IR spectrum of H SO species trapped in low temperature solid CO
2 4
and in CO containing matrices
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
The IR spectrum of the vapors above neat sulfuric acid trapped in CO enriched argon matrices was studied and compared to
their IR spectra in pure CO matrices. We assigned vibrational modes of (OC) É (H SO ), (OC) É (H O) É (H SO ), (OC) É (H SO )
2 4
2
2 4
2
2 4
and (OC) É (SO ) species isolated in argon matrices in all relevant spectral regions. Bonding between the moieties of the mixed
3
complexes is indicated. These species are formed by surface di†usion within the deposited layer and do not represent a vapor
phase equilibrium. No spectral evidence was found for any binary (H O) É (SO ) or ternary (CO) É (H O) É (SO ) complexes or
2 m
3n
l
2 m
3n
of ionic species, to indicate a proton transfer below 30 K between the H SO and H O moieties in either CO enriched or pure
2 4
2
CO matrices.
1 Introduction
We recently reported the IR spectra of H SO vapors trapped
2 4
in solid argon matrices1 as part of a continuing e†ort centered
on the vibrational spectroscopy of low temperature icy particles and frozen gas mixtures of relevance to environmental
and interstellar chemistry.2h6 These experiments showed the
existence of a variety of stable and metastable
(H O) É (H SO ) molecular species and, on the other hand,
2 n
2 4m
revealed the inability of an argon matrix to stabilize any
(H O) É (SO ) complexes, the assumed intermediates in the
2 m
3n
reaction chain forming H SO from SO in the vapor
2 4
2
phase.7h14
Here we study the spectrum of H SO vapors in argon
2 4
matrices enriched with an additional atmospheric trace component, CO, with the aim of investigating complexations
involving the latter. For completeness of CO bonding with
SO and H SO , a matrix isolation study of H SO in solid
3
2 4
2 4
CO is described. Previous spectral evidence for CO complexes
with H SO and SO is limited to a partial band assignment
2 4
3
of (OC) É (SO ) (produced by OCS photolysis in an oxygen
3
matrix).7
2 Experimental
Sulfuric acid was supplied by Prolabo (p.A.), and the Ar (5.7)
and CO (4.7) gases by AGA. The experimental details were
given previously.1 In short, the vapors for deposition were
taken from a drop of sulfuric acid placed in a furnace consisting of a quartz tube ending in a nozzle, wrapped by a heating
coil.
CO and argon matrix gases, either pure or premixed, were
passed through the H SO containing nozzle and warmed to
2 4
a maximum of 36 ¡C. These samples (typically of 1 : 4È1 : 1000
ratios) were sprayed onto a CsI window cooled to 5 K by an
Air Products HS-4 Heliplex cryostat employing two HC-4
MK 1 compressor modules. Temperatures were controlled by
a Lake Shore model 330 temperature controller using Si diode
sensors. Typical deposition times were of 1È3 h depending
upon sample dilution, while deposition rates were of several
mmol h~1. Temperature cycling of the samples was conducted
by a slow warming (1 K min~1) up to 38 K for argon matrix
and 33 K for CO matrix experiments, followed by quick
recooling to 5 K. IR spectra were recorded on a Bruker IFS
88 instrument employing a DTGS detector and coadding
32È128 scans at resolutions of 0.25È2 cm~1.
3 Results and Discussion
3.1 IR spectrum of gaseous H SO trapped in a CO matrix
2 4
The IR spectrum of H SO vapors trapped in CO matrices
2 4
demonstrates absorptions due to H SO , SO , and H O mol2 4
3
2
ecules as well as to solid CO. The normal modes involving the
OH group are expected to di†er signiÐcantly from those of
argon isolated molecules. Many of the H O, SO and H SO
2
3
2 4
absorption bands reÑect multiple trapping sites in the CO
solid.
Spectral data, summarized in Table 1, will be discussed
according to the absorbing components. In the text below,
bandwidths (fwhm, cm~1) are given in parentheses where signiÐcant and practicable (considering band intensities and
resolved band structure).
3.1.1 Pure H O species. Assignments are similar to those
2
previously reported for H O trapped in solid CO.5,15 Rele2
vant spectra and temperature cycling e†ects are shown in Fig.
1 and 2(a).
