Experimental study of the heterogeneous interaction of SO3 and H2O: Formation of condensed phase molecular sulfuric acid hydrates

Article abstract of DOI:10.1021/ja0210704

The interaction of SO₃ and H₂O at low temperatures upon an inert surface has been studied with infrared spectroscopy and compared to the predictions of recent computational studies. At low temperatures and low water partial pressures, amorphous deposits of molecular H₂SO₄ complexed with variable amounts of H₂O in a ratio of between 1:1 and 2:1 are formed. Upon annealing, this material ejects water and converts first to a 1:1 H₂SO₄·H₂O complex and subsequently to anhydrous H2SO4. Adding water to the amorphous molecular hydrate results in the formation of a new species, which on the basis of its thermal behavior and by comparison to theoretical predictions can be attributed to a molecular polymer with a repeat unit of (H₂SO₄·(H₂ O)₂)_n. Implications of these observations for the initial stages of the formation of sulfate aerosol in the atmosphere and their surface reactivity are discussed.

Full text of DOI:10.1021/ja0210704

Published on Web 01/25/2003
Experimental Study of the Heterogeneous Interaction of SO3
and H2O: Formation of Condensed Phase Molecular Sulfuric
Acid Hydrates
Suzanne B. Couling, K. Jessica Sully, and Andrew B. Horn*,†
Contribution from the Department of Chemistry, UniVersity of York, Heslington, York,
YO10 5DD, United Kingdom
Received August 12, 2002 ; E-mail: andrew.b.horn@man.ac.uk
Abstract: The interaction of SO3 and H2O at low temperatures upon an inert surface has been studied
with infrared spectroscopy and compared to the predictions of recent computational studies. At low
temperatures and low water partial pressures, amorphous deposits of molecular H2SO4 complexed with
variable amounts of H2O in a ratio of between 1:1 and 2:1 are formed. Upon annealing, this material ejects
water and converts first to a 1:1 H2SO4‚H2O complex and subsequently to anhydrous H2SO4. Adding water
to the amorphous molecular hydrate results in the formation of a new species, which on the basis of its
thermal behavior and by comparison to theoretical predictions can be attributed to a molecular polymer
with a repeat unit of (H2SO4‚(H2O)2)n. Implications of these observations for the initial stages of the formation
of sulfate aerosol in the atmosphere and their surface reactivity are discussed.
Introduction
database of vibrational spectroscopic fingerprints obtained under
appropriate conditions exists, the presence and concentration
Considerable interest has developed in the past decade on
the role played by sulfuric acid in the chemistry and physics of
the atmosphere and a range of experimental methods have been
applied to develop a deeper understanding of the formation,
stability, optical, and chemical properties of sulfuric acid
aerosol.1 However, there is still considerable uncertainty
concerning the detailed physical chemistry of the acid at low
temperatures and water partial pressures, particularly in the
fundamental stages of the formation of atmospheric sulfate
aerosols from the hydration of precursors such as SO2 and SO3.
This paper addresses aspects of the low-temperature chemistry
of sulfuric acid and compares experimental results to the
predictions of recent computational studies. Spectroscopic
evidence is presented of the structure and stability of molecular
H2SO4 hydrates and their potential role as intermediates in
atmospheric chemistry is discussed.
of these ionic species can often be extracted. However, this
process requires that each species shows a strongly characteristic
pattern of absorption bands and is frequently complicated by
the fact that the individual absorption features tend to vary in
a complex way with temperature, environment and physical
state. The bulk phase diagram is also well-understood and shows
a variety of both stable and metastable compositions.3 Of
potential interest in the atmosphere are the stable monohydrate
(SAM) and the tetrahydrate (SAT), whereas the dihydrate and
trihydrate are metastable states and not currently thought to be
significant components of atmospheric aerosol. It has been
suggested that the octahydrate may be a high relative humidity
form present in the stratosphere,4 although this phase has only
been observed in the laboratory in aerosol droplets which have
been cooled to 166 K, significantly lower that that experienced
in the stratosphere. Adding to the solid phase complexity, liquid
sulfuric acid aerosols are known to strongly supercool4 and are
observed to show a strongly temperature-dependent affinity for
water vapor and gaseous nitric and hydrochloric acids.5
Spectroscopic and other methods applied to bulk samples of
sulfuric acid/water mixtures have shown that condensed thin
films exist as a range of hydrates, with compositions dependent
on the temperature and partial pressure of water vapor.2 These
In the atmosphere, sulfuric acid is believed to be formed from
the hydration of SO2 and SO3. The gas-phase formation of H2-
SO4 has been extensively studied experimentally and theoreti-
hydrates are ionic in nature, containing H3O+, H5O2+, HSO4
,
-
and SO42- and in many cases readily interconvert upon variation
of ambient conditions. In situations for which a comprehensive
(2) (a) Tomikawa, K.; Kanno, H. J. Phys. Chem. A 1998, 102, 6082. (b) Horn
A. B.; Sully, K. J. Phys. Chem. Chem. Phys. 1999, 1, 3801. (c) Max, J.-J.;
Menichelli, C.; Chapados, C. J. Phys. Chem. A 2000, 104, 2845. (d) Nash,
K. L.; Sully, K. J.; Horn, A. B. Phys. Chem. Chem. Phys. 2000, 2, 2933
(e) Guldan, E. D.; Schindler, L. R.; Roberts, J. T. J. Phys. Chem. 1995,
99, 16 059.
(3) Gable, C. M.; Betz, H. F.; Maron, S. H. J. Am. Chem. Soc. 1950, 72, 1445.
(4) Krieger, U.K.; Colberg, C. A.; Weers, U.; Koop, T.; Peter, Th. Geophys.
Res. Letts 2000, 27, 2097.
† Now at Department of Chemistry, University of Manchester, Oxford
Road, Manchester M13 9PL, United Kingdom.
(1) See for example: (a) Hanson, D. A.; Ravishankara A. R.; Solomon, S. J.
Geophys. Res. 1994, 99, 3615. (b) Hanson D. R.; Ravishankara, A. R. J.
