations were calculated and fitted to
For the nitrate᎐water interactions a recently de- Eq. Ž3.:
veloped potential w 14x was used which had been
derived from about 400 points on the Hartree᎐Fock qk qi Ak i Bk i Ck i Dk i
Vfitn ᎐ p s Ý q q q q .
energy surface calculated with the 6-311qGŽd, p. k, i rk i R 2k i r k3i r k8i r k12i
basis set w 15, 16x . Water and nitrate anion in- Ž3.
tramolecular geometries had been kept rigid at the
experimental values w 17x with r N ᎐ O s 1.220 A, ˚ The values of the parameters in the polynomials
⬔O ᎐ N ᎐ O s 120⬚, rO ᎐ H s 0.957 and ⬔H ᎐ O ᎐ H s Ž2. and Ž3. are also listed in Table I. The powers of
104.5⬚. The potential function has the form the reciprocal distances in Eqs. Ž1. to Ž3. were
chosen to provide for the best fitting. The atomic
qk qi Ak i Bk i Ck i Dk i
Vfitn ᎐ w s Ý q q q q , partial charges for nitrate anion were q1.298 and
k, i rk i r k4i r k6i r k8i r k9i y0.766 for N and O, respectively. The charges of
Ž1. the MCYL potential w 22x , which was used for the
water᎐water interactions, are y1.434 and q0.717,
where A to D are the parameters to be fitted, qk respectively; the charge of Kq is q1.
and qi are the partial charges at the centers of To account for the intramolecular flexibility of
interaction k and i in the two molecules, and r k i is NOy w x
3 , a previously developed force field 14 was
their distance. The values of the parameters are employed. It expresses the energy as a harmonic
given in Table I. Details of the potential functions function of 6 internal coordinates of nitrate anion
are given in w 14x . plus the cubic contribution to the total symmetric

TABLE I
˚ ) of the analytical pair potentials for nitrate – water, potassium – water,
Values of the parameters (kcal / mol, A
and potassium – nitrate interactions [Eqs. (1) – (3)].

A B C D

O n ᎐O w 456.230953 y14346.495583 135881.693843 y163716.597217

O n ᎐H w y9.267009 509.57219 y1063.771480 654.054407
N n ᎐O w y141.352593 8378.086698 y52972.399360 60790.249131
N n ᎐H w y286.238598 1710.782613 y4740.608610 3454.023536
K +᎐O w 1859.406024 y11494.917116 21777.167880 y18263.765437
K +᎐H w y459.404780 1448.383938 y1282.260341 359.349644
K +᎐O n y34.474445 y148.359738 17963.462877 y53112.883010
K +᎐N n 199.694792 y261.718481 8595.372921 y17082.294320