3.1.1.1 Monomeric H O trapped in a stable solid CO site.
2
These intense, sharp (1.5È2.5 cm~1) bands [3707.7 cm~1 (l ),
3
3617.2 cm~1 (l ) and 1601.1 cm~1 (l bending)] decrease with
1
2
temperature cycling, but persist to close to the CO sublimation point (34 K).
3.1.1.2 Monomeric H O in unstable CO sites. These appear
2
at 3700, 3613 and 1595 cm~1, as shoulders to the stable site
absorptions, to disappear upon cycling to 20 K.
Neither site shows evidence for any rotational freedom of
H O monomers.
2
3.1.1.3 Dimeric (H O) bands. These 5 cm~1 broad fea2 2
tures, at 3674.7, 3597.6, 3494.1, 1637.5 and 1608.4 cm~1,6,15
appear as weak bands in the 5 K deposit and increase during
temperature cycling, up to 26 K. Further warming a†ects
intensity decreases.
3.1.1.4 T rimeric (H O) bands. These 5 cm~1 broad fea2 3
tures at 3685.5, 3649.6, 3438.3, 1652.8, and 1630 cm~1 are the
Ðrst to appear following the dimeric ones, usually at 20È30
K.15
3.1.1.5 Polymeric H O bands. The bands at 3660, 3361.1
2
and 3229.2 cm~1 of polymeric H O species,15 are the broadest
2
in the spectrum. They grow with temperature cycling and at
34 K change into bands of amorphous ice.
J. Chem. Soc., Faraday T rans., 1998, 94(16), 2277È2286
2277
Table 1 Band positions (cm~1) and assignments of species of H SO vapors trapped in a CO matrix at 5 K
2 4
frequency
3707.7 (vs)
3700 (w, sh)
3685.5 (w)
3674.7 (m)
3660 (w)
3649.6 (m)
3617.2 (m)
3613 (w, sh)
3597.6 (m)
3551.5 (w)
3494.1 (m)
3480 (w)
3438.3 (w)
3361.1 (s)
3320.8 (s)
3309.5 (m)
3293.5 (s)
3229.2 (m)
3182 (w)
2209 (s, broad)
2138 (vvs)
2146.9 (vs)
2112.4 (m)
2091.8 (vs)
2088.3 (s)
2039.9, 2027.5,
2002.8, 1995.8 (w)
1652.8 (w)
1637.5 (m)
1630 (w)
1608.4 (m)
1604.4 (w)
1601.1 (vs)
1595 (w, sh)
1456.6 (vw)
1439.9 (vs)
1436.7 (vs, sh)
1423 (w)
1419.2 (w)
1404.8 (vw)
1399.1, 1396.9 (s)
1394.2 (s)
1392.6 (w), 1390.3 (sh)
1387.4 (w)
1381.2 (sh)
1379, 1376.6 (w)
1347.1 (w)
1267.6 (w)
1229.5 (w)
1220.5 (m)
1207 (sh, s)
1202.7 (vs)
1193.6 (m, sh)
1174 (w)
1151 (w)
1060 (w)
1047.7 (w)
974.3 (w)
941.7
908.7 (vs)
897.1 (w)
882 (vw)
877 (vw)
861 (s)
856.1 (s)
836 (w)
829 (vw)
577.6 (m)
566.3 (m)
554.2 (s)
533.1, 531.4 (m)
530 (w, sh)
516.2 (vw)
503.5 (w)
473 (s)
2278
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
assignment
l antisymmetric stretch of monomeric H O
l3 antisymmetric stretch of monomeric H 2O, unstable site
3
2
l of (H O)
l3 of (H2O)3
3
2 2
polymeric water band
l of (H O)
3
2 3
l symmetric stretch of monomeric H O
l1 symmetric stretch of monomeric H 2O, unstable site
1
2
l of (H O)
1
2 2
l of (H O) É (H SO )
1
2
2 4
l of (H O)
l3 of (H2O)2
1
2 3
l of (H O)
1
2 3
polymeric (H O)
2 n
l (b) antisymmetric OH stretch of H SO unstable site
9
bonded
OH stretch of (H O) É (H SO 2) 4
2
2 4
l (b) antisymmetric OH stretch of H SO , stable site
9
2 4
(H O)
2 as stretch of bonded of (H SO )
l (OH)
1
2 42
combination
band of l(CO) ] translations
and rotations
l(CO) stretch of solid 12C16O
l(CO) perturbed by H O species
2
l(CO) stretch of 12C17O
l(CO) stretch of 13C16O
l(CO) stretch of 12C18O
not assigned
l bending mode of (H O)
2
2 3
l bending mode of (H O)
