Ammonia-Water Cation and Ammonia Dimer Cation

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Introduction
Extensive studies of dissociation of acids, bases, and salts
have been carried out to understand solvation phenomena.1-3
These phenomena are strongly involved in H-bonding and
proton transfer. For a better understanding of H-bonding and
proton transfer, it would be of interest to investigate such
phenomena in the ionized state, which can be easily observable
in the stratosphere. We are particularly interested in the
hydration of the ammonia cation, namely, (NH3 · · ·H2O)+.
Ammoinum cluster cations (NH3)n
+ and their hydrated
clusters [(NH3)n[(H2O)m]+ were experimentally produced under
special conditions in the gas phase.4 A few theoretical investigations
for ammonia, water, and ammonia-water cluster cations
were performed.5-8 The ammonia dimer cation and the
ammonia-water cation were experimentally discussed.9 For
the ammonia dimer cation, the disproportionated ionic structure
(NH4
+ · · ·NH2) was predicted to be more stable than the
hydrazine-like structure (H3N· · ·NH3).6 In previous work, one
of us reported that for the water dimer cation at high levels of
ab initio theory, the H3O+ · · ·OH structure is much more stable
than the H2O· · ·OH2 structure.10
Here, we investigate the structures, energetics, and spectra
of the ammonia-water cation (NH3H2O)+ and the ammonia
dimer cation (NH3)2
+ using high levels of ab initio theory. We
compare the DFT, MP2, and CCSD(T) results. Serious failures
for most DFT calculations are found, except for few functionals.
Calculation Methods
Before calculating the ionized structure of the ammonia-water
cluster and the ammonia dimer, we need to calculate their neutral
structure as the reference system. Since these structures are
already well-known,11,12 we calculated their structures and
energies at the CCSD(T)/aVQZ//CCSD(T)/aVDZ level of
theory. For the ionized structures, various structures of the
ammonia-water cation and the ammonia dimer cation are
optimized by using DFT methods with various functionals.
For the DFT calculations, we employ various functionals,
Becke’s exchange and Lee-Yang-Parr correlation functionals
(BLYP),13 Becke’s exchange and Perdew-Wang correlation
functionals (BPW91),14 Handy’s family functional including
gradient-corrected correlation (HCTH407),15 the local spin
density approximation, Vosko-Wilk-Nusair correlation and
Slater exchange functionals (LSDA: SVWN),16 semiempiricalcorrection
to BLYP for dispersion (BLYP-D),17 Tao-Perdew-
Staroverov-Scuseria exchange and τ-dependent gradientcorrected
functionals (TPSS),18 Becke’s three-parameters for
exchange and Lee-Yang-Parr correlation functionals
(B3LYP),19 Zhao and Truhlar’s parametrized exchange and
correlation hybrid meta-GGA M05-2X,20 Perdew-Burke-
E
zerhof hybrid functional (PBE1PBE),21 modified Perdew-
Wang one-parameter model/modified Perdew-Wang and Becke
one-parameter model for kinetics (MPW1K/MPWB1K),22,23
Becke’s half HF-LSDA (Hartree-Fock Local Spin Density
Approximation) exchange and Lee-Yang-Parr correlation
functionals (BH&H),24 and Becke’s half HF-LSDA-Becke
exchange and Lee-Yang-Parr correlation functionals
(BH&HLYP).25 For these DFT calculations, we used the
6-311++G** basis set.26 Then, as noted in the water dimer
cation,10 we also find that in the ammonia-water cation and
the ammonia dimer cation, DFT/MPW1K and DFT/ BH&HLYP
are reliable, while others give seriously wrong energy values,
as compared with the CCSD(T)/CBS values. Here, we compare
the DFT, MP2, and CCSD(T) results using the aug-cc-pVDZ,
aug-cc-pVTZ, and aug-cc-pVQZ basis sets27 (which will be
denoted as aVDZ, aVTZ, and aVQZ, respectively). The CBS
limit interaction energies were obtained with the extrapolation
* To whom correspondence should be addressed. E-mail: abcd0lhm@
postech.ac.kr.
J. Phys. Chem. A 2009, 113, 6859–6864 6859
10.1021/jp903093a CCC: $40.75  2009 American Chemical Society
Published on Web 06/02/2009
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Published on June 2, 2009 on http://pubs.acs.org | doi: 10.1021/jp903093a
scheme utilizing the fact that the electron correlation error is
proportional to N-3 for the aug-cc-pVNZ basis set (N ) 2: D;
N ) 3: T; N ) 4: Q) [ΔECBS ) (ΔENN3 - ΔEN-1(N - 1)3)/(N3
- (N - 1)3)].28 Here, the CBS energies, which would give the
most reliable values in these calculations, were obtained based
on the aVTZ and aVQZ results. In this way, we could compare
their CBS values and find the inherent errors in the DFT and
MP2 results.
For the DFT and MP2 calculations using the aVDZ and aVTZ
basis sets and the CCSD(T) calculations using the aVDZ basis
set, the geometries were fully optimized, and frequency calculations
were also carried out. A larger grid size (99, 974) than
the ultrafine grid (99, 590) was employed to eliminate the
imaginary frequencies for the DFT calculations. All of the
optimizations were done by minimizing the total energy without
any symmetry constraints. In the DFT and MP2 calculations
using the aVQZ basis set, the corresponding aVTZ geometries
were used, and the aVTZ frequencies were employed to obtain
the zero-point energies (ZPEs) and thermal energies, while
in the CCSD(T) calculations using the aVTZ and aVQZ basis
sets, the corresponding aVDZ geometries were used, and the
aVDZ frequencies were employed to obtain the ZPEs and
thermal energies. For the basis set, the 1s orbitals of oxygen
atoms were frozen in the correlation calculations. All of the
“d” and “f” orbitals used here are the spherical harmonic basis
functions (5d and 7f).
For the ionic structure, the basis set superposition energy
(BSSE) correction can be made. However, in the nonionic
structure, the positive charge is almost equally distributed
in two monomer species so that the BSSE correction cannot
be made properly. In order to compare the two structures at
equal conditions, it is better not to make the BSSE corrections.
Thus, the BSSE corrections are not considered in this
system.
We calculated the ZPE’s uncorrected total energy (ΔEe) at
the equilibrium states of the Bo
-Oppenheimer potential
surfaces and the ZPE-corrected total energy (ΔE0). The enthalpy/
free-energy changes (ΔHr/ΔGr) at room temperature and 1 atm
were obtained using the frequency calculations.
All of the calculations were carried out by using the Gaussian
03 suite of programs.29 The BLYP-D calculations were done
by using ORCA program30 and the M05-2X calculations by
Q-Chem program.31 The molecular structures were drawn using
the POSMOL package