Infrared matrix isolation study of H SO and its complexes
2 4
with H O
2
Aharon Givan,a Lars A. Larsen,b Aharon Loewenschussa* and Claus J. Nielsenb
a Department of Inorganic and Analytical Chemistry, T he Hebrew University of Jerusalem,
Jerusalem 91904, Israel
b Department of Chemistry, University of Oslo, Blindern, N-0315 Oslo, Norway
Infrared spectra of H SO vapors, which also include H O and SO , were trapped in argon matrices. Vibrational assignments for
2 4
2
3
monomeric H SO were established and additional bands were assigned to the (H SO ) , (H O) É (H SO ) and (H O) É (H SO )
2 4
2 42
2
2 4
2 2
2 4
species. For the (H O) É (H SO ) complex, several isomers are shown to exist, the most stable being H O É HOSO OH. Tem2
2 4
2
2
perature reversible changes in several bands of the latter complex are discussed in analogy to the trimeric (H O) complex. The
2 3
H O and H SO molecules dispersed in the argon solid do not a†ect the stabilization of a mixed (H O) É (SO ) complex. No
2
2 4
2 m
3n
spectral evidence for ionized species due to proton transfer was found in the matrix samples investigated.
1 Introduction
The interest in the properties of gaseous H SO during recent
2 4
decades is mostly due to its role in aerosol formation and acid
rain. Its atmospheric vapor-phase production mechanism
involves SO , which quickly interacts with H O in the gas3
2
phase or on the surface of aerosols, perhaps via an
(H O) É (SO ) adduct, to form H SO .1 However, ab initio cal2
3
2 4
culations indicate the energy barrier for the production of sulfuric acid from the bimolecular adduct to be over 80 kJ
mol~1.2,3 Morokuma and Muguruma4 suggest that formation
of H SO proceeds via the trimolecular cyclic complex
2 4
(H O) É SO in accord with mass spectrometric studies per2 2
3
formed by Kolb et al.5
Vibrational spectra of the gas phase,6h10 of the liquid and
aqueous solutions,11h16 as well as of crystalline H SO have
2 4
been reported.17 Matrix isolation data are limited to the
assignment of the most prominent H SO and (H O) É (SO )
2 4
2
3
bands appearing as by-products in the trapping of SO species
3
in Ne matrices18 and from photooxidation of H S in a low
2
temperature O solid.19 Considering the equilibrium existing
2
between H SO , SO and H O species in sulfuric acid
2 4
3
2
vapors,10 several monomers as well as pure and mixed polymers are expected to be stabilized in the low-temperature
matrix to produce a complex absorption pattern. Additional
complication of the spectrum may be caused by interactions
of the reactive gaseous species, especially SO (a powerful oxi3
dising agent), with the apparatus.
The subject of the present study is a detailed clariÐcation of
the infrared spectrum of H SO species trapped in argon
2 4
matrices. The ability of H SO and SO to form matrix stabil2 4
3
ized complexes with H O is also investigated in view of the
2
conÑicting reports of the existence of the (H O) É (SO )
2
3
complex in low-temperature solids.18h20
2 Experimental
Sulfuric acid was supplied by Prolabo (p.a.) and the Ar gas
(purity \ 99.9997%) by AGA. Water vapors were taken from
deionised water that had previously been submitted to several
degassing cycles. To produce the sulfuric acid vapors for
deposition, a drop of sulfuric acid was placed in a furnace
consisting of a quartz tube ending in a nozzle, wrapped
around by a heating coil. Prior to deposition, several short
degassing cycles were performed to a temperature of about
40 ¡C with the deposition window, still at room temperature,
turned away from the nozzle and protected by a polyethylene
bag. The vacuum was then broken by Ðlling the system with
argon and the protection was quickly removed while argon
was blowing through the cryostat. These measures were taken
to protect the internal CsI window from excessive corrosion
by exposure to the sulfuric acid and SO vapors during pump
3
down. After renewed pumping, the system was cooled for
deposition and the argon matrix gas, either pure or premixed
with water vapor, was passed through the H SO containing
2 4
nozzle, warmed to ca. 35 ¡C. The mixing ratios were estimated
from pressure measurements, Ñow rates and the calculated
partial pressure above liquid H SO (see below). Typical com2 4
positions ranging from 1 : 300 to 1 : 1000 argon : guest ratios
(within a factor of two or less) were sprayed onto a CsI
window, kept at 5È15 K and coated with a very thin layer of
silicone grease to prevent reaction with the acidic samples.
Additional experiments were carried out employing dual
nozzle deposition of ArÈH SO and ArÈH O mixtures.
2 4
2
Cooling was provided by an Air Products HS-4 Heliplex cryostat employing two HC-4 MK 1 compressor modules. Temperatures were controlled by a LakeShore model 330
controller using Si diode sensors. Typical deposition times
were 1È3 h depending upon sample dilution, while deposition
rates of Ar were of several mmol h~1. Temperature cycling of
the samples was conducted by slow warming (1 K min~1) up
to 38 K, followed by quick recooling to 5 K. Infrared spectra
were recorded on a Bruker IFS 88 FTIR spectrometer
employing a DTGS detector, coadding from 32 to 128 scans
at nominal resolutions of 0.5È1 cm~1.
