High-resolution spectroscopy of H2SO4, HDSO 4, and D2SO4 vapor in the region 1200-10,000 cm-1

Article abstract of DOI:10.1016/j.saa.2004.05.006

The high-resolution (0.05 cm^-1) spectra of gas-phase H ₂SO₄, HDSO₄, and D₂SO₄ were measured over the frequency region 1200-10,000 cm^-1 using Fourier-transform infrared spectroscopy. The increased resolution of this work compared with previous studies has lead to an improved vibrational analysis of H₂SO₄. This study has answered unresolved questions about combination bands and overlapping features from previous gas-phase spectroscopic studies of H₂SO₄ and marks the first experimental measurement of the ν₈ and ν₁₅ torsional vibrations in this molecule. This work leads to a brief discussion on vibrational mode mixing in sulfuric acid.

Full text of DOI:10.1016/j.saa.2004.05.006

Spectrochimica Acta Part A 61 (2005) 559–566
High-resolution spectroscopy of H SO , HDSO , and D SO
2
4
4
2
4
−
1
vapor in the region 1200–10,000 cm
a
b
b
b,∗
P.E. Hintze , K.J. Feierabend , D.K. Havey , V. Vaida
a
ASRC-15, Kennedy Space Center, Melbourne Beach, FL 32951, USA
b
Department of Chemistry and Biochemistry and CIRES, Campus Box 215, University of Colorado, Boulder, CO 80309, USA
Received 16 February 2004; accepted 6 May 2004
Abstract
Thehigh-resolution(0.05 cm−1)spectraofgas-phaseH
using Fourier-transform infrared spectroscopy. The increased resolution of this work compared with previous studies has lead to an improved
vibrational analysis of H SO . This study has answered unresolved questions about combination bands and overlapping features from previous
gas-phase spectroscopic studies of H SO and marks the first experimental measurement of the ν and ν15 torsional vibrations in this molecule.
This work leads to a brief discussion on vibrational mode mixing in sulfuric acid.
2004 Elsevier B.V. All rights reserved.
SO , HDSO , andD SO weremeasuredoverthefrequencyregion1200–10,000 cm
−1
2
4 4 2 4
2
4
2
4
8
©
Keywords: Sulfuric acid; Spectroscopy; Infrared; Combination bands; High-resolution
1
. Introduction
There are two major difficulties associated with obtain-
ing a vapor phase spectrum of H2SO4. The vapor pressure
of H2SO4 is very low [14] and spectra are complicated by
the presence of SO3 and H2O, which are in equilibrium
with H2SO4 and absorb radiation at similar energies. Conse-
quently this chromophore has been challenging for spectro-
scopic investigations. Recent experimental and theoretical
studies have done well in predicting vibrational frequencies
and intensities [1,15] and in examining the electronic spec-
troscopy of H2SO4 [1,16–18] but the vibrational spectrum
has remained a challenge. However, the difficulties can be
overcome, as shown in this paper, by increasing the vapor
pressure by working at elevated temperatures and by a care-
ful subtraction of SO3 and H2O reference spectra.
The fundamental spectroscopic properties of vapor phase
H2SO4 are required to evaluate the role of this atmospheric
chromophore. Previous low-resolution studies [1–4] have
succeeded in identifying IR and near-IR transitions and in
obtaining relative intensities for those transitions. However,
a high-resolution study was needed in order to address un-
resolved issues, such as unassigned combination bands and
overlapping transitions, in the vibrational spectrum. Vapor
phase H2SO4 has been shown to be an important atmo-
spheric system in the context of vibrationally mediated pho-
tochemistry [5]. The overtone-induced dehydration in the
stratosphere has been identified as an important process [5]
in generating the observed springtime sulfate layer [6–8].
Previous modeling studies had not considered this process
to be important [9–13]. Further spectroscopic studies on this
molecule can help to elucidate the mode coupling that is
necessary for this photodehydration reaction.
Structures of H2SO4, HDSO4, and D2SO4 have been
identified previously using microwave spectroscopy [19].
The structure of H2SO4 was found to have C2 symmetry.