806 VOL. 75, NO. 4 / 5

 AQUEOUS POTASSIUM NITRATE SOLUTION

stretching, which contains the largest part of the were rescaled every 10 timesteps to an average
anharmonicity: temperature of 2000 K, after which an equilibra-
tion phase at the intended simulation temperature
1 6 6 1
Vintra s f i i Di Di q f i j Di Dj q f 444 S43 of 293 K was performed for 0.5 ps. The data
2
Ý Ý 6 collection was then performed for 25 ps. On a SGI
is1 i/js1
Ž4. O2000 computer with a MIPS R10000 processor,
the elapsed time for one timestep was about 1.7 s.
with The conservation of the total energy was better
than 0.2%. The simulations were performed with a
S4 s Ž ⌬ r N ᎐ O1 q ⌬ r N ᎐ O 2 q ⌬ r N ᎐ O 3 . r'3 . modified version of the computer program KGN-
MCYL originally developed by Lie and Clementi
Here D1 to D6 denote three ⌬ r internal coordi- w 22x obtained from the MOTECC-89 collection of
nates, the two independent ⌬ ⬔ internal coordi- programs w 23x .
nates and the out-of-plane angle ⬔ŽOOP. . f 444 is the
anharmonic force constant of the total symmetric
stretching mode. The values of the force constants Results
are given in Table II.
STATIC AND STRUCTURAL PROPERTIES
Radial distribution functions ŽRDFs. g xy y have
Details of the Molecular been calculated for the different pairs of atoms
Dynamics Simulation where x refers to N or O of nitrate anion and y
refers to O or H of water molecule. The water᎐
The simulated system consisted of 1 potassium anion RDFs are displayed in Figure 1. The upper
cation, 1 nitrate anion, and 341 water molecules. part of the figure contains g N ᎐ O and g N ᎐ H
For interactions involving nitrate and potassium whereas in the lower one g O ᎐ O and g O ᎐ H are
ions, the potentials discussed above were used and shown. All RDFs are plotted together with their
the flexible MCYL potential w 22x was employed for running coordination numbers n x y :
water᎐water interactions. The cubic box had a side
˚ The concentration of 0.162 mol
length of 21.75 A. r
KNO3rkg H 2 O corresponds to an experimental n x y Ž r . s 4␲␳
H0 g xy
Ž r ⬘ . r ⬘ 2 dr ⬘, Ž5.
density of 1.00839 grcm3 for the solution at a
temperature of 293 K. Periodic boundary condi-
tions were employed and the long-range electro- where ␳ is the number density of solvent
static interactions were treated by the reaction molecules. The hydration number of an atom can
field method. For the short-range forces a spherical then be defined as the value of nŽ r . at the first
cutoff of half the box length and the shifted-force minimum of the corresponding radial distribution
method were employed. Newton’s equations of function.
motions were solved using a sixth-order gear pre- In the following discussion we use the sub-
dictor᎐corrector integration algorithm with a scripts n and w to denote atoms belonging to
timestep of 0.25 fs. During the first part of the nitrate anion and water molecule, respectively. The
equilibration period, the velocities of the molecules Nn ᎐Ow and On ᎐Ow RDF functions Žsolid lines in
Fig. 1., are qualitatively similar to RDFs of hy-
drated large monatomic anions such as iodide w 24x .
TABLE II
Force constants of the intramolecular NO3y
The first peak of g N ᎐ O has a maximum of 2.30 at
potential (atomic units).
˚ Correspondingly, the On ᎐Ow radial distri-
3.77 A.
bution function has its first maximum of 1.51 at
f r 1᎐r 1 0.4840 ˚ and shows a second broad peak around 4.8
2.92 A
f ␣1 ᎐ ␣1 0.7130 Å. One can compare the first maxima with the
foop 0.1748 ˚ ŽOn ᎐Ow . and 3.98 A
distances of 2.76 A ˚ ŽNn ᎐Ow .
f r 1᎐r 2 0.0583 which would result from ‘‘perfect’’ linear
f ␣1 ᎐ ␣ 2 0.3565 N—O ⭈⭈⭈ H—O hydrogen bonds Žassuming an
f ␣1 ᎐ r 1 0.0886 ˚ w 14x. . Therefore it seems
On ᎐H w distance of 1.80 A
f444 y1.3759
that water molecules with somewhat distorted hy-

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 807

EBNER ET AL.

water molecules already contributing with their

other hydrogen atom to the first peak. The On ᎐H w
RDF shows also two maxima, at distances of 2.02
and 3.50 A.˚ The low value of g Ž r . at the first
maximum results from the fact that the space
occupied by nitrate anion is not accounted for in
the spherical shell normalization of the RDFs.
Because the multitude of On ᎐Ow and On ᎐H w
pairs leads to complicated RDF functions, angular
distributions can further clarify the picture of
the hydration shell structure. The cosine distri-
butions of the two hydrogen bond angles
⬔ŽOn⭈⭈⭈ H w —Ow . and ⬔ŽNn —On⭈⭈⭈ H w . are shown
in Figure 2 Žsolid and dashed lines.. The angles
have been calculated for O ⭈⭈⭈ N distances less than
3.0 A˚ and a cosine axis has been chosen since as a
result of the angular metric this scaling leads to a
constant probability for a random distribution of
atoms. For an ideal hydrogen bond, all four atoms
Nn —On⭈⭈⭈ H w —Ow would be in a linear arrange-
ment with cosŽ ␣ . equal to y1 for both angles. The
angle ⬔ŽOn⭈⭈⭈ H w —Ow . is energetically the more
important one. Its distribution shows that, despite
the fact that there are only a few linear hydrogen
bonds, the majority of the water O—H bonds
interacts attractively with the nitrate oxygens. The
distribution of the second hydrogen bond angle,
⬔ŽNn —On⭈⭈⭈ H w ., is similar but broader and even
FIGURE 1. Radial distribution functions: N and O atoms Nn —On⭈⭈⭈ H w angles near 90⬚ are found.
of nitrate anion and water oxygen ( g N᎐O , g O ᎐O : top); N Figure 2 Ždotted line. shows the distribution of
and O atoms of nitrate anion and water hydrogen ( g N᎐H , Ow1⭈⭈⭈ Nn⭈⭈⭈ Ow 2 angles within the first hydration
g O᎐H : bottom). The radial distribution functions are shown shell. They were collected for all water molecules
together with the corresponding running integration w1 and w2 with N ⭈⭈⭈ Ow distances less than 4 A. ˚
numbers. For the corresponding RDF function in Figure 1, it
can be seen that most of the first hydration shell
falls within this cutoff. The distribution of the
drogen bonding to the oxygen atoms of NOy 3 are cosines is very broad and increases from y1 to a
the main constituents of the first hydration shell. shallow maximum at about O Ž ␣ s 90⬚.. A more
Since the three oxygen atoms in nitrate anion cause pronounced peak at 0.69, corresponding to an an-
a multitude of larger On ᎐Ow and On ᎐H w dis- gle of 46⬚ corresponds to Ow1⭈⭈⭈ Nn⭈⭈⭈ Ow 2 triplets
tances, the second peaks in g N ᎐ O and g N ᎐ H are with one short and one long Nn⭈⭈⭈ Ow distance
broad and unpronounced. where both water molecules can occupy positions
The Nn ᎐H w RDF differs from the ones usually with a partial H bond between them.
found with monoatomic anions since it shows two It can be concluded that the water molecules
peaks with maxima at distances of 3.02 and 4.30 A. ˚ prefer to coordinate to nitrate oxygen atoms and
The first peak contains about 12 hydrogen atoms not to the nitrogen atom from top and below the
Žcounted up to the first minimum at ; 3.7 A ˚ .. If plane of the nitrate anion. Conventional hydrogen
one estimates that g N ᎐ H up to the first minimum bonding dominates, but many other possible ar-
includes about 80% of those hydrogen atoms that rangements including strongly bent and bifurcated
constitute the first shell, this is consistent with hydrogen bonds as well as cyclic arrangements
about 16 oxygen atoms under the more pro- with two distorted H bonds can be found as well.
nounced first peak of g N ᎐ O . The second peak of The shell of water molecules around NOy 3 cation
g N ᎐ H is mainly caused by hydrogen atoms of is orientationally relatively unstructured with re-