3 Results and Discussion
Crystalline sulfuric acid belongs to the C2/c space group with
the SO tetrahedra possessing C symmetry.21 The stable con4
2
former of gaseous H SO monomer was determined by micro2 4
wave spectroscopy to be of C symmetry,22 in agreement with
2
theoretical investigations.23h25 Thus the 15 normal vibrations
of the molecule, all IR active, divide into : C \ 8a ] 7b.
vib
The vapor pressure above 99 wt.% H SO has been mea2 4
sured in the range 338È445 K and Ðtted to the equation :
ln P[atm] \ 16.259 [ 10156 ] T ~1,26 which extrapolates to
a value of 0.007 Pa at 310 K. Sulfuric acid and water form an
J. Chem. Soc., Faraday T rans., 1998, 94(7), 827È835
827
azeotrope and in the vapor phase the acid dissociates,
H SO (g) % SO (g) ] H O(g). The equilibrium constant for
2 4
3
2
this dissociation and its temperature dependence can be calculated from the thermodynamic functions of the pure components, K (310) \ 2.9 ] 10~9 atm.27 Assuming equilibrium
p
conditions above 99 wt.% sulfuric acid and that all water
vapor in the quartz oven originates from the azeotrope and
the dissociation of gaseous sulfuric acid, one arrives at a vapor
composition of 66% H SO (g), 19% H O(g) and 15% SO (g)
2 4
2
3
at 310 K. It has been postulated that sulfuric acid forms a
series of hydrates in the vapor phase, (H SO ) É (H O) , and
2 4
2 n
that the equilibrium constant for formation of the Ðrst of these
is 1364.0 atm~1.28 With a sulfuric acid partial pressure of
0.005 Pa (5 ] 10~8 atm) one would need around 1 Pa of
water vapor to convert 1% of the sulfuric acid to the monohydrate. We have therefore no possibility of verifying the existence of such hydrates in the vapor phase with the present
set-up and assume that sulfuric acidÈwater hydrates will be
formed only upon deposition on the cold window.
Survey spectra of gaseous H SO species trapped in argon
2 4
matrices are shown in Fig. 1. The IR bands are summarised in
Table 1. Several lines showing reversible temperature dependence of their intensities are listed in Table 2. The IR features
appearing in the spectrum, Fig. 1A, will now be attributed to
the di†erent possible chemical species. Assignments are based
upon relating intensity changes to the ratio of the pertinent
components (H SO : SO : H O), to e†ects of temperature
2 4
3
2
cycling of the deposited samples, and to the inÑuence of deposition at higher temperatures.
3.1 Monomeric and dimeric H SO species
2 4
The species of highest abundance above liquid sulfuric acid at
room temperature is monomeric H SO , and thus its bands
2 4
are the most prominent and sharp features in the spectrum of
a matrix sample deposited at 5 K, Fig. 1 (curve A). When
depositions were performed onto a window held at the higher
(18 K) temperature Fig. 1 (curve B), the intensities of the
bands assignable to monomers decreased due to adduct formation with the H O molecules migrating in the matrix. This
2
Fig. 1 IR spectrum of H SO vapors trapped in solid argon.
2 as4deposited at 5 K ; B, as deposited at
Ar : H SO ratio B 500 : 1. A,
18 K. 2 4
828
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
decrease is then paralleled with an increase of the bands
attributed to the binary and higher complexes formed, as discussed below in Section 3.3. Most lines are very similar in
wavenumber positions and relative intensities to those
obtained from spectra of H OÈSO mixtures in Ne matrices
2
3
by Bondybey and English,18 from spectra of H S photooxida2
tion in O matrices19 and from the various gas phase investi2
gations.6h10
The most intense band at 3566.7 cm~1 is obviously to be
assigned to the m (b) antisymmetric OH stretching mode. Its
9
bandwidth, 1.4 cm~1, is comparable to the other bands of sulfuric acid. This may be considered as evidence for the dilution
to be high enough to trap non-interacting H SO monomers
2 4
in the Ar : H SO [ 300 : 1 samples investigated here. The
2 4
symmetric OH stretching mode, m (a), was not observed in gas
1
phase6h10 or in previous matrix experiments.18,19 It is
expected to be of weaker intensity and, for comparison, even
in H O monomers, where the two OH bonds share a common
2
oxygen atom, the symmetric m stretch is of only weak inten1
sity compared to its antisymmetric m counterpart (Fig. 1,
3
Table 1).
The e†ect of intermolecular H-bonding on the frequency
di†erence between the antisymmetric and symmetric OH
stretching modes is well reÑected in H O species (Table 1). In
2
the argon isolated “ non-rotating monomer Ï its value is 98
cm~1, in the dimer donor moiety it increases to 134 cm~1,
whereas in the trimer donor moiety it amounts to 193 cm~1.
Returning now to solid H SO , Goypiron et al.17 attributed
2 4
two bands at 3092 and 2980 cm~1 to the antisymmetric and
symmetric stretches, respectively. Giguere and Savoie14 attributed absorptions at 2970 and 2450 cm~1 to these vibrational
modes both in the solid and liquid states, as did also Walrafen
and Dodd.12 On the basis of solid HSO F where the OH
3
stretching mode absorbs at 2940 cm~1, compared with 3500
cm~1 in the gas-phase, Giguere and Savoie14 assumed that
the antisymmetric m (b) OH stretch of gaseous H SO
9
2 4
absorbed at 3600 cm~1. For our argon matrix experiments,
we Ðnd the corresponding band position at 3566.7 cm~1. Considering the limited intermolecular coupling in the gas phase,
the previous authors14 suggested a frequency of 3500 cm~1
for the symmetric m (a) OH stretch. We Ðnd a weak band at
1
3563 cm~1, which we assign to this mode, an assignment supported by a very similar frequency di†erence between the two
stretches, found in recent ab initio calculations.29 Hence, the
two OH modes are almost degenerate due to their very weak
coupling. The values of 3566.7 and 3563 cm~1 compare well
with the single OH absorptions of the argon isolated HOSO
2
radical at 3539.9 cm~1,30 and of gaseous HOSO F and
2
HOSO CH at 3602 and 3610 cm~1, respectively,6 the latter
2
3
being of the same frequency as for gaseous H SO . Similar
2 4
near-degeneracy between symmetric and antisymmetric OH
stretching modes is found for gaseous (3610, 3607 cm~1) and
matrix isolated H O (3597.1, 3588.9 cm~1).31h33
2 2
The two sharp OxSxO stretches m (b) at 1452.4 cm~1
10
and m (a) at 1216.1 cm~1 are much less shifted than the OH
2
stretch from their gas phase values ; a 2.4 cm~1 blue shift for
the former and a 7 cm~1 red shift for the latter. The larger
frequency di†erence, as compared with the respective lines of
the SO radical at 1434 and 1267 cm~1, respectively,34 indi4
cates a larger angle between the two SxO bonds for the
hydrogen containing species. The combination of these two
stretches, m (a) ] m (b), is identiÐed as a very weak band at
2
10
2656.5 cm~1.