The four oxygen atoms are arranged in a near tetrahedral
shape around the sulfur. The hydrogen atoms are on op-
posite sides of the S–O2 plane. The microwave study [19]
−1
revealed that the C2 structure was within 0.5 kcal mol of
the lowest energy structure if that conformation was not the
lowest energy structure itself. The inertial c-axis was found
to coincide with the C2 rotation axis. For a particular vi-
brational mode, understanding the axis on which the dipole
∗
Corresponding author. Tel.: +1 303 492 8605; fax: +1 303 492 5894.
E-mail addresses: paul.hintze-1@ksc.nasa.gov (P.E. Hintze),
feierabe@colorado.edu (K.J. Feierabend), havey@colorado.edu
(D.K. Havey), vaida@colorado.edu (V. Vaida).
1
386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.05.006

5
60
P.E. Hintze et al. / Spectrochimica Acta Part A 61 (2005) 559–566
◦
moment oscillates can help to predict the shape of the band
and guide vibrational analysis.
A previous study reported the IR spectrum of gas phase
H2SO4 and D2SO4 [2]. This study did not report any
frequencies for HDSO4 and was done at a lower resolu-
tion than what is presented here. Another previous study
peratures of 130–150 C were required to give a measurable
concentration of H2SO4. However, at temperatures above
◦
150 C the reaction shifts to the reactants and reduces the
amount of sulfuric acid vapor that is present.
SO3 was acquired by distillation from fuming sulfuric acid
(∼25% SO3 in H2SO4). The IR spectrum of the distilled
SO3 showed no impurities of H2O (<0.001 vol.%) or SO2
(<0.01 vol.%). H2O was doubly distilled. The samples were
kept in glass containers that allowed the flow of a carrier gas
to pass over the sample or bypass the sample completely.
The carrier gas flows were kept constant even if one of the
reactants was left out to ensure that the temperature inside
the cell was constant. The amount of SO3 and H2O intro-
duced into the cell was maintained by keeping the sample
in a controlled-temperature bath and was varied to generate
different concentrations of H2SO4.
[15] reported the frequencies for many vibrational modes
of H2SO4 and some of its isotopomers in an Ar matrix.
They also reported vibrational frequencies calculated at the
MP2/6-311++G(2d,2p) level and results of a normal mode
analysis for each isotopomer. These results were used as an
aid in interpreting the gas phase spectra in this work where
applicable. The work presented here has allowed for a more
accurate assignment of some vibrational modes, the assign-
ment of combination bands, and vibrational assignments of
the near-IR spectrum for HDSO4 where possible.
The motivation for measuring the high resolution
0.05 cm ) IR spectrum of H2SO4 was to reinvestigate the
Light from the spectrometer was focused through the cell
with a path length of 100 cm. Spectra were recorded in the
range of 1200–10,000 cm with a resolution of 0.05 cm .
−
1
(
−
1
−1
vibrational spectrum, with a focus on assigning overlapping
features observed in low-resolution work [1] and to look at
combination bands across the near-IR. The low resolution
A quartz halogen source, InSb detector, and CaF2 beam-
splitter were used in this region. Six hundred scans were
coadded per spectrum and five spectra collected on different
days were averaged to give the data used in this work.
All bands reported in this work have been shown to have a
direct linear dependence on H2SO4 partial pressure. If small
H2SO4/H2O clusters had been present then their vibrational
bands would have been dependant on H2O partial pressure.
If any of the H2SO4 dimer was present then one would
have expected a nonlinear correlation between band intensity
and H2SO4 partial pressure. Analysis of the trends in the
intensity of vibrational bands against relative H2SO4 and
H2O partial pressures confirmed that all observed bands are
due to molecular sulfuric acid and not clusters or hydrates
of sulfuric acid.
−1
(
1 cm ) spectra reported previously [1] showed that there
were a number of bands that had rotational structure or
features that could not be fully characterized at that resolu-
tion. The high-resolution spectra in this work have allowed
for new and more accurate assignments as well as better
interpretation of the combination bands. This has yielded
information about coupling between vibrational modes that
may affect the dynamics of excited vibrational states. To aid
in identifying and interpreting the vibrational transitions the
spectra of HDSO4 and D2SO4 were taken and the results
are presented in what follows.