808 VOL. 75, NO. 4 / 5

 AQUEOUS POTASSIUM NITRATE SOLUTION

FIGURE 2. Distribution of the cosines of the O n⭈⭈⭈ H w ᎏ O w (full line) N n ᎐O n⭈⭈⭈ H w (dashed line) and angles up to an
˚ The dotted curve shows the cosine distribution of Ow 1⭈⭈⭈ N n⭈⭈⭈ Ow 2 up to a maximum N n⭈⭈⭈ Ow 1
O n⭈⭈⭈ H w distance of 3 A.
distance of 4 A.˚

spect to the possible binding sites. This feature is potential function in Eq. Ž3., which was specifically
in agreement with the interaction potential Žand constructed for potassium᎐water interactions, is
the results of ab initio geometry optimizations w 4, more accurate than the one used in w 25x .
5, 14x. since many attractive configurations with One can compare the structural results from our
Nn ᎐Ow distances between 3 and 4 A, ˚ including the simulation to data obtained from a simulation of
global energy minimum, have similar potential 4 AgNO3 molecules dissolved in 248 water
energies. In the simulation, due to water᎐water molecules w 8x . The g N ᎐ O function is similar to ours
interactions, temperature effects, and the restricted with the first maximum of about 3.0 at 3.6 A ˚ and
volume, these small differences become even less ˚
its first minimum at 4.1 A. As it is the case for our
important. simulations, this indicates that water coordinates
The Kq᎐Ow and H᎐H w RDFs ŽFig. 3. show the not to nitrogen but to oxygen atoms of nitrate
typical picture of the relatively weak hydration anion. There are, however, only eight oxygen atoms
structure of a large monovalent cation. The first under the first peak. g N ᎐ H looks different than in
peak of g K ᎐ O includes about 14 oxygen atoms up our simulation. The first peak has its maximum at
to its minimum at 4.67 A. ˚ There is no unusual about the same distance but contains all hydrogen
difference between our cation᎐water distribution atoms of the first-shell water molecules, and there-
functions and other simulation on systems contain- fore our second peak at 4.3 A ˚ is missing.
ing hydrated potassium cations. The height of the The main difference between w 8x and our work
first maximum of g K ᎐ O Ž2.97 A ˚ . s 2.65 is lower seems to be that in the former one most water
than, for example, found in w 25x with the molecules in the first hydration shell have both
CHARMM22 w 26x force field for water and Kq hydrogens pointing toward nitrate᎐nitrogen. In our
w g K ᎐ O Ž3.0 A
˚ . s 4.0x . It is, however, likely that the work, there are nearly twice as many water

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 809

EBNER ET AL.