The next two bands attributable to monomeric H SO by
2 4
their sharpness and annealing behavior are the weak absorptions 1156.9 and 1136.9 cm~1, minimally red shifted from gas
phase values around 1159 and 1138 cm~1.6,7 They resemble
the neon matrix bands at 1156.4 and 1134.2 cm~1,18 but di†er
substantially from the single liquid phase IR6 and Raman14
bands at 1170 and 1137 cm~1, respectively, and from the solid
Table 1 Assignments of IR absorption bands of H SO , H O and SO vapors trapped in argon matrices
2 4 2
3
band position/cm~1
assignment
3776.5, 3756.5, 3711 (s)
3735.7 (w)
3745.1 (m), 3723 (w)
3725.7 (w)
3708 (m)
3700, 3695 (m)
3669.8, 3653.5 (w)
3638 (w)
3633 (w)
3640 (w, sh)
3582 (w)
3577 (w)
3574 (m)
3572.6 (s)
3566.7 (vs)
3563 (w)
3527, 3517, 3514 (m)
3372, 3324 (w)
3180 (w)
m mode of rotating H O monomer
2 H O monomer
m3 mode of non-rotating
3
2
m mode of acceptor H O in (H O) É (H SO )
2 4
m3 mode of acceptor H 2O in (H2O)
3
2
2 2
m mode of donor H O in (H O)
3
2
2 2
m mode of (H O)
3
2 3
m mode of rotating H O monomer
2 H O monomer
m1 mode of non-rotating
1
2
m mode of acceptor H O in (H O)
1
2
2 2
m mode of acceptor H O in (H O) É (H SO )
1
2
2
2 4
H bonded OH stretch of H SO in (H O) É (H SO )
2 (H4 SO ) 2 2
2 4
free OH stretch of H SO in
2 4
2 42
m mode of donor H O in (H O)
1
2
2 2
H bonded OH stretch of H SO in (H O) É (H SO )
2 4
2
2 4
m (b) antisymmetric OH stretch of H SO monomer
4
m9(a) symmetric OH stretch of H SO2 monomer
1
2 4
m mode of donor H O in trimeric (H O) species
1
2
2 3
polymeric water bands
bonded OH stretch of H SO in (H SO )
2 4
2 42
2774 (vvw)
2656.5 (vw)
2438.6 (vw)
1660, 1636, 1624, 1608, 1573, 1556 (s)
1618 (w, sh)
1610.5 (s)
1601.7 (m)
1599.7 (w)
1593 (s)
1590 (w)
1452.4 (vs)
1449.1 (w)
1442 (w, broad)
1432.5 (w)
1417.4 (w)
1408.9 (w)
2m (e@) SO overtone
3
3
m (a) ] m (b) H SO combination
2 combination
4
m2(a@ ) ] m10(e@) SO
1 1
3
3
m bending mode of rotating H O monomer
2
2
m bending mode of donor H O in (H O)
2
2
2 3
m bending mode of donor H O in (H O)
2 O)
2
m2 bending mode of acceptor 2H O in (H
m2 bending mode of acceptor H 2O in (H2O)3É (H SO )
2
2
2
2 4
m bending mode of acceptor H O in (H O)
2 H O monomer
2 2
m2 bending mode of non-rotating
2 of H SO monomer
m2 (b) antisymmetric OxSxO stretch
10
2 SO
4 )
antisymmetric
OxSxO stretch in (H O) É (H
2
2 4
(H O) É (H SO )
2 2
2 4
antisymmetric OxSxO stretch in (H SO )
2 42
metastable (H O) É (H SO ) species
2
2 4
metastable (H O) É (H SO ) species
2
2 4
1371, 1367, 1355, 1351 (w)
1385.1 (m)
1389.8 (m)
1392.4 (m), 1396.5 (w)
site splitting of m antisymmetric stretch of SO
3
2
m (e@) mode of monomeric argon isolated SO
3
3
m (e@) mode of dimeric (SO )
3
32
m (e@) mode of SO perturbed by H O neighbours
3
3
2
1268.5, 1265.8 (m)
1216.1 (s)
1211 (w)
1204.8 (m)
1192 (w)
1156.9 (w)
1135.9 (w)
1080.2 (vw, broad)
1043.9 (w)
1037 (w)
1028 (w)
1023 (w)
956 (w)
950 (w)
929, 920 (w)
892.5 (w)
887 (w)
881.7 (vs)
869.4 (w)
847.5 (m)
834 (m)
831.4 (m)
827.1, 815 (m)
(H O) É (H SO )
2 n
2 4m
m (a) OxSxO symmetric stretch of monomeric H SO
2
2 4
OxSxO symmetric stretch in (H SO )
2 42
OxSxO symmetric stretch in (H O) É (H SO )
2
2 4
(H O) É (H SO )
2 4 antisymmetric bend of H SO
m 2(b) 2Sw(OH)
m11(a) Sw(OH) 2symmetric bend of H SO 2 4
3
metastable
(H 2O) É (H SO ) É (SO ) 2 4
metastable (H2O)n É (H2SO4)m É (SO3)p
metastable (H2O)n É (H2SO4)m É (SO3)p
n
2 by4 mH O neighbours
3p
2m (e@) of SO 2perturbed
4
3
2
2m (e@) of SO
2m4(e@) of SO3 in metastable (H O) É (H SO ) É (SO )
2m4(e@) of SO3 in metastable (H2O)n É (H2SO4)m É (SO3)p
3 )
2 n
2 4m
3p
(H4 O) É (H SO
2
n
2
4
m
Sw(OH) antisymmetric stretch of (H SO )
2 SO )
Sw(OH)2 antisymmetric stretch of (H 2O) É4(H
2
2 4
m (b) S(wOH)
antisymmetric stretch2 of H SO
12
2
Sw(OH)
antisymmetric
stretch of (H SO )2 4
2 )
Sw(OH)2 symmetric stretch of (H O) 2 É (H4 SO
2
2
2
2
Sw(OH) symmetric stretch of (H O) É (H SO )4
2
2 4 H SO
m (a) S(wOH)
symmetric stretch 2of monomeric
2
2 4
m4(a) of metastable
(H O) É (H SO )
4
2
2 4
576.2 (s)
558.0 (s)
554 (w)
548.1 (s)
528 (m), 531 (sh)
527.2 (m)
517.2 (w)
505.7 (m)
490.3 (m)
486.2 (w)
481.3 (w)
(H O) É (H SO )
m monomeric H SO
m 2(b) nSO 2rock4 of
13 rock 2of (H O) É (H SO )
2 4
SO
2 OxSxO 2bend of2 monomeric
4
m (a)
H SO
4 neighbours
m5(e@) SO in-plane bend perturbed by 2H O
3
m4(e@) in plane
bend of monomeric SO 2
3
m4 of SO trapped in argon
2 O) É (H
2 SO )
(H
2 A) nout-of-plane
2 4 m bend of monomeric SO
m (a
2 out-of-plane bend of SO perturbed 3by H O neighbours
m2(aA)
2 out-of-plane bend of dimeric
3
2
m2(aA)
(SO )
2 2
32
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
829
Table 2 Reversible temperature e†ects on IR bands of H SO , H O and SO vapor mixtures trapped in solid argon at 18 K and cooled to 5 K
2 4 2
3
(] \ intensity increase ; [\ intensity decrease)
frequency
change on cooling
3731.5 (s)
3729.9 (s, sh)
3574.2 (m)
3572.2 (s)
3557 (w)
3546.1 (s)