Phys. Chem. 1993, 97, 12309. (c) Clegg, S. L.; Brimblecombe, P.; Wexler,
A. S. http://mae.ucdavis.edu/wexler/aim.htm (d) Waschewsky, G. C. G.;
Abbatt, J. P. D. J. Phys. Chem. A 1999, 103, 5312. (e) Bierman, U. M.;
Luo, B. P.; Peter, Th. J. Phys. Chem. A 2000, 104, 783 (f) Morris, J. R.;
Behr, P.; Antman, M. D.; Ringeisen, B. R.; Splan, J.; Nathanson, G. M. J.
Phys. Chem. A 2000, 104, 6738. (g) Martin, S. T.; Salcedo, D.; Molina, L.
T.; Molina, M. J. J. Phys. Chem. B. 1997, 101, 5307.
(5) Finlayson-Pitts, B. J.; Pitts, J. N. Chemistry of the Upper and Lower
Atmosphere: Theory, Experiments and Applications; Academic Press: San
Diego, CA 2000.
9
1994
J. AM. CHEM. SOC. 2003, 125, 1994-2003
10.1021/ja0210704 CCC: $25.00 © 2003 American Chemical Society

Identification of Molecular Sulfuric Acid Hydrates
A R T I C L E S
cally.6,7 Although there are some discrepancies, there is
reasonable agreement in the literature as to the nature of this
process. Computational studies suggest that the reaction of SO3
with one water molecule has an activation barrier of about 28
kcal mol-1, with the high energy barrier being due to a sterically
hindered four centered transition state. The introduction of a
second water molecule reduces this energy barrier to about 13
kcal mol-1 through the formation of a six centered transition
state. Additional water molecules continue to reduce the barrier
either by acting as microsolvent stabilizing the transition state
or by allowing alternative reaction mechanisms (i.e., stepwise
rather than concerted proton transfers). The presence of 10 or
more water molecules effectively removes the barrier for the
reaction. These predictions are supported by experimental work.
For example, the gas-phase SO3 + H2O reaction has been shown
experimentally by Jayne et al to be second order with respect
to the partial pressure of water and to have a strong negative
temperature dependence.7 In the pressure range from 133 to
1000 mbar and temperature range from 283 to 370 K, Jayne et
al observed Arrhenius behavior with A ) 3.90 × 10-41 cm-6
molecule-2 s-1 and Ea ) -13.5 kcal mol-1 for the reaction.
However, at lower temperatures (243-268 K) and at SO3
densities of ∼1012 molecules cm-3, very fast decay rates showed
a trend to first-order dependence in water vapor pressures, which
may represent an efficient heterogeneous reaction between SO3
and acid water particles at reduced temperature.
Theoretical studies predict reaction products which involve
molecular sulfuric acid complexed with varying amounts of
hydrogen bonded water in the gas phase. This behavior is
confirmed in molecular beam experiments, with the principal
product being an un-ionized monohydrate complex for which
detailed structural parameters have been obtained.8 In order for
the ionic species observed in condensed films and aerosols to
be formed, these complexes must further hydrate and coalesce
during the formation of an aerosol particle. In this paper,
spectroscopic observations of the molecular products of the
heterogeneous hydration of SO3 collected on a cold ATR
element are presented, along with an analysis of the effects of
temperature and water partial pressure upon the subsequent
formation of intermediates and ionic hydrates.
valves with internal dosing tubes are directed at the open faced
of the IRE; this enables co-dosing of reactive gases. The gases
for dosing are prepared in separate gas lines to prevent pre-
reaction, and films are deposited by simultaneous dosing of SO3
and H2O (or D2O). SO3 gas is prepared by sublimation of solid
SO3 (Aldrich, U.K.) into a preconditioned, evacuated gas line.
Deionized H2O or D2O (Aldrich, U.K.) is vacuum distilled and
degassed by repeated freeze/pump/thaw cycles.
SO3 and H2O are co-dosed on to the IRE between 190 and
200 K, which is above the deposition temperature for either ice
or SO3 at partial pressures of less than 10-5 mbar. The partial
pressures of the reacting gases are monitored during deposition
by a nude ion gauge and a residual gas analyzer. Calibration of
the absolute pressures at the sample IRE face is performed by
measuring the frost point of water vapor at a set temperature.
The local pressures at the surface of the IRE are calculated from
a knowledge of the relationship between the measured chamber
pressure and the local pressure at the exit points of the dosing
tubes.
Results
Formation and Structure of Amorphous Sulfuric Acid
Films. At 190 K, neither SO3 nor H2O form stable single
component films of appreciable thickness at partial pressures
of less than 1 × 10-5 mbar. The deposition of material upon
the IRE surface must therefore be due to either a rapid gas-
phase reaction or a heterogeneous process with one or both
components adsorbed prior to reaction. Extrapolating the
reaction rate for H2O(g) + SO3(g) determined by Jayne et al.7
down to 180 K gives a very long half-life for the reaction, and
since the mean free path at pressures below 1 × 10-4 mbar is
of the order of meters there is not a significant probability of
the reaction occurring in the gas phase in the short distance
between the ends of the dosing tubes and the face of the IRE.
In their kinetic study, Jayne et al observed that at low
temperatures and at relatively high SO3 pressures (>∼1012
molecules cm-3) the reaction is very fast due to a heterogeneous
reaction at the surface of the acid-water film. Further evidence
for the likelihood of a heterogeneous process in these experi-
ments comes from an observation during sample preparation.
The Ge IREs used in this laboratory are cleaned prior to use by
etching in dilute HF. This renders the surface extremely
hydrophobic. Upon subsequent surface oxidation (either chemi-
cally or by standing in the atmosphere), the IRE becomes
hydrophilic in nature. In vacuo uptake of water is observed to
be retarded on the fresh hydrophobic surface, and the reaction
between SO3 and H2O does not occur at pressures below 1 ×
10-5 mbar. However, reaction is observed immediately upon
exposure above the hydrophilic surface and a film of 5 nm
thickness is deposited in ca. 5 min. This behavior is attributed
to the immobilization of a thin (possibly monolayer) film of
H2O by interaction between gaseous H2O and surface oxide and
hydroxide functionalities which initiates the heterogeneous
process. In these experiments, the hydrophilicity of the IRE
surface increases the lifetime of surface-bound H2O and permits
the SO3(g) + H2O(ads) reaction to be studied in detail.
