The Fourier-transform Fabry-Perot pulsed-molecular-beam microwave spectrometer at NIST was used to study the microwave spectra of a number of molecular dimers and trimers that may be present in planetary atmospheres. The weak van der Waals bonds associated with these species usually give rise to rotational-tunneling splittings in the microwave spectra. The microwave spectrum of the water dimer species was used to illustrate the complications that can arise in the study of the rotational spectra of these loosely bound species. In addition to the water dimer species, the microwave spectra of the following hydrogen-bonded and van der Waals complexes were studied: (CO2)2-H2O, CO2-(H2O)2, CO2-H2S, N2-H2O, CO-H2O, SO2-H2O, and O3-H2O

Lovas, F. J.

Suenram, R. D.

English

NASA Technical Reports Server

N90-26752
MICROWAVE SPECTRA OF VAN DER WAALS COMPLEXES OF
IMPORTANCE IN PLANETARY ATMOSPHERES
R. D. SUENRAM AND F. J. LOVAS
Molecular Spectroscopy Division, National Institute of Standards and
Technology, Gaithersburg, MD 20899
ABSTRACT
The Fourier-transform Fabry-Perot pulsed-molecular-beam microwave
spectrometer at NIST has been used to study the microwave spectra of a number
of molecular dimers and trimers that may be present in planetary atmospheres.
The weak van der Waals bonds associated with these species usually give rise to
rotational-tunneling splittings in the microwave spectra. We have used the
microwave spectrum of the water dimer species to illustrate the complications
that can arise in the study of the rotational spectra of these loosely bound
species. In addition to the water dimer species, the microwave spectra of the
following hydrogen-bonded and van der Waals complexes have been studied:
(CO2)2.H20 , CO2.(H20)2 , C02.H2S , N2.H20 , CO.H20 , SO2.H20 , and O3.H20.
INTRODUCTION
In a recent paper Jennings I summarizes the major and minor molecular
species present in planetary and lunar atmospheres. The most abundant species
are CO2, N 2, H2, and CH4, with more than 50 other molecular species being
present in smaller amounts. Fox and Kim 2 have recently attributed some
previously unassigned infrared features in the atmosphere of Titan as arising
from the H2.N 2 van der Waals complex.
Due to the complexity of the spectra caused by internal motions found in
most van der Waals and hydrogen bonded complexes and our inability to predict
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the structures of these species a priori, a thorough understanding of the
rotational spectra of such complexes is usually a prerequisite to the
understanding of their far-infrared and near infrared spectra. Over the past
five years at NIST we have developed a broad prog_ram aimed at the study of
numerous van der Waals and hydrogen bonded molecular species. Many of the
species we have analyzed could be important contributors to the infrared
spectra of planetary atmospheres. The analysis of the rotational spectra of
these species provides firm ground work_for attempts at understanding their
far-infrared and near infrared spectral features.
EXPERIMENTAL
a. Instrumentation. A pulsed-molecular-beam Fabry-Perot cavity Fourier-
transform microwave spectrometer has been used to study the species described
in this paper. A molecular beam of the species of interest is formed by
pulsing a 1% mixture of the species in an inert carrier gas (Ne or Ar) through
a pulsed solenoid valve. When the molecular beam pulse reaches the center of
the Fabry-Perot microwave cavity, a _/2 microwave pulse of several
microseconds duration is applied to the microwave cavity. This produces
Fourier components of the microwave radiation over a bandwidth of -i MHz around
the center frequency. If any of the molecular species in the beam have a
rotational transition within this 1MHz window, they are coherently excited by
the microwave radiation. The resulting free-induction decay signal emanating
r
from the microwave cavity is detected using a super heterodyne receiver. This
detected signal is digitized at 0.5 #sec intervals for 512 points. Generally
the digitized signals are averaged over hundreds or even thousands_of microwave
pulses. Once this average is obtained, it is Fourier transformed to give the
?9
power spectrum. The linewidths attainable with this instrument are typically
<20 kHz and frequency measurementsare considered accurate to 4kHz which is
the resolution element corresponding to the digitization channel width.
Further details of the construction and operation of the spectrometer are
provided in Ref. 3-5. Details pertaining to the chemistry and spectral fea-
tures of the species studied can be found in the papers referenced in Table I.
b. Water Dimer Spectrum. Over the past decade there has been substantial
interest in the microwave and infrared spectroscopy of (H20) 2. The early work
of Dyke and co-workers 6 provided the first preliminary analysis of the
rotational spectrum in 1980. Since then much additional work has been carried
out in the microwave, far-infrared, and infrared spectral regions. The history
of the spectroscopy of (H20)2 has been recently reviewed by Fraser, Suenram and
Coudert 7. The group theory necessary to describe the rotational spectrum of