3516.7 (s), 3519 (s, sh)
1601.9 (s)
1617.5 (m)
[
]
]
[
]
]
[
[
]
phase IR absorptions at 1240 and 1170 cm~1.14 Based on
their monomeric behavior (intensity being reduced by temperature cycling), low intensity and their width (2.5È2.8 cm~1)
being twice that of the OH and OxSxO stretchings, the
assignment of both to bending modes is very reasonable. The
Raman band for the liquid,14 recorded at exactly the same
position as our lower IR frequency value, supports its assignment to the m (a) symmetric SwOwH bending mode. The
3
assignment of the two strong and narrow (1.6È1.7 cm~1)
bands at 881.7 and 831.4 cm~1, very close to the gas phase
values, to the m (b) Sw(OH) antisymmetric and m (a)
12
2
4
Sw(OH) symmetric stretches respectively, is in accord with
2
previous studies.6,7 The same holds for the strong 558 and
548.1 cm~1 bands assigned to the m (b) SO rock and m (a)
13
2
5
OxSxO bend, respectively. In summary, only for the OH
stretch mode with its large amplitude anharmonic hydrogen
motion is a considerable (1.2%) red shift found in the argon
matrix as compared to the gas phase value.
Unlike gaseous SO samples which also consist of poly3
meric species (e.g. S O ),8 hydrogen bonded dimeric (H SO )
3 9
2 42
species are mainly formed by surface di†usion upon deposition. Of the species trapped in these experiment, only H O (or,
2
to a lesser extent, its dimer) may be expected to di†use
through the matrix as a result of annealing processes. The
probability of dimer production due to warming is therefore
low, and only small temperature induced intensity changes are
expected for bands attributed to them. In the assigning of
(H SO ) bands, the very slight dependency of their intensity
2 42
on the water content of the deposited gas is an additional
helpful characteristic. However, these bands are more intense
in an 18 K deposition than in a sample produced by deposition at 5 K. In the OH region two weak bands at 3577 and
3180 cm~1 correspond to these criteria and are assigned to
m(OH) of free and bonded (H SO ) dimers, respectively. The
2 42
frequency di†erence between the m (b) antisymmetric OH
9
stretch of the monomer and the bonded dimer m(OH) position
is about 65% of the parallel di†erence between the band positions in free monomer and in solid H SO . The analogous
2 4
ratio for H O monomer, dimer and solid is about 30%.35
2
In analogy to the H SO monomer, the 1432.5 and 1211
2 4
cm~1 bands are assigned to the OxSxO antisymmetric and
symmetric stretching modes, respectively, and the 892.5 cm~1
and 869.4 cm~1 bands to the bonded and non-bonded
Sw(OH) antisymmetric stretching modes.
2
3.2 H O species
2
H O species appear in all the low temperature deposited
2
argon matrices. Temperature cycling to above 15 K induces
signiÐcant intensity changes for these mobile species. The
assignments of (H O) species, observed in these experiments,
2 n
are summarized in Table 1 and below, where references are
given to some of the many studies of which they are the
subject.
Rotating H O monomers in solid argon : 3776.5, 3756.5,
2
3711.2 cm~1 (m antisymmetric stretch), 3669.9, 3653.5 cm~1
3
830
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
frequency
1612.3
1599.7
1450.6
1441.3
835.9
834.1
549.3
550.5
(s), 1611 (w, sh)
(s)
(s, sh)
(w)
(m)
(m)
(m)
(m)
change on cooling
]
[
[
]
]
[
[
]
(m symmetric stretch) and 1660, 1636, 1624, 1608, 1573, 1556
1
cm~1 (m bending mode).35h40
2
Non-rotating argon isolated H O monomers : 3735.7 cm~1
2
(m antisymmetric stretch), 3638 cm~1 (m symmetric stretch),
3
1
1590 cm~1 (m bending mode, shifting to 1589.4 cm~1 upon
2
temperature cycling).35h40 These frequencies are almost identical to those recorded for pure H OÈAr samples. Their rela2
tive intensity is higher in the more concentrated samples, in
agreement with the report that non-argon neighbors suppress
the H O vibration rotation lines.35
2
(H O) dimers : (3725.7, 3708), (3633, 3574), (1593, 1610.5)
2 2
cm~1, are the m , m and m modes of acceptor and donor
3 1
2
H O moieties of the open chain dimer, respectively.36h47
2
(H O) trimer bands : 3700 cm~1, m antisymmetric stretch
2 3
3
(donor H O moiety) ; m symmetric stretch, 3527 cm~1
2
1
(acceptor), 3516 cm~1, 3514 cm~1 (donor) ; m bending mode,
2
1618 cm~1 (donor), 1601.7 cm~1 (acceptor).35,48
Finally, (H O) polymeric water bands : 3372 and 3324
2 n
cm~1.35
3.3 (H O) Æ (H SO ) complexes
2 n
2 4m
As mentioned previously, mixed polymeric species are unlikely
to be formed in the gas phase, but they may be formed upon
deposition or by di†usion of H O, the lightest and hence the
2
most mobile species in the samples studied. The latter process
is also expected to occur as a result of temperature cycling of
the deposited Ðlm. As higher deposition temperatures have the
e†ect of markedly enhancing the intensity of the bands of
these polymers, it is safe to assume that the di†usion process is
responsible for most of their formation.