2
. Experiment
The absorption spectrum of H2SO4 vapor was measured
3. Results and discussion
−
1
in the IR/near-IR from 1200 to 10,000 cm with a BOMEM
DA3.002 Fourier-transform spectrometer (FTS). An optical
flow cell containing the sample has been described previ-
ously [1,16]. The cell was wrapped with copper foil, heating
tape, insulation, and a layer of aluminum foil. Thermocou-
ple gauges were placed on the cell walls between the glass
or copper foil and the heating tape to monitor the external
temperature. The temperature of the gas inside the cell was
measured by inserting a thermocouple into the cell while
gases were flowing through at conditions similar to those
used in the experiment. The cell utilized wedged CaF2 win-
dows to prevent internal reflections so that spectra could be
3.1. Interpretation of H SO , HDSO , and
2
4
4
D SO spectra
2
4
H SO has three different rotational constants and is an
2
4
asymmetric top. Asymmetric tops are characterized by the
asymmetry parameter κ, which depends on the three rota-
tional constants A, B, and C:
2
B − A − C
A − C
κ =
(1)
κ ranges from −1 for a prolate symmetric top to +1 for an
oblate symmetric top. The asymmetry parameter for H2SO4
is 0.03 and indicates that it does not approach any of the lim-
iting cases. Although the rotational lines of strongly asym-
metric molecules are generally irregular, spacings of 2C
and 2A may be seen in the P and R branches, respectively
[20]. The rotational constants and asymmetry parameters of
H2SO4, HDSO4, and D2SO4 are given in Table 1.
−
1
obtained at 0.05 cm resolution.
Gas phase H2SO4 was generated in a 1:1 reaction with
SO3 and H2O by titrating SO3 with H2O through a heated
side arm of a temperature controlled cell [1,16]. D2SO4 and
HDSO4 were made with the same procedure but substituted
D2O or a 1:1 mixture of H2O:D2O for pure H2O. The tem-
perature determined the vapor pressure of H2SO4. High tem-

P.E. Hintze et al. / Spectrochimica Acta Part A 61 (2005) 559–566
561
Table 1
Table 3
Rotational constants (cm 1) and asymmetry parameters, κ, for H2SO4
−
Observed transitions in H2SO4 and D2SO4
and its isotopomers [19]
ν (cm 1)
−
Symmetry
Band
H2SO4
HDSO4
D2SO4
H2SO4
D2SO4
A
B
C
κ
0.172
0.167
0.163
0.03
0.171
0.161
0.158
−0.46
0.168
0.155
0.154
−0.82
ν2
1222.2 (0.7)
1465.2
2278 (0.5)
2676.8
3328.5
3393.9
3609.6
3825.1
3890.3
4736 (5)
4760.8
1224.3 (0.7)
1452 (2)
A
B
A
B
ν10
2ν11
ν2 + ν10
ν9 − ν15
ν9 − ν8
ν9
B ← B
B ← A
2661.6
5240.2
B
B
A
B
A
A
Normal mode vibrations are by definition harmonic in na-
ν9 + ν8
ν9 + ν15
ν9 + ν3
ν9 + ν11
2ν9
ture. A summary of the normal vibrational modes in H2SO4
is shown in Table 2. When anharmonic effects need to
be considered, stretching vibrations are typically treated as
Morse oscillators. This allows for the vibrations to be fit to
an anharmonic potential and has proven to be good model for
overtone vibrations [1]. However, when dealing with com-
bination bands a further level of complexity is needed that
includes quartic terms in the potential energy. Neglecting
anharmonicity, combination bands have a frequency that is
equal to the sum of the fundamental vibrations. The equation
7060.6
−
1
Uncertainties in the frequency are 0.1 cm
parentheses.
unless otherwise noted in
larger intensities and greater anharmonic cross terms than
those that do not.