Experimental Section
The experimental apparatus used in this study has been
described previously.9 Briefly, a germanium internal reflection
element (IRE) is held on a thermostated sample mount in a high
vacuum chamber. The temperature of the IRE, measured by a
K type thermocouple, is controlled by the combination of liquid
nitrogen cooling and resistive heating. Three high precision leak
(6) (a) Hofman, M.; Schleyer, P.von R. J. Am. Chem. Soc. 1994, 116, 4947.
(b) Morukuma, K.; Muguruma, C. J. Am. Chem. Soc. 1994, 116, 10 316.
(c) Kolb, C. E.; Jayne, J. T.; Worsnop, D. R.; Molina, M. J.; Meads, R. F.;
Viggiano, A. A. J. Am. Chem. Soc. 1994, 116, 10 314. (d) Phillips, J. A.;
Canagaratna, M.; Goodfriend, H.; Leopold, K. R. J. Phys. Chem. 1995,
99, 501. (e) Lovejoy, E. R.; Hanson, D. R.; Huey, L. G. J. Phys. Chem.
1996, 100, 19 911. (f) Akhmatskaya, E. V.; Apps, C. J.; Hillier, I. H.;
Masters, A. J.; Palmer, I. J.; Watt, N. E.; Vincent, M. A.; Whitehead, J. C.
J. Chem. Soc., Faraday. Trans. 1997, 93, 2775. (g) Meyer, E. J.; Sprik,
M. J. Phys. Chem. A 1998, 102, 2893. (h) Loerting, T.; Liedl, K. R. Proc.
Nat. Acad. Sci. 2000, 97, 8874. (i) Loerting, T.; Kroemer, R. T.; Liedl, K,
R. Chem. Commun. 2000, 999. (j) Loerting, T.; Liedl, K. R., J. Phys. Chem.
A 2001, 105, 5137.
By variation of the ratio of the partial pressures of H2O and
SO3 above the IRE, the composition of the film which is
deposited on the cold IRE surface can be controlled. (NB, For
the purposes of simplicity, hereafter this ratio is defined as Rp,
(7) Jayne, J. T.; Po¨schl.; U, Chen, Y.; Dai, D.; Molina, L. T.; Worsnop, D. R.;
Kolb, C. E.; Molina, M. J. J. Phys. Chem. A. 1997, 101, 10 000.
(8) Fiacco, D. L.; Hunt, S. W.; Leopold, K. R. J. Am. Chem. Soc. 2002, 124,
4504.
(9) Horn A. B.; Sully, K. J. J. Chem. Soc.: Faraday Trans. 1997, 93, 2741.
9
J. AM. CHEM. SOC. VOL. 125, NO. 7, 2003 1995

A R T I C L E S
Couling et al.
(similar to the effects seen in the matrix spectra of Givan et
al.,11 who report evidence for the presence of both stable and
unstable environments for H2SO4 molecules held in a CO matrix
giving rise to a multiplicity of bands from a single vibrational
mode). These features confirm that the species involved is
molecular H2SO4 because such high frequencies are not
observed for any SO4 core modes of either bisulfate or sulfate
ions.
There are, however, a number of bands in the spectra which
are significantly shifted from those observed in the vapor phase.
In the high wavenumber region of the spectra where OH
stretching vibrations of water and the SO-H groups are
expected, the OH stretch appears at 3600 cm-1 in both the vapor
phase and in an inert matrix for the anhydrous acid, whereas it
occurs at ca. 2845 cm-1 in Figure 1. For comparison, this
vibration occurs at about 2960 cm-1 in liquid H2SO4 and at
3010 cm-1 in condensed solid films.12 The red shift of this mode
implies there is a strong interaction with neighboring H2SO4
and/or H2O molecules. There are a number of vibrational
frequency calculations for molecular complexes such as (H2-
SO4)‚(H2O)n which are discussed in more detail below which
show this effect, typically yielding red shifts of the order of
10-17%. Furthermore, the visible features in the OH stretching
region are very broad, also commensurate with strong hydrogen
bonding. Although less easy to assign unequivocally, the lower
wavenumber features are also shifted from the gas phase and
matrix values. The main bands in this region in Figure 1 occur
at 970, 864, and 752 cm-1, and fall within the region where
coupled and independent S-O stretches occur. Given that the
H2SO4 is formed from gaseous SO3 and H2O, these spectral
changes can be rationalized by considering the established
reaction mechanism. Theoretical and experimental studies have
shown that at least 2 water molecules are required per SO3,
with the result that any H2SO4 formed must initially be in the
presence of at least one other water molecule. At the low
temperatures utilized in these experiments, there is a significant
likelihood that additional water molecules are also incorporated
into the deposit. This is confirmed by Figure 1b, which shows
an IR spectrum of a film formed at higher water pressures (Rp
) 14). Compared to Figure 1a, the additional features present
in this spectrum are seen at 1284, 1086, and 788 cm-1. These
bands are variable in intensity and are observed in both the
normal and deuterated spectra. Their intensity relative to the
acid bands shows a strong dependence on water content. It will
be shown below that these features are due to the formation of
a higher molecular hydrate of H2SO4 with a characteristic
structure. Similarly, very little surface product is observed on
the experimental time scale in the presence of only trace amounts
of water, since the reaction is limited by water availability. At
Figure 1. ATR-IR spectra of the formation of sulfuric acid species from
the reaction of gas-phase H2O and SO3 on a Ge ATR element at 190 K. (a)
H2O ) 1 × 10-7 mbar, SO3 ≈ 7 × 10-6 mbar.; (b) H2O ) 5 × 10-7 mbar,
SO3 ≈ 7 × 10-6 mbar; (c) D2O ) 1 × 10-7 mbar, SO3 ≈ 7 × 10-6 mbar.
Spectra are offset for clarity.
where the subscript p denotes partial pressure ratio). For
example, for an H2O:SO3 ratio of 1:5, Rp ) 5]. At high values
of Rp, the concentration of the surface film will tend to favor
sulfate species over water-containing species. Figure 1 shows
the ATR-IR spectra of films deposited at 190 K with Rp ) 70
(specifically, pH2O (and pD2O) ≈ 1 × 10-7 mbar and pSO3 ≈
7 × 10-6 mbar as determined by the ion gauge) along with a
more hydrated deposit. Film thicknesses, calculated using an
optical property of the ATR process described elsewhere,9 are
ca. 11 nm.