At least two open chain as well as one cyclic dimeric species
may be envisioned for (H O) É (H SO ). In the open chain
2
2 4
species the water molecule may act either as a proton acceptor, H OÉ É ÉHÉ É ÉOSO OH, or as a proton donor,
2
2
HOHÉ É ÉOSO(OH) , according to the part of the H SO
2
2 4
moiety to which it attaches. Recent ab initio calculations
support the existence of energy minima for both of these structures.49 The cyclic dimer would have both H-atoms of H O
2
coordinated to the two free SxO bonds of H SO . Tem2 4
perature cycling induces both a transformation of the less
stable structures to the most stable one, and the formation, via
H O di†usion, of trimeric (H O) É (H SO ) and polymeric
2
2 2
2 4
(H O) É (H SO ) species, with m O 2 as noted above. These
2 n
2 4m
two processes have opposing e†ects on the mixed dimer concentration so that the intensities of its bands do not change
signiÐcantly by temperature cycling. Several structural possibilities also exist for the trimeric and polymeric
(H O) É (H SO ) species. In addition to temperature cycling,
2 n
2 4m
distinctions between these species are indicated by varying
H O : H SO ratios. Related previous spectral information is
2
2 4
of solid hydrates of CaSO 50 and of H SO in aqueous solu4
2 4
tions,51 dealing with the relevant ionic species. IR data
regarding molecular (H O) É (H SO ) species is limited to
2 n
2 4m
several bands recorded and assigned by Bondybey and
English in their studies of SO in neon matrices.18
3
The mixed dimer (H O) É (H SO ) is formed even without
2
2 4
added water vapor due to the gas phase equilibrium :
H SO (g) % SO (g) ] H O(g), detailed above. The spectral
2 4
3
2
assignment of its H O moiety bands is not straightforward, as
2
previously
discussed
for
similar
H O
containing
2
dimers :42,45,52,53 in the m region two weak peaks are identi3
Ðed for it at 3745.1 cm~1 and 3723 cm~1, Fig. 2(a). These
values are very similar to three absorptions assigned to the m
3
band of (H O) by Ayers and Pullin for H O in solid argon
2 2
2
samples.40 The splitting was explained in terms of combinations of m of the proton acceptor water molecule of
3
(H O) and large amplitude dimeric torsional motions, which
2 2
modulate the dipole moment changes of the antisymmetric
stretch but not those of the m and m modes. This phenome1
2
non was also found for the proton acceptor H O of the
2
(H O) É (HCl) complex.45 The close analogy to the present case
2
leads to the assignment of the two above mentioned bands to
the m antisymmetric stretch mode of (H O) É (H SO ). The
3
2
2 4
non-split m symmetric stretch of H O in (H O) É (H SO )
1
2
2
2 4
emerges at 3640 cm~1 as a shoulder of the m band of non1
rotating water monomer at 3638 cm~1. Similarly, the 1599.7
cm~1 shoulder, Fig. 2(b), is assigned to the m bending mode
2
of this mixed dimer.
We now consider the OH modes of the H SO moiety of
2 4
the mixed dimer. The 3572.6 cm~1 band, which lies very close
to the 3574 cm~1 H-bonded absorption of the (H O) , is
2 2
attributed to the analogous bonded OH mode of H SO in
2 4
the mixed dimer. The intensity exchange upon temperature
cycling between the 3566.7 cm~1 OH line of non-bonded
H SO and the 3572.6 cm~1 absorption of (H O) É (H SO ) is
2 4
2
2 4
clearly demonstrated in Fig. 3.
There is a strong similarity in frequencies between the H O
2
moiety in (H O) É (H SO ) to those of the hydrogen acceptor
2
2 4
H O molecule of (H O) , i.e. 3745.1 and 3723 cm~1 vs. 3724.7
2
2 2
cm~1, 3640 cm~1 vs. 3633 cm~1, and 1599.7 cm~1 vs. 1593
cm~1. Likewise, the two bonded OH stretching modes of the
donor moieties, 3572.6 cm~1 vs. 3574 cm~1, indicate a similarity in the H-bonding, suggesting that the stable isomer is
the open chain H OÉ É ÉHOSO OH. This conclusion is also
2
2
supported by the ab initio calculations.49
In spectral proximity of the monomeric H SO absorptions,
2 4
several lines show a strong intensity dependence upon the
H O concentration. This is illustrated in Fig. 4, and may be
2
attributed to the H SO sub-unit of (H O) É (H SO ). Hence2 4
2
2 4
forth, this assignment is supported by the weak e†ects of temperature on their intensity, as discussed above. The 1449.1 and
1204.8 cm~1 bands are assigned to the (H O) É (H SO ) anti2
2 4
symmetric and symmetric OxSxO stretches, red shifted by
3.4 and 11.2 cm~1, respectively, from the analogous monomeric frequencies. The 887 and 847.5 cm~1 bands, with
respective blue shifts of 16.1 and 5.3 cm~1, from the parallel
H SO absorptions, are assigned to the antisymmetric and
2 4
symmetric Sw(OH) stretches of (H O) É (H SO ), respec2
2
2 4
tively. Of the lower frequency bands only the weak 554 cm~1
Fig. 2 (a) Detailed view of the H O antisymmetric stretching, m ,
2 O bending mode, m , region.
3
region. (b) Detailed view of the H
2 : H SO sample. 2
A, 400 : 1 Ar : H O sample ; B, 500 : 1 Ar
2
2 4
Fig. 3 Temperature cycling e†ect on the OH stretching region of
H SO trapped in solid argon. A, As deposited at 5 K ; B, after tem2 4 cycling to 25 K.
perature
line meets the required criteria and is attributed to the SO
2
rock of (H O) É (H SO ), red shifted by 4 cm~1 from the
2
2 4
monomer line. In summary, the following bands are attributed
to the mixed (H O) É (H SO ) dimer : 3745, 3723, 3640, 3572.6,
2
2 4
1599.7, 1449.1, 1204.8 ,887, 834 and 554 cm~1.