The hydrogen atoms of H2SO4 were substituted with deu-
terium to aid in identifying unknown features. When this
substitution is made the fundamental OD stretch shifts rel-
ꢁ
ꢂ
ꢀ
1
2
G(υ1, υ2, υ3, . . . ) =
ωi υi +
i
ative to the OH stretch in a predictable manner given by:
ꢁ
ꢂ ꢁ
ꢂ
ꢀ
ꢀ
ꢃ
1
2
1
2
ωOH
ωOD
µOD
µOH
+ i
x
υi +
υk +
ik
=
= 1.37
(3)
k
(2)
To calculate the shift in an overtone level the shift in the
anharmonicity must also be known.
introduces anharmonic constants, xik, that are a function of
the two vibrational levels [20]. The experimental frequency
of combination bands is less than the sum of the fundamen-
tal vibrations by an amount dependent on xik. The differ-
ence between the experimental frequency and the sum of
the fundamental frequencies is a measure of the anharmonic
cross term between the two vibrations. This difference can
be thought of as a measure of the coupling between the two
modes. Modes that couple well together will typically have
ωxOH
ωxOD
µOD
µOH
=
= 1.89
(4)
Modes such as S=O2 stretches will not be greatly affected
by this isotopic substitution. Note that the harmonic and an-
harmonic constants in these equations are for Morse oscil-
lator energy levels.
For reference, summaries of all observed transitions for
H2SO4, D2SO4, and HDSO4 are given in Tables 3 and 4.
Table 2
3.2. The ν O–H asymmetric stretch and related bands
9
Notation and description of the vibrational modes in H2SO4
Symmetry
Mode
Description
The ν9 O–H asymmetric stretch of H2SO4 has a Q branch
−
1
centered at 3609.6 cm . Previous lower resolution studies
a
ν1
ν2
ν3
ν4
ν5
ν6
ν7
ν8
OH symmetric stretch
S=O2 symmetric stretch
SOH symmetric bend
S–O2 symmetric stretch
S=O2 bend
O–S=O twist
O–S=O bend
−1
showed the Q branch centered at 3609.2 [1] and 3610 cm
Table 4
Observed transitions of HDSO4
−1
Mode
Description
νa (cm
)
Symmetric torsion
ν4
S=O2 symmetric stretch
=
S O2 asymmetric stretch
OD stretch
1223.5 (0.7)
1463 (3)
2663.9
b
ν9
OH asymmetric stretch
S=O2 asymmetric stretch
SOH asymmetric bend
S–O2 asymmetric stretch
O–S=O rock
S=O2 wag
Asymmetric torsion
ν3
ν10
ν11
ν12
ν13
ν14
ν15
ν2
ν1
OH stretch
3611.3
2ν2
2ν1
Overtone of OD stretch
Overtone of OH stretch
5237.5 (1)
7059.6 (1)
−
1
Uncertainties in the frequency are 0.1 cm
parentheses.
unless otherwise noted in

5
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P.E. Hintze et al. / Spectrochimica Acta Part A 61 (2005) 559–566
Fig. 1. An enlargement of the ν9 transition in H2SO4. H2O has been subtracted from the spectrum.
[2,3]. This work reveals that the band has a progression of
The overall appearance of the ν9 band in D2SO4, shown
evenly spaced clusters of rotational lines as seen in Fig. 1.
in Fig. 3, is similar to that in H2SO4. However, the Q
branch at 2661.6 cm is less prominent and visibly wider.
−
1
−1
The spacing is 0.32 ± 0.2 cm and is approximately equal
to twice the ground state rotational constant. The presence
of a prominent Q branch and an apparent change in dipole
along the a inertial axis suggests that ν9 is primarily an
A-type transition. Spectra of the ν9 O–H asymmetric stretch
of H2SO4 and the corresponding O–H stretch of HDSO4 are
shown in Fig. 2.
The width of the Q branch is a measure of the difference
between the ground and excited state rotational constants.