In Figure 1a and 1c, the strongest peaks in the spectra below
are seen at 2845(2262), 1460(1441), 1214(1213), 970(955), 864-
(850), and 752(750) cm-1 (deuterated species in parentheses).
Of these major bands, those at 1460 and 1214 cm-1 correlate
well with those previously reported for sulfuric acid vapor
spectra10 and matrix isolated sulfuric acid, assigned to the
antisymmetric and symmetric stretching modes of the OdSdO
group, respectively. Furthermore, there appears to be some
evidence of splitting of the peaks at 1460 and 1214 cm-1, seen
mainly as shoulders to the main features. This is likely to be
due either to the presence of two separate species or to a
difference in local environments within the deposited film
higher temperatures and higher water pressures (> 195 K, Rp
< 1), extended ionized sulfuric acid monohydrate (H3O+HSO4
)
-
films are formed directly on the time scale of the IR experiment.
This is commensurate with the theoretical predictions of Larson
et al., who observed a direct channel to the H3O+HSO4 ion
-
pair in their simulation of 4:1 H2O:SO3 clusters.13
The spectroscopic data presented above indicate that surface
reaction of H2O and SO3 at cold temperatures (190 K) produces
(11) Givan, A.; Larsen, L. A.; Loewenschuss, A.; Nielsen, C. J. J. Chem. Soc.:
Faraday Trans. 1998, 94, 2277.
(12) Nash, K. L.; Sully, K. J.; Horn, A. B. J. Phys. Chem. A. 2001, 105, 9422.
(13) Larson, L. J.; Kuno, M.; Tao, F.-M. J. Chem. Phys. 2000, 112, 8830.
(10) Stopperka, Von K.; Kilz, F. Z. Anorg. Allgem. Chemie. Soc. 1969, 88, 723.
9
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Identification of Molecular Sulfuric Acid Hydrates
A R T I C L E S
an amorphous film in which the individual acid molecules are
associated with surrounding water molecules through hydrogen
bonding and have many of the spectroscopic characteristics of
the acid vapor and of matrix-isolated species. Such an environ-
ment clearly perturbs the SdO vibrations of H2SO4 very little
compared to the gas phase, with the main influence being
through the vibrational modes of the OH groups through which
hydrogen bonding occurs, accompanied by a less marked effect
on the S-OH modes. These amorphous sulfuric acid films are
only formed when the conditions are very water limited although
it is clear that more than one water molecule must be involved
in the heterogeneous process as for the gaseous one. The
material deposited on the ATR element surface under these
conditions is clearly more closely related to the gas-phase
products of the SO3 + H2O reaction than a genuine condensed
solid phase, producing species in a form analogous to that
observed in a matrix isolation experiment. As will be shown,
an increase in water pressure rapidly converts this film to other
species. In the following sections, it will also be shown that
the as-yet unassigned bands (1284, 1086, and 788 cm-1) can
be attributed to a number of the stable molecular hydrate species
(for the moment labeled “species X”) predicted by theoretical
methods.
Thermal Behavior and H2SO4‚H2O Complex Formation.
To investigate the composition of the deposit, thin films of
amorphous sulfuric acid were warmed slowly in vacuo from
190 to 225 K. Changes in the condensed film were followed
by IR spectroscopy along with simultaneous monitoring of the
partial water pressure in the chamber with a residual gas
analyzer. Figure 2 shows spectra recorded at 190, 210, 217,
and 225 K during such an annealing procedure. Figure 2a shows
an initial amorphous film at 190 K. Above 205 K, a significant
increase in water pressure in the chamber is observed as excess
water is driven from the sample. The IR spectrum of the film
at 210 K (Figure 2b) reveals a reduction in the bands at 1284,
1086, and 788 cm-1 attributed to the as-yet unidentified
hydrates. The amorphous sulfuric acid bands at 1460 and 1214
cm-1, attributed to the OdSdO antisymmetric and symmetric
stretching modes, broaden and shift down to ca. 1400 and 1180
cm-1. Similarly, strong broad features at ca. 965 and 802 cm-1
grow in along with a shoulder at 911 cm-1. These new bands
clearly have their origin in some new species. The broad O-H
stretching peak at 2845 cm-1 shifts up to 2926 cm-1 as the
concentration of sulfuric acid in the film increases.
Figure 2. ATR-IR spectra showing the effect of annealing the as-deposited
thin film to successively higher temperatures: (a) as-deposited, 190 K; (b)
210 K; (c) 217 K; (d) 225 K.
The material present upon the surface can now be identified
using the above observations in conjunction with aspects of the
theoretical work in the literature.13 The work of Re et al.14b and
Bandy and Ianni14a is based upon DFT methods, reputedly
accurate for hydrogen bonded complexes. Both studies clearly
identify a number of stable molecular hydrates, as shown in
Figure 4, and both support the notion that spontaneous ionization
is unfavorable for less that four water molecules per H2SO4.
The most stable 1:1 H2O‚H2SO4 complex has the H2O hydrogen
bonded across one HO-SdO angle in a six-membered ring, as
shown in Figure 4a. Two possibilities exist for the 2:1 complex,
with either two H2O molecules across one HO-SdO angle in
an eight-membered ring (Figure 4b) or one H2O across each
HO-SdO angle (Figure 4c). According to Re et al., the former
is the more stable by 0.7 kcal, although at 190 K this barrier is
likely to be insignificant. It therefore seems probable that the
species initially formed upon reaction at the IRE surface is a
mixture of these species, since the direct formation of anhydrous
H2SO4 is not feasible. Because heating the initially formed
material drives off a proportion of excess water to form a stable
species still containing molecular H2SO4 and H2O, it seems
likely that the initial material consists of a mixture of the
Continued heating shows further transformation as the
remainder of the excess water leaves the film and eventually
the ejection of water ceases and the chamber pressure returns
to background levels. The partial pressure of water during these
experiments is shown in Figure 3, where the base pressure of
water is 5 × 10-8 mbar. Inspection of the spectrum of the film
at 217 K in Figure 2c during this process shows the film to be
predominantly liquid sulfuric acid with characteristic broad
peaks at 1359, 1164, 965, and 906 cm-1. The peaks at 1400
and 802 cm-1 are still present but have diminished in intensity
(the related bands at 1180 and 965 cm-1 cannot be discerned
explicitly because of overlap). On heating to 225 K, these minor
peaks disappear as the ejection of water vapor ceases and the
liquid sulfuric acid peaks sharpen as the film crystallizes, as
can be seen in Figure 2d.