l2 bending mode of (H 2O)2
l2 bending mode of (H 2O)3
l2 bending mode of (H 2O)2É (H SO )
2
2
2 4
l bending mode of monomeric H O
2
2
l bending mode of monomeric H O, unstable site
2
2
unstable (H SO )
2 42
l (b) SxO antisymmetric stretch of monomeric H SO
10
2
2 4
l (b) SxO antisymmetric stretch of H SO monomer, unstable site
10
2
2 4
(H SO )
2 42
antisymmetric SxO stretch of (H O) É (H SO )
2
2
2 4
(SO ) /(SO )
33
3n
l (e@) antisymmetric stretch of SO , stable CO site
3
3
l (e@) antisymmetric stretch of SO , unstable CO site
3
3
l (e@) antisymmetric stretch of SO , H O enriched stable CO site
3
3
2
l (e@) antisymmetric stretch of (SO )
3
32
l (e@) antisymmetric stretch of SO , H O enriched unstable CO site
3
3
2
l (e@) antisymmetric stretch of SO , H O enriched stable CO site
3
3
2
l (e@) antisymmetric stretch of (SO )
3 2 mode of monomeric H SO
l3 (b) SwOH antisymmetric bending
l11(a) SwOH symmetric bending mode of monomeric H SO 2 4
3
2 4
symmetric
SxO stretch of (H O) É (H SO )
2
2 4 H SO , unstable site
l (a) SxO , symmetric
stretch2of monomeric
l2(a) SxO2 , symmetric stretch of monomeric H 2SO4
2
2
2 4
SxO
, symmetric
stretch (H O) É (H SO )
2
2
2
2
4
(H SO )
(H2SO4)2
2 SO ) É (SO )
(H2O) 4É (H
(H2O)n É (H2SO4)m É (SO3)p
3p
(H2O)n É (H2SO4)m
2 n
2 4m
Sw(OH)
antisymmetric
stretch (H O) É (H SO )
2
2 2of monomeric
2 4
l (b) Sw(OH)
antisymmetric stretch
H SO
12
2
2 4
Sw(OH) antisymmetric stretch (H O) É (H SO )
2
2
2
4
(H SO )
(H2SO4)2
2 4 2 antisymmetric stretch (H O) É H SO
Sw(OH)
2
2 2
l (a) Sw(OH)
symmetric stretch of2 monomeric
H SO
4 SO )
2
2 4
(H
2
4
2
(H SO )
2 SO )
(H2O) 4É (H
4 m of monomeric H SO
l 2(b) nSxO2 rock
l13(a) SxO 2bend of monomeric H 2SO 4
2 bend of monomeric SO
2 4, stable site
l5(e@) in plane
l4(e@) in plane bend of monomeric SO 3 , unstable site
4 )
3
(SO
3 2 É (H SO )
(H O)
2 A) nout of
2 plane
4 m bend of monomeric SO
l (a
2 2
3
Fig. 1 IR spectrum of matrix isolated H O species from a CO : H SO \ 250 : 1 mixture. A, As deposited at 5 K. B, Temperature cycled to 23
2
2 4
K and recooled to 5 K. C, Temperature cycled to 26 K and recooled to 5 K. D, Temperature cycled to 30 K and recooled to 5 K. (a) The l (H O)
3 2
antisymmetric stretch region. (b) The l (H O) symmetric stretch region. (c) The l (H O) bending mode region.
1 2
2 2
3.1.2 CO bands. The bands comprise the strong l(12C16O)
solid band, lines of isotopic CO species and a 2146.9 cm~1
feature of CO perturbed by water molecules.15 The broad
(about 50 cm~1) 2209 cm~1 band is of combinations between
l(CO) and translational and rotational phonon modes.16h18
Weak bands near 2000 cm~1 may be due to small amounts of
metal carbonyls formed during deposition of the corrosive gas
mixture.
3.1.3 H SO species. The relevant spectral regions are
2 4
reproduced in Fig. 2. Table 2 compares wavenumber values of
both H SO and SO monomers in a CO matrix to values for
2 4
3
the vapor phase,19h24 and in other matrices.1,7,8 In the OH
stretch region [Fig. 2(a), trace A], only two strong bands of
almost equal intensity, not belonging to any (H O) related
2 n
species, appear in the 5 K deposit at 3320.8 and 3293.5 cm~1.