The increased width of the Q branch in D2SO4 is expected
since the rotational constants will change more when the
OD bonds stretch than when the OH bonds stretch. The
Q branch appears to have two shoulders at slightly higher
energies. These shoulders do not have the same appear-
ance as the features in the R branch of H2SO4. The po-
sitions of the two shoulders have been assigned to 2663.9
The most interesting aspect of the ν9 band is the presence
of a series of at least three features of significant intensity
in its R branch. The most obvious features appear at 3613.2,
−
1
−1
3
615.9, and 3620.2 cm . This work assigns these features
and 2666.2 cm . Assuming the features in the R branch of
as being caused by different rotamers of H2SO4. Evidence
for this comes from the results of the D2SO4 study and will
be discussed below.
H2SO4 are rotamers of sulfuric acid, the transitions should
shift upon deuteration by the same amount as the transi-
tion in the C2 conformation. The highest energy feature
Fig. 2. The ν9 band of H2SO4 and the O–H stretch of HDSO4. H2O has been subtracted from both spectra.

P.E. Hintze et al. / Spectrochimica Acta Part A 61 (2005) 559–566
563
Fig. 3. The ν9 band of D2SO4 and the O–D stretch of HDSO4. D2O has been subtracted from both spectra.
−
1
3
.2.1. The first overtone of the ν9 O–H asymmetric stretch
on the R branch of H2SO4 is 3620.2 cm and would fall
−
1
The first overtone of the ν9 O–H asymmetric stretch of
at 2668.1 cm in D2SO4, which is close to the observed
shoulders. All of these features from the different rotamers
would be very close in energy and harder to resolve in the
deuterated compound. The proposition that the features seen
in H2SO4 are caused by rotamers of sulfuric acid would be
consistent with the spectrum of D2SO4. D2SO4 does not
show the same rotational structure as H2SO4, even though
the signal to noise ratio is high enough to allow it if it were
present.
The ratio of the frequencies of the ν9 mode in
H2SO4 to D2SO4 is 1.36 and is consistent with a gen-
eral knowledge for OH-containing molecules. Accord-
ing to Eq. (3), the harmonic frequency should change
by 1.37 in very good agreement with the experimental
value.
−
1
H2SO4 occurs at 7060.6 cm and can be seen in Fig. 4.
The Q branch for 2ν9 is significantly broader than it is
for ν9 and the bandshape has a different appearance with
a pronounced maximum in the R branch at 7073.7 cm
The features that were observed in the R branch of ν9
are not present in 2ν9. The origin of the 2ν9 transition
for D2SO4 occurs at 5240.2 cm . Again the Q branch
broadened but the bandshape in this case is very similar
to ν9 of D2SO4. The H2SO4/D2SO4 ratio of the peak po-
sitions of this mode is 1.35 and is very close to the ratio
for the fundamental ν9 region. From combination differ-
ences, the frequency and anharmonicity of this mode in
−1
.
−
1
−1
D2SO4 were calculated to be 2744.6 and 41.5 cm , re-
spectively. The OH/OD stretches in HDSO4 were shifted
Fig. 4. The 2ν9 O–H overtone transitions of H2SO4 and HDSO4. H2O has been subtracted from both spectra.

5
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P.E. Hintze et al. / Spectrochimica Acta Part A 61 (2005) 559–566
Fig. 5. The ν9 + ν3 and ν9 + ν11 combination bands of H2SO4.
to lower energy than the respective stretches in the pure
isotopomers.
acteristic doublet structure while the fourth transition (ν9
+ ν ) may have interference from H2O. In each case the
1
5
−
1
doublets are split by about 4 cm . Although the ν8 and ν
1
5
3
.2.2. Combination bands involving the ν9 O–H
fundamental transitions have not been observed in the gas
phase, their energies are assigned here using the combina-
tion bands. The symmetric ν8 mode is calculated to be lower
in energy than the asymmetric ν15 mode [15]. To unambigu-
ously assign these modes the difference between all maxima
asymmetric stretch
The ν9 O–H asymmetric stretch of H2SO4 is seen in
combination with many of the other fundamental normal
modes. The most intense of these combination bands is ob-
served at 4739 cm . Previous low-resolution work [1] had
assigned the band in this region to either ν9 + ν3 sym-
metric SOH bend or ν9 + ν11 asymmetric SOH bend. This
high-resolution study has revealed that there are actually two
transitions occurring in this region. Fig. 5 shows a shoulder at
−
1
−1
and the ν9 transition at 3609.6 cm was checked. The lower
energy maxima in two of the doublets were almost exactly
−1
equidistant to the ν9 mode, 215.7 and 215.5 cm for the
difference and combination band, respectively. The energies
of the three transitions were calculated in terms of the har-
monic and anharmonic constants. Coupling constants to the
other modes all cancel out. The difference between the ν9
fundamental and the ν9 − ν8 difference band is exactly the
frequency of ν8. Since the anharmonic constant is negative
the difference between the ν9 + ν8 combination band and
the ν9 mode will be slightly less than the frequency of ν8.