(14) (a) Bandy, A. R.; Ianni, J. C. J. Phys. Chem. A. 1998, 102, 6533. (b) Re,
S.; Osamura, Y.; Morokuma, K. J. Phys. Chem. A. 1999, 103, 3535. (c)
Beichert, P.; Schrems, O. J. Phys. Chem. A. 1998, 102, 10 540. (d) Arstila,
H.; Laasonen, K.; Laaksonen, A. J. Chem. Phys. 1998, 108, 1031. (e)
Beichert, P.; Schrems, O. Phys. Chem. Chem. Phys. 1999, 1, 5459.
9
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A R T I C L E S
Couling et al.
Figure 3. Partial pressure of H2O in the vacuum chamber during film
annealing from 190 K.
monohydrate and one or both of the dihydrate isomers. Taking
the dihydrates as an example, the theoretical structures can then
be used to guide the IR band assignments. As a result of the
weak hydrogen bonding interaction between the SdO bonds
and H2O in either dihydrate structure, the SdO bonds are
approximately the same length in both cases at ca. 0.146 nm.
These two bonds couple to give the strong symmetric and
antisymmetric stretching modes described above at 1214 and
1460 cm-1 for both isomers. However, the effect of hydrogen
bonding between the water molecules and the SO-H bonds
leads to rather different S-O bond lengths between the two
isomers. For isomer I (Figure 4b), the interacting S-O bond
shortens and the noninteracting bond lengthens, whereas for the
symmetrically bonded isomer II (Figure 4c), both bonds are of
equal length. Given that the bond lengths affect the vibrational
frequencies, the resonances from these two isomers will
therefore be different both from each other and from the free
gas-phase molecule. In isomer I, the bonds are different lengths
and will not necessarily couple, giving a high frequency for
the vibration of the short bond and a lower one for the longer
bond. For isomer II, the bonds are still very similar in length
and strength to each other, so coupling to give a symmetric
and antisymmetric doublet should occur. If both species are
present, then up to four bands should be observed in the S-O
stretching region provided that neither species possesses high
symmetry (more will be expected if the monohydrate is also
present, although considerable overlap is to be expected). On
the basis of these observations, the bands at 970, 864, and 752
Figure 4. Structures for the theoretically predicted 1:1 and 2:1 H2O‚H2-
SO4 complexes from Re et al. (ref 14b) (a) 1:1 complex, (b) 2:1 complex:
Isomer I, (c) 2:1 complex: Isomer II.
cm-1 can be assigned to stretching modes (coupled or individual
bond displacement) of S-O bonds in a hydrogen bonded
configuration. The pattern of three modes rather than four
suggests that either there are coincidences or that one or more
modes of the most symmetrical isomer (II) is IR-forbidden. That
these modes are due to S-O vibrations is confirmed by the
fact that they do not shift significantly upon full deuteration.
By analogous reasoning, the 1:1 complex of H2O and H2SO4
as shown in Figure 4a can therefore be envisaged as a step in
the dehydration process. Although it is also likely to be present
at variable concentrations in the as-deposited film as discussed
above, the material may not be sufficiently well-ordered to
produce a characteristic spectrum. The microwave spectrum of
this material in the gas phase8 reveals that the single water
molecule in the 1:1 complex is bound by two hydrogen bonds
across the OdS-OH moiety, just as predicted by theoretical
methods. As before, the SdO bonds are short and relatively
free from hydrogen bonding, giving rise through coupling to
the doublet at ca. 1400 and 1180 cm-1. It is also likely that
these modes are softened somewhat compared to the dihydrate
isomers by virtue of increased long range order induced by
annealing the film. The band at 802 cm-1 can be assigned to
the librational mode (frustrated rotation) of the bound H2O in
9
1998 J. AM. CHEM. SOC. VOL. 125, NO. 7, 2003

Identification of Molecular Sulfuric Acid Hydrates
A R T I C L E S
Table 1. Summary of Vibrational Frequencies and Assignments for the Initial Deposit at 190 K and Its Thermal Products at 210 and 217 Ka
thermal
product I
210 K
thermal
product II
217 K
amorphous
deposit 190 K
(Figure 1a)
deuterated
deposit 190 K
(Figure 1c)
gas phase
(anhydrous)
(ref 10)
H SO (s)
2 4
210 K
(ref 12)
(Figure 2b)
(Figure 2c)
2845 (m, br)
1460 (s)
1284
1214 (s)
1086 (w)
970 (m)
864 (s)
2262 (m, br)
1441 (s)
1302
1213 (s)
1091
3610
1450
2926 (s, br)
1400 (m)
OH stretch
νas(OdSdO)
species X
νs(OdSdO)
species X
νas(HO-S-OH)
νs(HO-S-OH)
complexed H2O
species X
1365 (s)
1170 (s)
1358 (s)
1165 (s)
1223
1180 (s, br)
955 (m)
850 (sh)
883
834
965 (s, br)
911 (w, sh)
802 (vs)
969 (s)
907 (s)
965 (s)
905 (s)
788 (s)
752 (s)
780 (m)
750 (s)
752 (m)
undefined O-S-O related
a (Peak intensities indicated by: (s) strong; (m) medium; (w) weak; (br) broad; (sh) shoulder).
a highly ordered system, analogous to the H2O librational mode
observed in crystalline ice at 810 cm-1. The fact that this mode
is due to an OH-related vibration is verified by deuteration
experiments, in which it is absent (although it is not possible
to identify this band at a shifted position in the deuterated sample
as it lies below the low-wavenumber cutoff of the optical system
used here). Fiacco et al.8 observed a significant degree of
rotational freedom for the H2O molecule in this configuration,
which lends weight to the assignment. The residual features in
Figure 2b arise from the 2:1 isomers, and decrease as the
reaction dehydration continues. Figure 2c represents further
dehydration, i.e., water loss from the 1:1 complex as anhydrous
sulfuric acid is formed. This IR spectroscopic evidence therefore
supports predictions of the stability of the 1:1 complex, and
that fact that a range of products are observed from the initial
reaction also supports the assertion that the free energies of
formation of the lower ratio isomers are similar. Under these
conditions (i.e., reduced temperature and water concentration),
the formation of the higher hydrates is favored energetically
but the amounts actually formed are limited by the availability
of H2O.