FIGURE 3. Water ᎐cation radial distribution functions g K ᎐O and g K ᎐H are corresponding running integration numbers.

molecules in the hydration shell, most of them distributions been calculated, and both works use
coordinating to the anion with one hydrogen atom rigid molecules since their emphasis has been in
only. We tried to trace back these results to the calculating nuclear magnetic resonance ŽNMR.
different water᎐nitrate potentials, but it turned properties. It can be assumed from these simula-
out that they, despite their completely different tions and ours that the details of the hydration
origin, are quite similar. In fact, in our potential shell depend quite sensitively on the interaction
w 14x the simultaneous binding of two water hydro- potentials, and therefore the construction and use
gens is even more favored Žby 2 kcalrmol. than in of a nontransferable, specifically constructed po-
the empirical potential used by Laaksonen and tential function for the nitrate interactions seems
Kovacs w 8x . Since we could perfectly reproduce warranted. Figure 4 shows two typical configura-
their results by using their potentials, the reason tions of the water molecules in the hydration shell
for the above-mentioned differences in the hydra- of nitrate anion.
tion shell structure remain unclear and can proba-
bly only be explained by a detailed analysis of all
the different terms in the potential functions. INTRAMOLECULAR GEOMETRY AND
VIBRATIONS OF NITRATE ANION
In the only other MD simulation on aqueous
nitrate solutions we could find in the literature w 9x , Thermal motion and vibrations cause instanta-
the Caravetta᎐Clementi w 27x model for water was neous deviations from the D 3 h equilibrium geome-
used and another empirical Lennard-Jones poten- try of nitrate anion in solution. The distribution of
tial for nitrate anion. The radial distribution func- the out-of-plane angle Ždefined as the angle be-
tions of this study are very similar to ours. For tween any N—O bond and the plane constituted
example, the two first peaks of g N ᎐ H are to the left by N and the other two oxygen atoms. show a
and to the right of the first peak of g N ᎐ O . The half-height width of about "6⬚ w Fig. 5Ža.x . The
number of oxygen atoms under the first peak of half-height width of the distribution of the in-
g N ᎐ H is 10᎐11. Neither in w 8x nor w 9x have angular tramolecular O—N—O angles w Fig. 5Žb.x , centered

810 VOL. 75, NO. 4 / 5

 AQUEOUS POTASSIUM NITRATE SOLUTION

FIGURE 4. Snapshots of the first hydration shell of nitrate anion at timesteps (a) 1 and (b) 40000 after the equilibration
period.

at 120⬚ is "4⬚. The distribution of the N—O bond anion shows a frequency spectrum consisting of 4
length w Fig. 5Žc.x is symmetric with a half-height- bands ␯ 1 to ␯4 at 1050, 830, 1380, and 716 cmy1 .
width of about "0.03 A. ˚ Its maximum almost They belong to modes with AX1 , AY2 , and EX Žtwo-
coincides with the potential energy minimum at fold. symmetry w 28x and can be described as sym-
˚
1.27 A. metric stretching, out-of-plane bending, asymmet-
The vibrational properties of the anion were ric stretching and asymmetric bending vibrations,
calculated from the velocity autocorrelation func- respectively.
tions ŽVACF. of the nitrogen and oxygen atoms as One of the reasons for looking at intramolecular
defined by: nitrate anion frequency shifts is the experimental
finding that in dilute aqueous alkali metal nitrate
VN Ž 0 . *VN Ž t . solutions a splitting of the ␯ 3 band of about 56
VACFN Ž t . s ¦ w VN Ž 0 .x 2 ; t
Ž6.
cmy1 occurs w 29x . This splitting is contributed to a
symmetry lowering of nitrate anion to C2 v due to
and its interaction with the solvent molecules. In Table
III, the frequencies obtained from our molecular
Ý3ks 1 VO kŽ 0 . *VO kŽ t . dynamics ŽMD. simulation and those from ab ini-
VACFO kŽ t . s
¦ Ý3ks 1 VO kŽ 0 .
2 ;
, Ž7. tio calculations w 14x are compared with experimen-
tal data. Two peaks each appear for the out-of-
plane mode and the asymmetric stretching mode.
where VO k is the velocity of the k th oxygen atom All gas ª solution frequency shifts from the simu-
of nitrate anion and VN is the velocity of the lation are small and toward higher frequencies.
nitrogen atom. The brackets denote the average They amount to 13, 8, 8, and 14 cmy1 for ␯ 1 to ␯4 ,
over time origins. The Fourier transform of the respectively, if arithmetic average is taken for the
VACF delivers the spectral densities Žpower spec- splitted peaks.
tra. of the vibrations of nitrate ion ŽFig. 6.. Experi- These upshifts can be compared with the results
mentally, the Žhypothetically. undisturbed nitrate of ab initio reaction field calculations. We per-

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 811

EBNER ET AL.