The energy of the ν8 torsional mode was found to be 215.7 ±
−1
−1
4
762 cm to the high-energy side of the peak at 4739 cm .
−
1
The main peak at 4739 cm is assigned to ν9 + ν3 and the
shoulder is assigned to ν9 + ν11. These assignments are jus-
tified by examining the frequencies and symmetries of the
ν3 and ν11 fundamental modes. In the infrared, the ν11 SOH
asymmetric bend occurs at 1157.1 cm [1] while the ν3
SOH symmetric bend occurs at approximately 1140 cm
−
1
−
1
−
0.2 cm . The energy of the ν torsional mode was found to
15
1
[
2,3]. The ν9 + ν11 transition is totally symmetric and the
−
1
ν9 + ν3 transition is totally asymmetric. Asymmetric tran-
sitions generally have a greater change in dipole moment
and therefore a higher infrared intensity. These arguments
suggest that ν9 + ν11 will appear at higher frequency and
lower intensity than ν9 + ν3. Given that the dipole change
of ν9 + ν3 is primarily along the b inertial axis, the “double
hump” bandshape of this combination band is attributed to
a B-type transition.
be 281.1 ± 0.2 cm using the same arguments for the com-
bination and difference bands involving ν . This is the first
15
experimental measurement of the torsional mode vibrations
in gas phase H2SO4. The two assigned energies for ν8 and
ν
compare as expected with the calculated gas phase en-
5
1
−
1
−1
ergies of these modes (ν8 = 226 cm , ν15 = 288 cm ) as
−1
well as the values observed in matrix work (ν8 = 208 cm
,
−
1
ν
= 187.4 cm ) [15].
15
The ν8 and ν15 torsional modes are also seen in combi-
nation with the ν9 O–H asymmetric stretch. The difference
3.3. The ν and ν S=O symmetric and asymmetric
2
10
2
−
1
band of ν9 − ν8 was observed at 3393.9 cm and the dif-
stretches and related bands
−
1
ference band of ν9 − ν was observed at 3328.5 cm . The
15
−
1
combination band of ν9 + ν8 was observed at 3829.1 cm
and the combination band of ν9 + ν was observed at
The ν symmetric S=O stretch for each isotopomer is
2
2
1
5
shown in Fig. 6. The overall band shape is that of nearly
symmetric P and R branches and a relatively broad Q branch.
−
1
3
890.3 cm . Three of the four transitions showed a char-

P.E. Hintze et al. / Spectrochimica Acta Part A 61 (2005) 559–566
565
Fig. 6. The ν2 bands of H2SO4, D2SO4, and HDSO4.
the Q branch is at 1465.2 cm 1. There are some secondary
maxima in the P branch in H2SO4. It is unclear if these sec-
ondary maxima or the shoulder on the P branch originate
from different transitions or are simply caused by the inten-
sity of the rotational lines.
−
The rotational contours of all isotopomers are similar. There
was very little change in the energy of this transition be-
tween the three isotopomers with H2SO4 having the lowest
energy and D2SO4 having the highest. This indicates that
deuteration does not drastically affect this transition. For all
isotopomers, the exact position of the Q branch was not
assigned. Unlike other transitions of this molecule, the Q
branch did not have a sharp onset but rather a rounded peak.