(H20) 2 has been given by Coudert and Hougen 8. Briefly, the rotational energy
levels are first split by a tunneling motion involving a C2 rotation of the
acceptor H20 molecule in the complex. This splitting is quite large (230 GHz
for the K_I=0 levels) and as yet not well determined. These two levels are
then further split (22.55437 GHz for the AI/B 1K_I=0 levels) by an exchange
motion of the two H20 subunits in the complex as they interchange their proton
donor-acceptor roles within the complex. The energy level diagram is shown
schematically in Figure i. The rotational analysis provided by Fraser, Suenram
and Coudert 7 has greatly aided the far-infrared and near infrared analysis of
the (H20) 2 spectrum 9,10.
c. Other Molecular Species. In general, the rotational spectra of the
additional species mentioned in the abstract are not as complicated as the
80
Table I. Structural Summariesand Electric Dipole Momentsfor
Weakly BoundMolecular Complexesof Interest in
Planetary Atmospheres.
Species Ref. Dipole Summary
N2,H20 ii #a=0.833
CO,H20 12 #a=i.055
S02,H20 13 #a=1.984
_c=0.488
03-H20 14 #a=l.014
_c=0.522
C02,H2S 15 #a=0.410
_c=0.822
(CO2)2-H20 16 _b=1.982
C02,(H20)2 17 _a=1.571
_b=0.761
Four tunneling states observed due to
tunneling of both the H20 and N2 subunits
The spectral splitting caused by the H20 is
-ten times greater than that caused by the
N2. Linear heavy atom geometry.
Two tunneling states observed due to
internal motion of the H20 subunit. Linear
heavy atom structure with C atom hydrogen
bonded to H atom.
Two tunneling states observed due to
internal rotation of the H20 subunit. Sand-
wich-shaped structure with the S-O distance
shorter than the O-H distance.
Twotunneling states observed due to the
internal motion of the H20 subunit. Upper
state does not conform to rigid rotor
theory. Cross shapedcomplex with water
atoms in the plane of the center 0 atom of
ozone. Nohydrogen bonding is evident.
Multidimensional tunneling similar to the
water dimer spectrum. Large tunneling
splittings (12.5 GHz) are observed for the
L-type transitions. T-shaped complex with
the H2Splane -90° to the linear CO2 axis.
Twosymmetry states observed with 3:1 in-
tensity ratio. Structure has C2 symmetry
axis with the water oxygen located symmetri-
cally below the plane of the slipped
parallel CO2 units.
Two states observed with small spectral
splittings. Structure is a triple hydrogen
bonded system in which all heavy atoms are
planar and the two water oxygens and the
carbon atom of CO2 are located at the
apices of a triangle.
81
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82
water dimer spectrum but spectral splittings still occur. There are almost
always two states observable due to an internal motion in complexes which
contain H20. In most cases this is caused by the rather facile motion of water
about its C 2 axis within the complex. Structural summaries and electric
dipole moments for the complexes which have been studied that are of planetary
atmospheric interest are listed in Table I.
CONCLUSION
In this paper, we have summarized the spectral results that have been
obtained from the rotational analysis of a number of weakly bound complexes
containing CO 2 and/or H20. The data presented on the rotational spectra of
these species should be useful to experimentalists attempting to analyze the
far-infrared and near infrared spectral features of these complexes and also to
those investigators involved in the modeling of their spectra in planetary and
lunar atmospheres.
REFERENCES
i. D.E. Jennings, J. Quant. Spectrosc. Radiat. Transfer, 40, 221
(1988).
2. K. Fox and S. J. Kim, J. Quant. Spectrosc. Radiat. Transfer, 40, 177
(1988).
3. F.J. Lovas and R. D. Suenram, J. Chem. Phys., 87, 2010 (1987).
4. R.D. Suenram, F. J. Lovas, G. T. Fraser, J. Z. Gillies, C. W.
Gillies, and M. Onda., J. Molec. Spectrosc., 137, 127 (1989).
5. L.H. Coudert, F. J. Lovas, R. D. Suenram, and J. T. Hougen, J.
Chem. Phys., 87, 6290 (1987).
6. J.A. Odutola and T. R. Dyke, J. Chem. Phys., 72, 5062 (1980).
7. G.T. Fraser, R. D. Suenram, and L. H. Coudert, J. Chem. Phys., 90,
6077 (1989).
83
8. L.H. Coudert and J. T. Hougen, J. Molec. Spectrosc., in press.
9. Z.S. Huangand R. E. Miller, J. Chem.Phys., 88, 8008 (1988).
i0. K. L. Busarow, R. C. Cohen, G. A. Blake, K. B. Laughlin, Y. T. Lee,
and R. J. Saykally, J. Chem. Phys., 90, 3937 (1989).
Ii. H. O. Leung, M. D. Marshall, R. D. Suenram and F_ J. Lovas, J. Chem.
Phys., 90, 700 (1989).
12. D. Yaron, K. I. Peterson, D. Zolandz, W. Klemperer, F. J. Lovas, and
R. D. Suenram, J. Chem. Phys., in press (1989).
13. K. Matsumura, F. J. Lovas, and R. D. Suenram, J. Chem. Phys., in
press (1989).
14. J. Z. Gillies, C. W. Gillies, R. D. Suenram, F. J. Lovas, T. Schmidt
and D. Cremer, in preparation.
15. J. K. Rice, L. H. Coudert, K. Matsumura, R. D. Suenram, F. J. Lovas,
and W. Stahl., in preparation.
16. K. I. Peterson, R. D. Suenram, and F. J. Lovas, J. Chem. Phys., 90,
5964 (1989).
17. K. I. Peterson, R. D. Suenram, and F. J. Lovas, in preparation.
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Microwave spectra of van der Waals complexes of importance in planetary atmospheres

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