These 15 cm~1 broad absorptions which decrease with temperature cycling, are attributed to the l (b) antisymmetric OH
9
stretch of monomeric H SO . Upon warming [Fig. 2(a),
2 4
traces B, C] the doublet shifts to 3309.5 and 3285 cm~1, with
the higher frequency band losing most of its intensity. This
dual band behaviour, typical for most monomeric H SO
2 4
bands, suggests a “ two sites Ï explanation, with 3293.5 cm~1
and 3320.8 cm~1 as stable and unstable sites, respectively.
Several characteristics indicate a stronger interaction of the
OH groups with the CO matrix than with other matrices :1,7,8
(1) a large red shift of the stable site band, from the vapor
Fig. 2 IR spectrum of matrix isolated H SO species from a CO : H SO \ 250 : 1 mixture. A, As deposited at 5 K. B, Temperature cycled to 23
4 to 26 K and recooled2 to 45 K. D, Temperature cycled to 30 K and recooled to 5 K. (a) The OH
K and recooled to 5 K. C, Temperature 2cycled
stretching mode region. (b) The antisymmetric SxO stretch region. (c) The symmetric SxO stretch region. (d) The antisymmetric and sym2 region.
2
metric Sw(OH) stretch region. (e) The low frequencies
2
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
2279
Table 2 Band positions (cm~1) of H SO and SO monomers in the vapor and in solid matrices
2 4
3
vapora
neonb
oxygenc
argond
COe
3603.3
1461.2
1222
1156
1134.6
887
835
3591.6
1455.5
1218
H SO
2 4
3610
1450
1223
1159
1138
883
834
568
550
3566.7
1452.4
1216.1
1156.9
1135.9
881.7
831.4
558
548.1
3293.5, 3320.8f
1439.9, 1436.7f
1202.7, 1206.9f
1267.6
1229.5
908.7
856.4
566.3
554.2
SO
1380
529
497
1390.8
529.5
494.7
1385.1
527.2
490.3
1399.1, 1396.9, 1394.9f
533.1, 531.4
473
3
889.5,
842.3,
578,
549.5,
884.6
834.3
560
545.8
1385.5
528
490.8
a Ref. 19. b Ref. 8. c Ref. 7. d Ref. 1. e This work. f Site e†ect.
phase value (316.5 cm~1, 8.77%), vs. B1% (Table 2) ; (2) a
bandwidth larger by a factor of 10 ; (3) the site splitting, 27.3
cm~1 compared to 6 cm~1, at most, for other H SO , H O
2 4
2
and SO absorptions, (Table 1). Similar strong interactions
3
were found for l(OH) of phenol in (CH ) CO solutions as
32
compared to CCl as solvent.25 The red shift of l(OH) of
4
H SO in a CO matrix is almost 50% of that in the H-bonded
2 4
H SO solid.20,24 By an empirical correlation relating the
2 4
strength of hydrogen bond to the square root of the frequency
shift,26 the interaction in solid CO is about 70% of that in
solid H SO (23 and 33 kJ mol~1, respectively).
2 4
The l (b) antisymmetric and l (a) symmetric SxO
10
2
2
stretches [Fig. 2(b) and 2(c)] also show the two band pattern.
The pairs, 1439.9, 1436.7 cm~1 and 1202.7, 1207 cm~1 are
attributed to stable (1439.9 and 1202.7 cm~1) and unstable
(1436.7 and 1207 cm~1) sites. These much narrower and less
shifted lines, indicate a limited interaction with their environment.
In an argon matrix, the l (b) antisymmetric and l (a) sym11
3
metric SwOH bending modes, appear as weak bands at
1156.9 and 1135.9 cm~1, respectively,1 close to their vapor
phase,19h23 neon matrix8 and liquid state values.24 The crystalline phase values are 1240 and 1170 cm~1, respectively.24
In solid CO, they are signiÐcantly blue shifted with respect to
the vapor phase and inert matrix values, to 1267.6 (9.5%) and
1229.5 (8%) cm~1, respectively, again indicating the strong
OH interaction with the CO environment [Fig. 2(c)]. They
compare to the H SO solid with its much higher intermolec2 4
ular interactions (1240 cm~1, 7.3% and 1170 cm~1, 3%).24
Similarly, the Sw(OH) l (b) antisymmetric and l (a) sym2 12
4
metric stretches at 908.7 (4.6) and 856.1 (2.9) cm~1, respectively, are blue shifted from the vapor phase (2.9% and 2.7%,
respectively) and from other matrix values but less than in the
H SO solid [to 967 cm~1 and to 907 cm~1 respectively,
2 4
Table 2 and Fig. 2(d)]. As these modes are mainly SwO
motions, their relative shifts are lower than for l(OH) modes.