The ν10 asymmetric S=O2 stretch is shown in Fig. 7. This
transition is characterized by a more intense P branch than
Q branch. The less intense Q branch could be an indication
that this transition is primarily a B-type transition. The in-
tense P branch is typical of asymmetric modes in which the
rotational constants for the excited states differ greatly from
those of the ground state, as would be expected in this tran-
sition. The P branches of the pure isotopomers also have a
broad shoulder at lower energy. The frequency of this shoul-
der is highest for H2SO4 and lowest for D2SO4. In H2SO4,
The Q branch of the ␯10 asymmetric S O2 stretch in
D2SO4 has a maximum intensity less than that of the P
branch. In fact, the Q branch is not very well defined at
all. The asymmetry parameter of D2SO4 indicates that it
is much closer to being a symmetric top than either of the
other isotopomers. Thus, it is not unusual to expect that the
rotational bands would have a different appearance.
For both the ν2 and ν10 transitions, it is difficult to analyze
the HDSO4 spectrum, but the Q branch of the ν10 transition
has been assigned to 1460.9 cm . The HDSO4 spectrum
contains features of both of the other isotopomers as well
as lines from the HDO bending mode, which have not been
subtracted.
=
−
1
Fig. 7. The ν10 bands of H2SO4, D2SO4, and HDSO4. H2O has been subtracted from the H2SO4 spectrum.

5
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P.E. Hintze et al. / Spectrochimica Acta Part A 61 (2005) 559–566
3
.3.1. Combination bands involving the ν2 and ν10 S=O2
Features in the spectra, especially in the ν9 region, sug-
symmetric and asymmetric stretches
gest that different rotamers of H2SO4 may be significantly
−
1
◦
In the 1 cm resolution studies of previous work [1] a
populated at 150 C. The presence of these low-intensity
−
1
band at 2665 cm was assigned to the ν2 + ν10 combina-
features shifted to slightly higher energies than the ν9 O–H
stretch suggests that there is more than one stable geometry
for sulfuric acid. This hypothesis is supported by the shoul-
ders observed in measured spectra of D2SO4. In addition,
unexplained features observed in the ν10 band may also arise
from contributions from different rotamers. If different ro-
tamers are contributing to the spectra of H2SO4 and D2SO4
one would expect them to be observed mostly near the most
intense bands of ν10 and ν9. Previous microwave work [19]
did not obtain more than one minimum energy configura-
tion for H2SO4. However, the study was not able to rule out
the possibility that other stable configurations could exist.
−
1
tion band. The 0.05 cm resolution spectrum of this band
has allowed for a more detailed analysis. There is a lo-
−
1
cal maximum at 2676 cm although the center frequency
of the band is at a slightly lower energy. After measuring
the spectrum of HDSO4 and D2SO4 it was observed that
ν2 + ν10 overlaps with the OD stretch in each of those
molecules. The natural abundance of deuterium could be
sufficient to account for the formation of some HDSO4
−
1
and a peak at 2664 cm . The band shows features sim-
−
1
ilar to HDSO4. The local maximum at 2676 cm is not
seen in either HDSO4 or D2SO4 suggesting it is a tran-
sition of H2SO4. The energy of the ν2 + ν10 transition
−
1
is 2684 cm when anharmonicity is neglected. This sup-
−
1
ports the assignment of the band at 2676 cm to ν2 + ν10
since the experimental frequency is slightly less than this
value.
Acknowledgements
P.E.H. would like to thank CIRES for a graduate fellow-
ship. V.V. thanks NSF for funding. K.F. thanks NASA ESS
division for fellowship funding. D.H. is thankful for being
funded by a Sewall fellowship.
3
.4. Other features
Previous low-resolution studies of H2SO4 [1] assigned a
−
1
broad peak at 2278 cm to either 2ν3, 2ν11, or ν2 + ν10.
This work has identified this feature as primarily 2ν11.
High-resolution spectra show that this broad peak has a
bandshape indicative of a single transition. In addition,
spectra of HDSO4 contain this transition while spectra of
D2SO4 do not. This confirms that the transition must be the
overtone of the SOH bending mode and therefore assigned
as 2ν11.