Fig. 4 E†ect of H O concentration on the IR spectra of H SO in
2 at 5 K. (a) The OH stretching region. 2(b) 4The
solid argon deposited
OxSxO symmetric stretching region. (c) The 1100È900 cmv1 region.
(d) The Sw(OH) antisymmetric and symmetric stretching region.
A, H O : H SO2 : Ar \ 2.5 : 1 : 1000 ; B, H O : H SO : Ar \
2
2 4
2
2 4
1 : 1 : 1000.
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
831
The weak bands at 1417.4, 1408.9 and 827.1 cm~1 also
show an intensity dependence upon both water and sulfuric
acid concentrations. Contrary to the previously discussed
(H O) É (H SO ) bands, their intensity decreases irreversibly
2
2 4
upon warming the sample or raising the deposition temperature. Fig. 5 shows the antisymmetric OxSxO stretching
region and one notes the increase of the 1449.1 cm~1 stable
(H O) É (H SO ) band versus the decrease of the 1417.4 and
2
2 4
1408.9 cmv1 bands in going from trace A to trace B. The
1417.4, 1408.9 and 827.1 cm~1 bands are therefore assigned to
unstable isomers of open chain species.
Other, broader absorptions showing an intensity dependence upon both H O and H SO concentrations demon2
2 4
strate polymeric behavior, i.e. irreversible intensity
enhancement and broadening with temperature cycling.
Several would only emerge upon warming of the 5 K deposited sample. We therefore ascribe them to (H O) É (H SO )
2 n
2 4m
species. According to their order of appearance during temperature cycling, the less broad bands at 3582, 1442, 1192 and
847.5 cm~1 are attributed to the (H O) É (H SO ) trimer
2 2
2 4
whose H O modes most likely are masked by (H O) and
2
2 n
(H O) É (H SO ) absorptions. The additional 9.4 cm~1 blue
2
2 4
shift of the bonded OH frequency from the (H O) É (H SO )
2
2 4
value indicates an H OÉ É ÉH OÉ É ÉHOSO OH structure rather
2
2
2
than an H OÉ É ÉHOSO OHÉ É ÉOH structure. Owing to the
2
2
2
negligibly small coupling between the two OH bonds of sulfuric acid, no such shift is expected for the latter conÐguration.
Broader and much less well deÐned bands are recorded at
1268.5, 1265.8 (sh), 1080, 1043.9, 1037, 929, 920, 576.2 and
505.7 cm~1. All show a dependence upon H O and H SO
2
2 4
concentrations and are therefore associated with higher
(H O) É (H SO ) mixed species. On the other hand, the
2 n
2 4m
group of weak bands around 1043 cm~1 loses intensity upon
temperature cycling or deposition at higher temperatures,
contrary to the majority of lines attributed to polymeric
species. Possibly the 1080, 1043.9, 1037 cm~1 bands belong
to some SO -containing unstable mixed polymers
3
(H O) É (H SO ) É (SO ) , in which the m (a ) mode of SO
2 n
2 4m
3p
1 1
3
Fig. 5 Temperature cycling e†ect on unstable H O É H SO bands.
A, As deposited at 5 K ; B, after temperature cycling2to 30 2K. 4
832
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
becomes IR active in the cluster. Several similar frequencies
were also reported for SO in solid neon spectra18 and
3
ascribed to (H O) É (H SO ) species, e.g. 928.8 and 924.9 cm~1,
2
2 4
and to SO polymers, e.g. 1090.8 and 1068.6 cm~1.
3
We Ðnally emphasise : there is no spectral evidence of ionic
species formed to indicate that a proton transfer has taken
place between the H SO and H O moieties at temperatures
2 4
2
below 30 K, whereas ionic species resulting from such transfer
may well predominate in the heterogeneous atmospheric
nucleation processes at higher temperatures.
3.4 SO species
3
SO monomers (planar, D symmetry) are present in all
3
3h
samples, as implied by the gas phase equilibrium between
H SO , SO and H O. The three IR active fundamentals,
2 4
3
2
recorded previously in the gas phase and in Ne, Ar, Xe and
O matrices,7h10,18h20 appear in our 5 K argon matrix as
2
singlets at 1385.1 cm~1, m (e@) antisymmetric stretch, 527.2
3
cm~1, m (e@) in-plane bend, and 490.3 cm~1, m (aA) out-of4
2 2
plane bend. These values deviate only slightly (\0.5 cm~1)
from those of Bondybey and English,18 possibly due to long
range inÑuences of the other guest species in the matrices
studied here. Bands of mixed complexes between SO and the
3
other species in the matrices were also observed. Vibrational
frequencies of the (H O) É (SO ) complex were reported for
2
3
neon,18 for O 19 and for N 20 matrices. Only in the nitrogen
2
2
matrix does the complex bond activate the IR forbidden m (a@ )
1 1
mode of SO . A low concentration matrix study of a ca.
3
1 : 1 : 1000 H O : SO : Ar mixture opposes the possibility of
2
3
(H O) É (SO ) formation.16 However, an ab initio calculation
2
3
on the (H O) É (SO ) complex reports a complexation energy
2
3
of [33 kJ mol~1 for an electrophilic charge transfer bond
between the sulfur atom and the electron donating water
oxygen.2 Morokuma and Muguruma4 suggest a cyclical
ternary (H O) É (SO ) complex with a stabilization energy of
2 2
3
about twice that of the binary complex. Geometries of such
weakly bound complexes are expected to be sensitive to their
environment, possibly explaining its stabilization only in the
cylindrical N Ðeld.20 We found no H O absorptions to be
2
2
associated with (H O) É (SO ) complex formation by relating
2 m
3n
their temperature cycling induced intensity changes with those
of the SO bands.