The l (b) SxO rock at 566.3 (4.5) cm~1 and the l (a)
13
2
5
SxO bend at 554.2 (4.3) cm~1 [Fig. 2(e)], show only minor
2
shifts relative to vapor phase and other matrix values (Table 2).
CO matrices shift vibrational frequencies of species capable
of hydrogen bond interactions like H O, NH 15 and hydro2
3
gen halides,27,28 far more than inert gas matrices. The large
shifts of vibrations involving the OH group versus the minor
e†ects on other modes, guide the interpretation of the spectral
features of the discrete mixed molecular complexes,
(OC) É (H SO ), (OC) É (H SO ) and (OC) É (H O) É (H SO ) in
2 4
2
2 4
2
2 4
an argon matrix where the modes of these species appear very
close to the positions reported above for H SO in solid CO.
2 4
(H SO ) dimers are formed mainly during deposition at 5
2 42
K.1 Their low intensity changed only very slightly by temperature cycling. Peaks belonging to either SxO or the
2
Sw(OH) groups are in proximity to the corresponding
2
2280
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
monomeric absorptions. Bands of the bonded groups are
shifted more signiÐcantly (Table 1). The 3182 cm~1 band
assigned to the dimer bonding OH stretch is similar to the
value for the HNO dimer in an argon matrix (3216 cm~1).29
3
It is interesting that the position of this mode is similar in
both argon and CO matrices. For comparison, the di†erence
in shifts for the “ free OH Ï and of the OH bonded to H O
2
stretches is over 273 cm~1 and over 260 cm~1, respectively,
see Table 1 and ref. 1. This may indicate that the (H SO )
2 42
dimer structure is the more rigid cyclical one, with both
bonded OH groups belonging to the same H SO moiety.
2 4
Such a structure, with its dipole higher than that of a dimer
with a single bonded OH per moiety, may be better stabilized
in a polar CO solid. Cyclical structures were also suggested
for nitric acid and formic acid dimers.30 Using the above
relationship26 we get a value of 25.5 kJ mol~1 for the bond
enthalpy in the (H SO ) dimer as compared to a value of
2 42
22.7 kJ mol~1 for the (HNO ) dimer. The higher acidity of
32
H SO is also evident in the gas phase free energies for disso2 4
ciation into ions, 1264.4 kJ mol~1 for H SO 34 and 1329.8
2 4
kJ mol~1 for HNO .32
3
3.1.4 SO species. The spectrum of monomeric SO com3
3
prises three IR active bands : the degenerate l (e@) in plane
4
bending mode and l (aA) out of plane bending mode, both
2 2
seen in Fig. 2(e), and the degenerate l (e@) antisymmetric
3
stretching mode, the dominating band in Fig. 3.
Dimerization and site splitting e†ects and the presence of
adjacent H O molecules are evident in the stretching mode
2
Fig. 3 IR spectrum of the l (e@) antisymmetric stretch region of
3 : H SO \ 250 : 1 mixture. A, As
matrix isolated SO from a CO
2 to4 23 K and recooled to 5 K.
deposited at 5 K. B, 3Temperature cycled
C, Temperature cycled to 26 K and recooled to 5 K. D, Temperature
cycled to 30 K and recooled to 5 K.