On the basis of the above observations, it is proposed that
the conversion of the as-deposited sulfuric acid hydrate film to
condensed anhydrous sulfuric acid film proceeds largely from
two isomers of molecular H2SO4‚(H2O)2 through H2SO4‚H2O
(thermal product I) to molecular H2SO4 (thermal product II).
Thus Figure 2a shows a mixture of the mixed dihydrates with
a small amount of disordered 1:1 complex, Figure 2b is
predominantly the 1:1 complex with some liquid sulfuric acid
present and Figure 2c shows predominantly a film of liquid
sulfuric acid. It was not possible in these experiments to stabilize
a film of the 1:1 complex, suggesting that the structure is only
Figure 5. ATR-IR spectra of the product of exposing the film with spectra
shown in 1(a) and 1(c) to 1 × 10-5 mbar H2O at 190 K until complete
conversion has occurred.
weakly stable as a condensed film under vacuum conditions.
Table 1 summarizes the vibrational frequencies and assignments
presented above.
Hydration of Amorphous Sulfuric Acid at 190 K. To
further investigate the structure and stability of the molecular
complexes identified above, additional water was added. Given
that the lower ratio hydrates are formed largely as a result of
the restricted amount of water available for reaction, this would
be expected to facilitate the formation of higher hydrates. When
the water pressure above an amorphous sulfuric acid film is
raised above 1 × 10-5 mbar, significant changes in the film
spectra are observed as the condensate rapidly converts to a
new species. Figure 5 shows IR spectra of this transition and
Figure 6 shows examples of the hydration product and its
deuterated analogue. The spectra are characterized by a number
of strong absorption bands in the SO4 core region. Bands at ca.
1260(1275), 1092(1092), 1038(1039), 785(783), and 750(745)
cm-1 are common to both normal and deuterated materials, with
additional features of significant intensity at 1007, 931, and 873
cm-1 in the undeuterated film. Additionally, the OH stretching
region is dominated by strong broad absorptions at ca. 3380
cm-1, due to the addition of less strongly bound H2O molecules
in the deposit. At temperatures below 190 K, this transformation
is not observed: upon water exposure, the amorphous acid peaks
remain unchanged and molecular H2O is observed to be
9
J. AM. CHEM. SOC. VOL. 125, NO. 7, 2003 1999

A R T I C L E S
Couling et al.
Figure 6. ATR-IR spectra of (a) normal and (b) deuterated hydration
product at 190 K produced directly from gaseous SO3 and H2O at Rp ) 10.
Spectra are offset for clarity.
Figure 7. ATR-IR spectra showing the result of annealing the hydration
product to successively higher temperatures. (a) 190 K; (b) 195 K; (c) 200
K; (d) 205 K; (e) 208 K; and (f) 210 K.
incorporated into the film as crystalline ice. By careful control
of the water and SO3 pressures around Rp ) 10, this material
can be deposited directly. Films produced by this method are
generally free of lower hydrate contamination, as can be seen
from the spectra presented in Figure 6.
film has been converted to SAM, it is not possible to regenerate
the original species.
Composition of the Hydration Product. The changes in the
OH vibrations on conversion to SAM in Figure 7 along with
the predictions from the computational studies give some clues
as to the character of the new species. Comparison with
reference spectra of sulfuric acid tetrahydrate9 (SAT) suggest
that the material does not contain tetrahedral SO42-, which
Although the direct analysis of this material by assignment
of the spectral features is difficult without reference materials,
the approximate composition can be deduced by observation
of its thermal behavior. Heating the undeuterated film slowly
from 190 to 220 K results in an irreversible transformation to
sulfuric acid monohydrate, as shown in Figure 7. Starting from
the unidentified material in Figure 7a at 190 K to the end product
SAM in Figure 7f at 210 K, several clear transformations are
observed. The broad absorption at ca. 3380 cm-1 characteristic
of liquidlike molecular H2O is lost, to be replaced by the broad
features at ca. 2860, 2202 and 1699 cm-1 characteristic of the
H3O+ ion. Concomitantly in the sulfate region, the broad features
at ca. 1260 and 785 cm-1 and the sharp features at 1092 and
1038 cm-1 are also gradually lost as the characteristically strong
and broad features of the bisulfate ion at 1130, 1034, and 902
cm-1 grow in. Simultaneous monitoring of the gas phase above
this film reveals the ejection of water during this transformation
as shown in Figure 8. The rate of this ejection peaks at ca. 202
K, commensurate with the midpoint of the transformation from
the unidentified species to crystalline SAM. No sulfur-containing
species are ejected during this process. Furthermore, once the
would give rise to an intense, sharp band at ca. 1070 cm-1
.
Although the sulfate ion could be present in a different
coordination environment, the spectral features are not compat-
ible with any of its known distortions in complexes.15 Further-
more, there is no convincing evidence in the initial spectrum
(Figure 7a) for the presence of H3O+ (or higher proton hydrate)
counterions which would necessarily accompany any ionized
sulfur-containing anion. Figure 8 clearly shows that some water
is driven off during the conversion to SAM, whereas Figure
7a-f show production of H3O+ simultaneous with this loss.
This implies that some of the water associated with the hydrate
is being converted to the hydronium ion and some is lost to the
vacuum. Because SAM (H3O+HSO4-) is effectively an ionized
form of the H2SO4‚H2O complex, the product must contain more
water than either of these materials (obviously, it has more than
(15) (a) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordina-
tion Compounds; John Wiley-Interscience, New York, 1997. (b) Yamaguchi,
T.; Jin, T.; Tanabe, K. J. Phys. Chem. 1988, 90, 3148.
9
2000 J. AM. CHEM. SOC. VOL. 125, NO. 7, 2003

Identification of Molecular Sulfuric Acid Hydrates
A R T I C L E S
studies suggest that the free energy of ionization of an acid
complex is positive at room temperature even with n ) 6.