3
In a “ normal Ï 5 K deposition, the 1385.1 cm~1 m (e@) SO
3
3
monomer band is accompanied by a triplet of weak features at
1389.8, 1392.4 and 1396.5 cm~1, Fig. 6(a). Raising the deposition temperature or depositing with increased water concentration or warming the sample to 34 K produce spectra in
which the intensities of the 1385.1 cm~1 absorption and of the
1389.8 and 1392.4 cm~1 lines are about equal, Fig. 6(a). The
latter intensity distribution resembles better the N and O
2
2
matrix spectra,19,20 which demonstrate doublet absorptions in
this region. In these studies the two bands observed were
associated with m (e@) of the SO sub-unit of (H O) É (SO ),
3
3
2
3
Fig. 6 Deposition temperature e†ect on the m (e@) antisymmetric
stretch and on the m (e@) bending mode of SO . 3(a) m (e@) SO anti3
symmetric stretch. (b)4m (e@) SO bend. A, Deposition
at35 K ; B,3depo4
3
sition at 18 K.
split by a lowered symmetry upon complex formation, a phenomenon previously reported for other SO containing
3
species.54 We found, however, that the 1389.8 cm~1 band
intensity depends more on SO concentration, whereas the
3
1392.4 and 1396.5 cm~1 band intensities are more H O
2
dependent. We therefore assign the 1389.8 cm~1 band to m (e@)
3
of (SO ) species, whereas the 1392.4 and 1396.5 cm~1 bands
32
are attributed to m (e@) of SO monomer perturbed by its H O
3
3
2
neighbours. Moreover, if the observation of the 1389.8 and
1392.4 cm~1 lines were to represent a symmetry lowering
splitting due to complex formation, the same symmetry lowering should have an e†ect on the m (SO ) mode activity ; this
1
3
was not observed.
Fig. 6(b) shows the m (e@) SO in-plane bend appearing at
4
3
527.2 cm~1 in a 5 K deposited sample and being Ñanked by a
band at 528 and a shoulder at 531 cm~1, both growing in for
water-rich samples or 18 K depositions. As above, these lines
are also ascribed to SO dimers and monomers perturbed by
3
H O.
2
Also in the region around the 490.3 cm~1 out-of-plane
bending mode two additional weak bands are recorded at
486.2 and 481.3 cm~1. As can be seen in Fig. 7, the 486.2
cm~1 band is a water concentration dependent feature with its
intensity increasing upon temperature cycling and thus is
ascribed to SO perturbed by H O neighbours. The second
3
2
line, increasing when warmed to 20 K and diminishing at
higher temperatures, is attributed to the (SO ) dimer. In the
32
similar doublet, recorded in the SO in solid neon study,18 the
3
assignment is reversed. Thus for all IR active SO modes,
3
additional water concentration dependent bands are recorded.
However, the suggested 1 : 1 (H O) É (SO ) complex,2 dis2
3
cussed above, would indicate this e†ect to be most prominent
in the latter m (aA) mode, contrary to our experimental observ2 2
ations. An analogous situation exists for H O in experiments
2
with solid CO,55 where a CO band was attributed to the e†ect
of neighbouring H O molecules but no bands could be assigned
2
to the speciÐc 1 : 1 (CO) É (H O) complex.
2
Two sharp, weak lines at 1023 and 1028 cm~1 are SO con3
centration dependent, with the latter showing a slight H O
2
concentration dependence. They are assigned to the IR active
Fig. 7 E†ect of H O concentration on the m (aA) out-of-plane
2 in solid argon. A and B samples
2 2 recorded as
bending mode of H SO
4
deposited at 5 K. H2O concentration
ratios, [A] : [B] \ 1 : 2.5.
2
overtone components of m (e@) of SO and of water perturbed
4
3
SO , observed at 527.2 and 528 cm~1, respectively. The very
3
weak 2438.6 cm~1 band, resembling previously recorded SO
3
absorptions at 2447.8 cm~1 in neon matrix18 and at 2443
cm~1 in the gas phase,56 is attributed to the m (a@ ) ] m (e@)
1 1
3
combination band. Another very weak band, recorded at 2774
cm~1, and resembling the gas phase and neon matrix bands at
2773 and 2776.7 cm~1, respectively,18 is assigned to the 2m (eÏ)
3
SO overtone. Finally, a group of very weak lines at 1371,
3
1367, 1355, and 1351 cm~1 and a very weak absorption at 517
cm~1 are assigned to m and m modes of monomeric SO in
3
2
2
solid argon.20 SO formation may result from SO oxidation
2
3
reactions with experimental system parts.
In summary, the only SO mode exhibiting spectral inÑu3
ences of neighbouring water molecules is the antisymmetric m
3
mode, and even more signiÐcant, no H O bands may be
2
associated with a formation of SO complexes. We therefore
3
assign the m (SO ) satellite bands to SO “ perturbed Ï by
3
3
3
neighbouring water molecules (Table 1) and not to complex
formation.
3.5 Depositions at 18 K and temperature reversible
phenomena
Samples with increased water content, produced by premixing
the argon with 0.4% of H O, and deposited at a higher tem2
perature, e.g. 18 K as shown in Fig. 1 (curve B), reveal marked
di†erences compared with those formed by a “ normal Ï 5 K
deposition. In addition, several bands of such samples show
prominent reversible intensity changes when cooled to 5 K
and rewarmed to 18 K (in contrast to the irreversible adduct
formation changes induced by annealing a 5 K deposited
sample to 18 K).