Table 3 Band positions (cm~1) and assignments of CO containing species in Ar/CO/H SO matrices
2 4
frequency
assignment
3723.4 (m)
3720 (w)
3701 (s)
3690 (s)
3627.7 (m)
3622.5 (w)
3618 (w)
3616 (w)
3613.2 (s)
3400 (w)
3380 (w)
3371.1 (w)
2167.1 (w)
2163 (w)
2158.6 (w)
2152 (w)
2148.6 (s)
2142.5 (s)
2138.5 (vs)
2136.8 (s)
2111.4 (w)
2101.5 (vw)
2091.4 (m)
2087.3 (w)
1615.5 (m)
1610.5 (m)
1605.5 (s)
1601.8 (vs)
1595.5 (m)
1448 (w)
1442.7 (w)
1437 (w)
1396.7 (m)
1394.6 (m)
1391.4 (m)
1214.1 (w)
1209 (w)
1204 (w)
907 (w, sh)
898.6 (w)
892 (w)
844.8 (w)
841.2 (w)
835.5 (w)
563.7 (w)
562 (w)
560 (w)
555.5 (w)
553.2 (w)
550.7 (w)
530.6 (m)
532.6 (m)
474 (m)
l (H O) of (OC) É (H O)
l3(H2O) of (OC) É (H2O) É (H SO )
3 2
2
2 4
l (H O) of (CO) É (H O)
l3(H2O) of (OC)3 É (H2 O)
3 2
m
2 n
l (H O) of (OC) É (H O)
1 2
2
l (H O) of (OC) É (H O)
1 2
2 2
l (H O) of (OC) É (H O)
l1(H2O) of (OC)2É (H 2O) É (H SO )
1 2
2
2 4
l (H O) of (CO) É (H O)
1 2
3
2
l (b) antisymmetric OH stretch of H SO in (OC) É (H SO )
9
2 4
2
2 4
l (b) antisymmetric OH stretch of H SO in (OC) É (H O) É (H SO )
l9 (b) antisymmetric OH stretch of H 2SO4 in (OC) É (H2SO ) 2 4
9
2 4
2 4
l(CO) of (OC) É (H SO )
2 4
l(CO) of (OC) É (H SO )
2
2 4
l(CO) of (OC) É (H O) É (H SO )
2 4
l(CO) of (OC) É (H2O)
2 2
l(CO) of (OC) É (H O)
2
l(CO) of (OC) É (H O)
2 monomers
2
l(CO) of 12C16O
l(CO) of 12C16O clusters
l(CO) of 12C17O monomers
l(CO) of (13C16O) É (H O)
2
l(CO) of 13C16O monomers
l(CO) of 12C18O
l (H O) of (OC) É (H O)
2 2
2 2
l (H O) of (OC) É (H O) É (H SO ) and of (H O)
2 4
2 2
l2(H2O) of (OC) É (H2O)
2 CO
2
l2 (H2 O) monomer in
2 2
l (H O) of (OC) É (H O)
2
l2 (b)2 SxO antisymmetric
stretch of (OC) É (H SO )
l10(b) SxO2 antisymmetric stretch of (OC) É (H 2O) É4(H SO )
10
2
l (b) SxO antisymmetric stretch of (OC) É (H2 SO ) 2 4
10
2
2
2 4
l (e@) antisymmetric stretch of (OC) É (SO )
3
3
l (e@) antisymmetric stretch of (OC) É (SO )
3
3
l (e@) antisymmetric stretch of SO monomer in H O rich environment
3
3
2
l (a) SxO symmetric stretch of (OC) É (H O) É (H SO )
2
2
2
2 4
l (a) SxO symmetric stretch of (OC) É (H SO )
2
2
2 4
l (a) SxO symmetric stretch of (OC) É (H SO )
2
2
2
2 4
l (b) Sw(OH) antisymmetric stretch of (OC) É (H SO )
12
2
2
2 4
l (b) Sw(OH) antisymmetric stretch of (OC) É (H SO )
12
2
2 4
l (b) Sw(OH) antisymmetric stretch of (OC) É (H O) É (H SO )
12
2
2
2 4
l (a) Sw(OH) symmetric stretch of (OC) É (H SO )
4
2
2 4
l (a) Sw(OH) symmetric stretch of (OC) É (H SO )
4
2
2
2 4
l (a) Sw(OH) symmetric stretch of (OC) É (H O) É (H SO )
4
2
2
2 4
l (b) SxO rock of (OC) É (H SO )
13
2
2
2 4
l (b) SxO rock of (OC) É (H SO )
13
2
2 4
l (b) SxO rock of (OC) É (H O) É (H SO )
l13(a) SxO 2bend of (OC) É (H2 SO ) 2 4
2 )4
l5(a) SxO2 bend of (OC)2É (H SO
5
2
2
l (a) SxO bend of (OC) É (H O) É4(H SO )
2 bend of (OC) É (SO
2 ) 2 4
l5(e@) in plane
l4(e@) in plane bend of (OC) É (SO 3)
l4(aA) out of plane bend of (OC) É3(SO )
2 2
3
region with its eleven distinct absorptions (Fig. 3). At 5 K, a
triplet of strong bands at 1399.1, 1396.9 and 1394.2 cm~1 and
three weaker ones at 1404.8, 1387.4 and 1347.1 cm~1 were
recorded. The Ðrst two strong sharp lines are of a split l (e@)
3
band trapped of SO monomer in a stable CO site. The third
3
peak at 1394.2 cm~1, annealed away at 26 K, is of an SO
3
monomer trapped in an unstable site. This unstable site being
represented by a single band may be indicative of its higher
symmetry.