Consequently, it is probable that at least 5 water molecules are
required before the spontaneous ionization is favorable, even
at temperatures of 180 K. Re et al arrived at similar conclusions
that several water molecules are required before ionization
becomes favorable; although Larson et al have identified an
alternative mechanism for n ) 4. The spectroscopic observations
presented here are therefore discussed in the light of these
predictions.
Given the stability of hydrogen bonded molecular complexes
of H2SO4 and H2O, possibly comprising of loosely bound
(H2SO4‚3H2O)2 dimers or higher chains with the form
(H2SO4‚2H2O)n‚2H2O, the following observations can be made
to propose a structure for the hydrated product and an assign-
ment of its vibrational modes as seen in Figures 6 and 7a. In
the OH stretching region, there is a strong, broad feature at 3380
cm-1 and a weaker (broad) shoulder centered at ca. 3000 cm-1
.
The calculated lengths of the O-H bonds of both the SO-H
and the H-O-H moieties in these higher ratio complexes are
similar those observed in the 1:1 complex and are somewhat
longer than those found in the isolated H2O. The broad OH
feature at ca. 3380 cm-1 is therefore due to the O-H stretching
modes of complexed H2O molecules and the broad shoulder at
ca. 3000 cm-1 arises from the OH stretches of H2SO4. The
intensity of the water-related OH stretches compared to those
of H2SO4 support the assertion that the hydrate contains a
significant amount of water. The OH-related features are also
seen at appropriately shifted values in the spectrum of the
deuterated compound. These assignments are supported by
literature observations of the spectral signatures of thin films
of vapor deposited ice, which are characterized by broad,
structured absorption bands comprising of a number of overlap-
ping resonances. These composite bands typically show maxima
at ca. 3240 cm-1 with shoulders at ca. 3380 and 3160 cm-1 in
the amorphous form. On heating such films, the features at 3240
and 3160 cm-1 increase in intensity at the expense of that at
3380 cm-1, which implies that the shoulder at 3380 cm-1 is
associated with H2O in an environment where it is not fully
coordinated in a regular ice lattice. The assertion that the 3380
cm-1 band may be due to weakly coordinated water is supported
by recent experimental work by Schriver-Mazzuoli et al.,16 who
have deposited a thin ice film showing a asymmetric band with
a maximum at 3380 cm-1 and a secondary maximum at 3230
cm-1. This film was grown under slow deposition conditions
and is similar is shape and position to that seen in the product
hydrate.
Figure 8. Partial pressure of H2O in the vacuum chamber during annealing
the hydration product from 190 K.
the latter because it was formed from hydration of the
H2SO4‚(H2O)n (1 < n < 2) complex(es)). If the hydrated product
contains intact molecular H2SO4, the SO4 core is clearly not in
the same form as in either the anhydrous condensed form (solid
or liquid), the amorphous form (1 < n < 2) or the 1:1 complex.
The spectroscopic evidence points to the fact that the hydrated
product must therefore be a molecular complex of H2SO4 and
H2O in a ratio greater than 2:1
The DFT studies of Bandy and Ianni14a for the hydrates of
sulfuric acid in small clusters of up to 7 water molecules and
for the dimer of the trihydrate of sulfuric acid (H2SO4‚3H2O)2
point the way to possible structures for this material. As in the
Re et al. work,14b they found that many neutral clusters of
H2SO4‚nH2O for n ) 1-6, were stable in the form of hydrogen-
bonded molecular complexes of H2SO4 and H2O and contain
no H3O+ ions. They also determined that there is no free energy
barrier to the formation of larger hydrates below 248 K. The
calculations suggest that the species (H2SO4‚3H2O)2 had the
greatest Gibbs free energy of formation of the species studied
and on the basis that water molecules may be coordinated
between several H2SO4 molecules to produce an extended
structure, the authors speculate that most atmospheric ultra-fine
particles in the 1-100 nm radius range might be entirely
composed of long chains of (H2SO4‚2H2O)n. They also con-
sidered the probability of neutral clusters of H2SO4‚nH2O
isomerizing to form H3O+HSO4-‚(n-1)H2O by calculating the
Gibbs free energy of isomerization of the hydrates of sulfuric
acid (into the ionized form). In contrast to bulk solutions, these
Assignment of the SO4 core modes is more problematical
but can again follow similar reasoning to that invoked for the
1:1 and 2:1 complexes. For example, from the calculated
structure of the (H2SO4‚3H2O)2 dimers, there are considerable
changes in the lengths and inter-bond angles of the SdO and
S-OH bonds. We propose the following assignments on the
basis of the generalized structural features observed in the higher
hydrates. Typically, both of the SdO bonds are lengthened
through hydrogen bonds leading to considerable downward
frequency shifts from the free acid. The broad, structured feature
at ca. 1260 cm-1 (1275 cm-1 in the deuterated analogue) is
(16) Schriver-Mazzuoli, L.; Schriver, A.; Hallou, A. J. Mol. Struct. 2000, 554,
289.
9
J. AM. CHEM. SOC. VOL. 125, NO. 7, 2003 2001

A R T I C L E S
Couling et al.
Table 2: Summary of Vibrational Frequencies and Assignments
for the Hydration Product at 190 K and for the Thermal Product at
210 K
hydration
product
(Figure 6a)
D O
product
(Figure 6b)
thermal
product III
(Figure 7f)
2
assignment
3380 (s, br)
1723 (br)
2524 (s, br)
-
νH2O
2860 (br)
1699 (br)
1274 (sh)
νH3O+
δH3O+
δO-H (bisulfate)
1260 (s)
1246 (s)
1275 (s)
1261 (s)
νas(OdSdO) (acid)
νs(OdSdO) (acid)
νasS-OH (bisulfate)
νas(O-S-O)-2 (acid)
νas(O-S-O)-1 (acid)
νasSO3 (bisulfate)
Figure 9. Reaction scheme and interconversion of sulfuric acid species
with temperature and water exposure (ejection).
1130 (s)
1034 (s)
902 (s)
1092 (m)
1038 (s)
1092 (m)
1039 (s)
Figure 9 summarizes the observed forms of sulfuric acid and
its hydrates and their interconversions.