The Ðrst characteristic feature of such samples is a prominent enhancement of the band peaking at 3731.5 cm~1 with a
shoulder at 3739 cm~1 as seen in Fig. 8(a). This peak replaces
the 3735.7 cm~1 of the antisymmetric stretch of non-rotating
H O monomers in the 5 K deposited sample. While the band
2
may, in part, be due to the higher concentration of the nonrotating H O molecules, its large intensity is due to the
2
m (H O) mode of the increased amount of the (H O) É (H SO )
3 2
2
2 4
complex. The parallel m (H O) mode appears as a strong
1 2
doublet at 3640 and 3642.5 cm~1, compared with a weak
shoulder at 3640 cm~1 at 5 K. The very strong trimeric
(H O) doublet at 3519 and 3517 cm~1 is the only prominent
2 3
polymeric water line, Fig. 2. As expected, the monomeric and
dimeric H SO absorptions are almost identical to those
2 4
recorded for the 5 K deposited sample, whereas the
(H O) É (H SO ) bands at 3640, 3572.2, 1599.7, 1450.7 and
2
2 4
887.3 cm~1 are strongly enhanced. Some of the latter show
reversible intensity changes with temperature and are listed in
Table 2, with the spectral e†ects demonstrated in Fig. 8. Additional such bands (not listed in the Table) represent population dependent vibrationÈrotation transitions of monomeric
H O (3776.5, 3756, 3711, 3669.8, 3653.5, 1660.4, 1636, 1624,
2
1608, 1573 and 1556 cm~1).36h38
The reversible changes in the (H O) bands are : (i) a
2 3
decrease of intensity upon cooling to 5 K in the 3519 and
3517 cm~1 doublet, Fig. 8(c) ; (ii) a decrease of intensty in the
1601.9 cm~1 band in the m (H O) mode region [Fig. 8(d)] ;
2 2
and (iii) a parallel increase of the 3557 cm~1 band and especially of the 3546 and 1617.5 cm~1 lines. These changes
cannot represent a simple temperature dependent equilibrium
between two species, as it is not evident in a warming cycle
starting and ending at 5 K. A similar situation was found by
Fredin et al.57,58 for the (D O) dimer in solid nitrogen and
2 2
was related to a reversible IR light induced transformation.
We propose an alternative explanation in the present case,
noting that the bands of samples deposited at 5 and 18 K are
only somewhat similar and certainly not identical to each
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
833
Fig. 8 Reversible temperature cycling e†ects on the spectrum of (H SO ] H O) vapor mixture trapped in an argon matrix. (a) On the 3730
2 4
2
cm~1 band. (b) On the 3572 cm~1 band. (c) On the 3547 cm~1 and 3516 cm~1 bands. (d) On the m (H O) bending mode. (e) On the anti2 2
symmetric OxSxO stretching mode. (f) On the S(wOH) stretch mode. (g) On the 548 cm~1 OxSxO bending mode. A, As deposited at 18 K ;
2
B, cooled to 5 K ; C, rewarmed to 18 K.
other. We tentatively suggest that the forms of the H O trimer
2
and of the (H O) É (H SO ) complex (discussed below) trapped
2
2 4
at the two window temperatures have a di†erent conÐguration ; the 18 K deposited form being capable of a thermally
induced reversible change, e.g. a transition from an open to a
more closed structure. Fredin et al.57,58 claim a cyclic structure based upon only three frequencies observed by them,
3707.2, 3516 and 1602.3 cm~1, which are similar to those
obtained by us for the 18 K deposition. Bentwood et al.35
argue for an open chain conÐguration from the observation of
lines at 3700, 3695, 3528, 3516, 1620 and 1602 cm~1, which
bear a similarity with our 5 K deposition bands at 3527 and
3516 cmv1 but do not agree with the high frequency band
positions recorded in our experiments cooling from 18 to 5 K,
i.e. the prominent band growing in at 3546 cm~1 and the
weak line at 3557 cm~1.
The (H O) É (H SO ) related features at 3731.5, 3572.2,
2
2 4
1599.7, 1450.6, 834.1 and 549.3 cm~1, recorded for an 18 K
deposition, decrease in intensity upon cooling to 5 K, with a
parallel increase of the peaks at 3729.9, 3574.2, 1612, 1441.3,
835.9 and 550.5 cm~1, Fig. 8. Except for the m (b) anti10
symmetric OxSxO stretch, the H SO bands are only
2 4
slightly shifted in this process. Of the H O modes, only the m
2
2
bending mode shifts signiÐcantly from 1599.7 to 1612 cm~1
for the 18 K deposited layer upon cooling to 5 K. The former
H O wavenumber resembles that of the acceptor sub-unit in
2
the dimer (H O) , 1601.7 cm~1,35 whereas the latter falls
2 2
exactly on the position of the m bending mode of the donor
2
moiety. This may indicate a reversible change in the bonding
between H O and H SO in the complex. At 18 K the H O
2
2 4
2
moiety is attached to the OH of the acid, whereas at 5 K, via a
simple rotation around the H-bonded OwH bond, it
approaches the OxSxO oxygen and acts as an H-donor ; the
width of the m band of the water molecule might, in part,
3
mask a wavenumber change in it caused by this transformation.
Conclusion
The vapors over concentrated sulfuric acid were trapped in
solid argon and we have identiÐed spectral features from the
isolated H O, SO and H SO monomers as well as of dimers
2
3
2 4
834
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
and complexes consisting of H O and H SO moieties. Our
2
2 4
spectral results do not support the existence of (H O) É (SO )
2 n
3m
discrete complexes and there are no indications of ionic
species being formed by proton transfer from sulfuric acid at
temperatures below 30 K.
The absorption bands of the (H O) É (H SO ) complex
2
2 4
show reversible variations with temperature indicating several minima on the potential surface, and that the more
stable form of this complex is the open chain
H OÉ É ÉHOwSO wOH. Spectral features of higher com2
2
plexes were also observed and we suggest the existence
of an (H O) É (H SO ) complex where sulfuric acid acts as
2 2
2 4
proton donor to only one of the water molecules,
H OÉ É ÉH OÉ É ÉHOwSO wOH.
2
2
2
A. L. acknowledges a visiting scientist grant from the Norwegian Research Council.
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