Calculated OH-Stretching Vibrational Transitions in the Water-Nitrogen and
Water-Oxygen Complexes
Henrik G. Kjaergaard,*
,²
Geoffrey R. Low, Timothy W. Robinson, and Daryl L. Howard
Department of Chemistry, UniVersity of Otago, P. O. Box 56, Dunedin, New Zealand
ReceiVed: February 26, 2002; In Final Form: June 18, 2002
We have calculated the fundamental and overtone OH-stretching vibrational band intensities of the water-
nitrogen (H
2
O‚N
2
) and water-oxygen (H
2
O‚O
2
) complexes. The calculations use the harmonically coupled
anharmonic oscillator local mode model with local mode parameters obtained from scaled ab initio calculations
and ab initio calculated dipole moment functions. The H
2
O‚N
2
and H
2
O‚O
2
complexes are weakly bound and
the individual molecular units are only slightly perturbed by complexation, unlike what is found for the water
dimer (H
2
O‚H
2
O) and the water-nitric acid complex (H
2
O‚HNO
3
). The fundamental OH-stretching intensity
in H
2
O‚N
2
is enhanced and the first overtone intensity weakened compared to H
2
O as an effect of the hydrogen
bonding. In H
2
O‚O
2
the OH-stretching intensities are comparable to those of H
2
O. On a per water unit basis,
the calculated OH-stretching intensities of the higher overtones of H
2
O‚N
2
and H
2
O‚O
2
are similar to those
of H
2
O‚H
2
O. The possible effect of H
2
O‚N
2
and H
2
O‚O
2
on the atmospheric absorption of solar radiation is
discussed.
Introduction
The discrepancy between observed and modeled atmospheric
absorption of solar radiation is a long-standing problem in
atmospheric science.
1-4
Water is the principal absorber of solar
radiation, and in the near-infrared and visible regions its
absorption spectrum is dominated by OH-stretching overtone
transitions. Weakly bound van der Waals complexes, especially
those containing water, have been suggested as possible
contributors to the excess or anomalous solar absorption.
5-9
In
the atmosphere, where the gas is not strongly pressurized, the
principal complexes formed are the 1:1 complexes.
9
Previously,
we have investigated the OH-stretching overtone spectra of the
water dimer
10
(H
2
O‚H
2
O), and in the present article we
investigate the water-nitrogen (H
2
O‚N
2
) and water-oxygen
(H
2
O‚O
2
) complexes.
The large-amplitude motion associated with OH-stretching
vibrations can be described in terms of local modes and the
harmonically coupled anharmonic oscillator (HCAO) local mode
model.
11-13
Overtone intensities have been successfully pre-
dicted with the HCAO local mode model and ab initio calculated
dipole moment functions at modest ab initio levels.
14-17
Calculations on the water monomer have shown that the simple
HCAO local mode model is an adequate vibrational model
compared to a full variational calculation,
18
and the results
obtained are in good agreement with the experimental intensities
of the HITRAN database.
19
Basis sets of triple- quality
including diffuse and polarization functions were required to
obtain accurate absolute intensities for the water molecule.
20
We have previously observed for monomeric species that the
inclusion of electron correlation is important for the accurate
prediction of fundamental intensities, but less so for the overtone
intensities.
21
We have recently suggested a method that allows the
calculation of OH-stretching overtone spectra without the need
of input from experimental spectra.
10
The method is based on
suggestions by Sowa et al.
22
to obtain the local mode parameters
from ab initio calculated potential energy curves.
10
We have
used this method to calculate the OH-stretching transitions for
the water dimer and trimer
10
and for the water-nitric acid
(H
2
O‚HNO
3
) complex.
23
These calculated overtone spectra
indicate spectral regions that are favorable for experimental
investigations of these complexes.
Infrared (IR) spectra of H
2
O‚N
2
trapped in cold Ar matrixes
have been recorded,
24,25
and the vapor phase structure has been
determined by microwave studies.
26
No vibrational spectra of
H
2
O‚O
2
have been recorded, nor has the experimental structure
been determined. However, recent neutralization-reionization
mass spectroscopy studies have found evidence for the presence
of both H
2
O‚O
2
and the charge-transfer complex H
2
O
+
‚O
2
-
,
with the neutral species having a lifetime exceeding 0.5 µs.
27
There have been a number of theoretical investigations
regarding the structure and IR spectra of the H
2
O‚N
2
and
H
2
O‚O
2
complexes.
25,28-31
The theoretical predictions for
H
2
O‚N
2
compare reasonably well with the experimental matrix
isolation IR spectra
24,25
and with the microwave determined
structure.
26
Theoretical studies of H
2
O‚O
2
have focused pre-
dominantly on its role in the photonucleation of water vapor in
the presence of oxygen, a first step in cloud formation. It is
suggested that UV radiation incident on the H
2
O‚O
2
complex
stimulates an electron-transfer process creating a charge-transfer
product H
2
O
+
‚O
2
-
that is thought to function as an aggregation
point for nearby polar water molecules.
32,33
We report HCAO calculated fundamental and overtone OH-
stretching band positions and intensities for H
2
O‚N
2
and H
2
O‚O
2
and compare our results with calculations for H
2
O, H
2
O‚H
2
O,
and H
2
O‚HNO
3
. We compare our results for H
2
O‚N
2
with the
available experimental IR data
24,25
and for H
2
O‚H
2
O with the
molecular beam IR data
34,35
and the recent IR and near-IR (NIR)
matrix isolation data.
36,37
* Corresponding author. Telephone: 64-3-479-5378. Fax: 64-3-479-
7906. E-mail: henrik@alkali.otago.ac.nz.
²
On sabbatical at Cooperative Institute for Research in Environmental
Sciences (CIRES), University of Colorado, Boulder, CO 80309-0216.
8955 J. Phys. Chem. A 2002, 106, 8955-8962
10.1021/jp020542y CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/30/2002