FIGURE 6. Power spectra of the nitrogen and oxygen

atoms of NO 3y. The resolution of the frequency axis is
6.5 cm y1 .

scribed above ŽTable IV.. If all water molecules are

taken into account, the three bands correspond to
bending, symmetric, and asymmetric stretching
appear, within the spectral resolution, unchanged
from the values for pure water. There is consider-
able exchange of water molecules during the simu-
lation. To obtain the spectral densities only for
water molecules in the vicinity of one of the ions,
we selected those parts of the water trajectories
˚ Ž3.3 A
with an Nn⭈⭈⭈ Ow distance less than 4.0 A ˚ for
Kq ⭈⭈⭈ Ow . for at least half of the timesteps. The
average trajectory length were 3310 timesteps for
water near NOy 3 and 1970 timesteps for the hydra-
FIGURE 5. Average geometry of the NO 3y anion: (a)
out-of-plane angle; (b) intramolecular O ᎏ N ᎏ O angles,
tion shell water of Kq. From Table IV it can be
and (c) N ᎏ O distances. seen that nitrate hydration shell water experiences
an upshift of 22 and 28 cmy1 for the symmetric
and asymmetric stretching modes Žcompared to
formed such a calculation by means of the self-
pure water., respectively. Normally stronger hy-
consistent isodensity polarized continuum model
ŽPCM. method w 30x for the reaction field with the drogen bonding lowers the O᎐H force constant
B3LYP density functional and the aug-cc-pVTZ and leads to downshifted stretching frequencies.
basis set. A dielectricity constant of ␧-78.85 was Therefore, this is in accordance with the discussion
assumed for the surrounding medium. For the of the structural features of the hydration shell in
symmetric stretching Ž ␯ 1 . and the asymmetric the section above and shows that the water᎐water
bending Ž ␯4 . modes, the reaction field causes up- interactions are preferred over the water᎐nitrate
shifts of 7 and 9 cmy1 whereas the other modes interactions. For Kq a slightly smaller upshift can
stayed unchanged. be seen as well. There, however, the hydrogen
bonds point away from the ion, and therefore it is
the interaction between first and second hydration
FREQUENCIES OF THE WATER MOLECULES
shell water molecules that is weaker than in the
Power spectra for the water molecules were bulk phase. All these shifts are much smaller than
calculated analogous to the nitrate spectra de- the gas ª liquid downshifts of our MCYL water

812 VOL. 75, NO. 4 / 5

 AQUEOUS POTASSIUM NITRATE SOLUTION

TABLE III
Vibrational frequencies of nitrate anion.

Method Ref. ␯4 ␯2 ␯1 ␯3

Ab initio, gas phase [14] 689 816 996 1379

(MP4 / aug-cc-pVTZ)
Ab initio, gas phase [14] 707 844 1061 1364
(B3LYP / aug-cc-pVTZ)
Ab initio, reaction field This 714 844 1070 1364
(B3LYP / aug-cc-pVTZ) work
MD, solution This 703 814, 833 1009 1380, 1393
work
MD, isolated anion This 689 816 996 1379
work
Experiment [29] 719 825 1049 1348, 1404
Experiment [31] 720, 740 ᎏ ᎏ 1340, 1460

TABLE IV power spectra of nitrate anion show small gas ª

Vibrational frequencies (cm y1) of the water liquid frequency shifts. The doubling of the ␯ 3
molecules in the first hydration shell of nitrate band amounts to 13 wavenumbers. The stretching
anion and potassium cation as obtained from the
modes of the water molecules in the anionic hy-
MD simulation.
dration shell show upshifts of about 22 and 28
Symmetric Asymmetric wavenumbers, indicating weaker hydrogen bond-
Bending stretching stretching ing than in the bulk water. Comparison with other
simulations shows that hydration number and ori-
Pure H 2 O 1740 3648 3752 entations of the water molecules respond sensi-
All H 2 O 1742 3650 3751 tively to details of the potentials.
molecules
NO 3y hydration 1751 3670 3780
shell ACKNOWLEDGMENTS
K + hydration 1764 3656 3772
shell Financial support from the Austrian FWF Žpro-
ject P10106-MOB. is gratefully acknowledged.

model Ž198 and 203 cmy1 for the O᎐H stretches

w 22x. . References
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London, 1976, Vol. 2, Chapter 6, p 212.

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3. de P. Nicholas, A. M.; Wasylishen, R. Can J Chem 1987, 65,
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by an intramolecular anharmonic force field. The Phys 1990, 93, 3379; Chem Phys 1991, 151, 187.
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