References
[1] P.E. Hintze, H.G. Kjaergaard, V. Vaida, J.B. Burkholder, J. Phys.
Chem. A 107 (2003) 1112.
[
[
2] S.M. Chackalackal, F.E. Stafford, J. Am. Chem. Soc. 88 (1966) 723.
3] K. Stopperka, F. Kilz, Z. Anorg. Allg. Chem. 370 (1969) 49.
[4] R.F. Majkowski, R.J. Blint, J.C. Hill, Appl. Opt. 17 (1978) 975.
[5] V. Vaida, H.G. Kjaergaard, P.E. Hintze, D.J. Donaldson, Science 299
(2003) 1566.
[
[
[
[
6] D.J. Hofmann, J.M. Rosen, Geophys. Res. Lett. 12 (1985) 13.
7] D.J. Hofmann, Rev. Geophys. 26 (1988) 113.
8] D.J. Hofmann, Geophys. Res. Lett. 17 (1990) 357.
9] M.J. Mills, O.B. Toon, S. Solomon, Geophys. Res. Lett. 26 (1999)
4
. Conclusions
1
133.
The high-resolution spectroscopic study presented in
[
10] D.K. Weisenstein, G.K. Yue, M.K.W. Ko, N.D. Sze, J.M. Rodriguez,
C.J. Scott, J. Geophys. Res.-Atmos. 102 (1997) 13019.
11] C.P. Rinsland, M.R. Gunson, M.K.W. Ko, D.W. Weisenstein, R.
Zander, M.C. Abrams, A. Goldman, N.D. Sze, G.K. Yue, Geophys.
Res. Lett. 22 (1995) 1109.
this work reexamines the vibrational spectrum of sulfuric
acid and has answered unresolved questions from previous
low-resolution work [1] concerning overlapping transitions.
High-resolution spectra allowed the assignment of the band
[
−
1
[12] J.X. Zhao, O.B. Toon, R.P. Turco, J. Geophys. Res.-Atmos. 100
1995) 5215.
at 2278 cm as being primarily 2ν11 and were essential
in separating ν9 + ν11 from ν9 + ν3. These assignments
build on the fundamental spectroscopic knowledge base
established in previous work [1–4,15].
(
[
13] S. Bekki, M.P. Chipperfield, J.A. Pyle, J.J. Remedios, S.E. Smith,
R.G. Grainger, A. Lambert, J.B. Kumer, J.L. Mergenthaler, J. Geo-
phys. Res.-Atmos. 102 (1997) 8977.
Many combination bands were observed over the region
200–10,000 cm . Of notable interest are the combination
[14] G.P. Ayers, R.W. Gillett, J.L. Gras, Geophys. Res. Lett. 7 (1980) 433.
[15] A. Givan, L.A. Larsen, A. Loewenschuss, C.J. Nielsen, J. Mol. Struct.
−
1
1
5
09 (1999) 35.
16] J.B. Burkholder, M. Mills, S. McKeen, Geophys. Res. Lett. 27 (2000)
493.
bands of ν9 with ν3, ν8, ν11, and ν . All of these combi-
15
[
nations occur with observable intensity which suggests that
mode coupling with the O–H stretch is important in H2SO4.
Using these combination bands the energies of the ν8 and
2
[17] T.W. Robinson, D.P. Schofield, H.G. Kjaergaard, J. Chem. Phys. 118
(2003) 7226.
[18] S.J. Wrenn, L.J. Butler, G.A. Rowland, C.J.H. Knox, L.F. Phillips,
J. Photochem. Photobiol. A-Chem. 129 (1999) 101.
ν
torsional were extracted. This is the first measurement of
5
1
these energies for gas phase H2SO4. The torsional frequen-
cies are in good agreement with gas phase calculations and,
as expected, are higher than those measured in low temper-
ature matrices [15].
[19] R.L. Kuczkowski, R.D. Suenram, F.J. Lovas, J. Am. Chem. Soc. 103
(1981) 2561.
[20] G. Herzberg, Molecular Spectra and Molecular Structure, Van Nos-
trand, New York, 1950.