1007 (w)
931 (w)
Conclusions
νsSO3 (bisulfate)
873 (w)
785 (s)
750 (m)
By controlled variation of temperature and reaction condi-
tions, various low-ratio H2SO4/H2O species have been con-
densed and characterized. Amorphous mixtures of sulfuric acid
and water can be formed at low temperatures under conditions
of low water availability. These deposits show a vibrational
spectrum very similar to that observed for H2SO4 in dilute inert
gas matrixes. That anhydrous H2SO4 cannot be formed directly
under these conditions (i.e., water is always present in the
condensed film) and is only accessible by dehydration supports
gas-phase experimental and theoretical observations that the
reaction of SO3 with H2O requires at least two H2O molecules,
even when the reaction occurs heterogeneously. It would appear
that the principal role of the surface in this reaction is to
immobilize one or both of the reactants, thereby increasing the
reaction efficiency. The stoichiometry of this heterogeneous
process cannot therefore be determined directly from the gas-
phase measurements without a knowledge of the temperature-
dependent sticking probabilities of H2O and SO3.
Upon heating, this amorphous sulfuric acid/water mixture
evolves the excess water trapped in the film and transforms first
into an ordered 1:1 molecular complex of H2O and H2SO4 and
then into molecular sulfuric acid. The released water can be
observed in the gas phase until there is complete conversion to
the molecular acid. The 1:1 complex is characterized by H2-
SO4 vibrations which, although lower in frequency than those
for the initially formed amorphous acid/water mixture, are
clearly different from those observed for condensed anhydrous
H2SO4. Additionally, this complex is characterized by a readily
identifiable vibration of the coordinated H2O molecule. The
vibrational spectroscopic observation of such a complex provides
further evidence in support of the conclusions of microwave
and theoretical studies that such a complex is stable at low
temperatures in clusters.
783 (s)
745 (m)
νs(O-S-O)-1 (acid)
νs(O-S-O)-2 (acid)
therefore assigned to the overlapped symmetric and antisym-
metric (OdSdO) stretches. There are clearly more than two
bands under this profile, which is similar for both the normal
and deuterated material: this is most likely due to the presence
of at least two environments for the H2SO4 molecules within
the polymeric structure. The shoulder at ca. 1200 cm-1 is also
present in the deuterated analogue, confirming its association
with the SO4 core (although the broad δOD modes of D2SO4
and D2O will also underlie this region of the spectrum).
The microwave study of the 1:1 H2SO4‚H2O complex by
Fiacco et al.8 reveals evidence for changes in the inter-bond
angles that might be expected as more water is added to the
hydrate and the H2SO4 moiety experiences additional hydrogen
bonding. These changes are particularly pronounced for the
HO-S-OH angles, in conjunction with the predicted shortening
of the S-O bonds in a hydrogen bonded chain, this effect would
serve to increase the frequency spacing between the antisym-
metric and symmetric stretching modes. Consequently, the
strong sharp bands at 1038 and 785 cm-1 (1039 and 783 cm-1
in the deuterated material) are tentatively assigned to these
modes (respectively). These bands are enveloped by weaker
features at 1092 and 750 cm-1, which are attributable to the
same vibrational modes of a second H2SO4 molecule in a slightly
different environment (probably with a slightly wider O-S-O
bond angle). These pairs of modes are always observed with a
fixed relative intensity ratio with respect to each other in the
spectra. Furthermore, it is unlikely that the lower frequency
modes at ca. 780 cm-1 are due to OdSdO deformation modes,
given that the highest frequency modes of this type in the spectra
of matrix isolated H2SO4 occurs at 554 cm-1. Suggested
vibrational assignments are given in Table 2.
The addition of water to the amorphous H2SO4/H2O mixture
results in the formation of a new material with a reproducible
and well-defined vibrational spectrum. Through the use of a
combination of literature theoretical predictions, thermal be-
havior and spectroscopic arguments, this material is identified
as a stable higher ratio molecular H2SO4/H2O complex with a
probable structure of poly(H2SO4(H2O)2). Given additional
In summary, although the above tentative assignment requires
further theoretical work to ascertain the validity of the vibrational
mode assignment, the hydrated product is clearly a higher ratio
molecular H2SO4/H2O complex. Upon heating, it thermally
decomposes to ionic sulfuric acid monohydrate with the loss
of water (but does not lose any sulfur-containing moiety). Upon
overlaying the spectra of the molecular to ionic hydrate
conversion shown in Figure 5, an isosbestic point is evident at
ca. 800 cm-1, indicating a straightforward conversion of one
species to another (namely, H2SO4 to HSO4-). The diagram in
energy, this material irreversibly transforms into an ionized
sulfuric acid hydrate, namely the monohydrate H3O+HSO4
.
-
Further heating produces the anhydrous molecular acid as seen
in previous experiments. This study presents the first experiment
observation of this un-ionized complex.
9
2002 J. AM. CHEM. SOC. VOL. 125, NO. 7, 2003

Identification of Molecular Sulfuric Acid Hydrates
A R T I C L E S
There are atmospheric implications as a result of the stability
of these compounds. Because the majority of atmospheric sulfate
aerosol are thought to be formed from both the gaseous
nucleation of SO3 and water and through the heterogeneous
hydration of SO3 on water droplets, it is likely that these species
are involved directly in the process of aerosol formation,
although it is unlikely that they are long-lived in the bulk aerosol
under typical ambient conditions. However, the reproducibility
and stability of formation of these materials gives experimental
credibility to the suggestion by Bandy and Ianni14a that
un-ionized acid hydrates may be important surface species on
aerosols because these molecular species are potentially far more
stable at the aerosol/vacuum interface than any ionic species.
An understanding of the nature of the hydrate complexes, their
ionization energetics and their thermal stability will clearly be
essential for detailed models of aerosol formation in the
atmosphere. The direct observation of these intermediates in
sulfate aerosol formation processes provides a challenge for
future experiments.
Acknowledgment. This work was supported by the “Core-
Strategic Measurements for Atmospheric Science” (COSMAS)
program of the United Kingdom Natural Environment Research
Council under research grant GST/02/2703. A.B.H. acknowl-
edges the support of the United Kingdom Engineering and
Physical Sciences Research Council through the award of an
Advanced Research Fellowship.
JA0210704
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J. AM. CHEM. SOC. VOL. 125, NO. 7, 2003 2003