Two of the other weaker bands (1387.4, 1347.1 cm~1) as
well as the low frequency 516.2 cm~1 band, increase in intensity upon warming to 23 K (with little e†ect of further
warming), indicating a subsequent augmentation by warming
of initially trapped dimers. Their assignment is to the l anti3
symmetric stretch (1389.8 cm~1 in solid argon1) and l
2
bending mode of (SO ) , respectively. The third weak band at
32
1404.8 cm~1, growing only slightly by warming, is tentatively
assigned to (SO ) (or to a higher polymer). Similar values
33
were reported for the vapor phase20,21 and in neon matrices.8
The other Ðve bands, a doublet at 1392.6 and 1390.3 cm~1
and a triplet at 1381.2, 1379 and 1376.6 cm~1, essentially replicate the above pattern with the wavenumber shift attributed
to the presence of water molecules in the trapping sites. The
wavenumber di†erences in the doublet 1392.6, 1390.3 cm~1
(2.3 cm~1) and between the two components at 1379, 1376.6
cm~1 (2.4 cm~1) are very similar to the monomeric SO
3
absorption at 1399.1, 1396.9 cm~1 (2.2 cm~1). The 1392.6,
1390.3 cm~1 doublet, emerging at higher temperature depositions, is assigned to SO species in a cage richer in water mol3
ecules. The 1381.2 cm~1 absorption, disappearing at 26 K,
belongs to an SO monomer in an unstable, water containing
3
site.
The complexity of the SO l (e@) antisymmetric stretch
3 3
absorption and its sensitivity to site e†ects and their composition resembles that of the l antisymmetric stretch of CS in
3
2
various matrices.33
The l (e@) in plane bending mode [Fig. 2(e)] shows up as a
4
doublet of almost equal intensity at 533.1 and 531.4 cm~1,
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
2281
Fig. 4 IR spectra of CO and H SO matrix isolated in argon at 5 K. A, Ar : H SO \ 200 : 1. B, Ar : H SO : CO \ 250 : 1 : 1. C,
2 4
2 stretching
4
Ar : H SO : CO \ 500 : 2 : 125. D, 2CO 4: H SO \ 250 : 1. (a) The l (H O) and l (H O) antisymmetric
and symmetric
modes region.
4 of the l (H O) bending mode.2 (c) 4The region of the l(OH)
3 2stretching1 modes
2
(b) The2 region
of H SO . (d) The region of the SxO antisymmetric
2 2. (e) The region of the SxO symmetric stretching mode of H SO . (f2 ) The
4 region of the Sw(OH) antisymmetric
2
stretching mode of H SO
and
4 of H SO . (g) The region of
2 low frequencies. (h) The region 2of the
4 l (e@) antisymmetric stretch mode
2
symmetric stretching 2mode
of SO . (i) The
2
4
3
3
region of the deformation modes of SO .
3
split by a lower symmetry of the trapping site. On the other
hand, in its position [close to the vapor phase and to other
matrices (Table 2)], this mode is the least sensitive to the trapping environment.
Theoretical calculations10h13 suggest that the formation of
binary (H O) É (SO ) or ternary (H O) É (SO ) mixed complexes
2
3
2 2
3
involves an interaction between the H O oxygen with the
2
central S atom of SO . Such a bond would primarily a†ect
3
the l (aA) bending mode. However, the l (aA) bending appears
2 2
2 2
as a sharp (2.5 cm~1) single band at 473 cm~1 with a slight
asymmetry on its red side. The latter anneals away [Fig. 2(e)]
and is attributed to an SO monomer in an unstable CO site.
3
The simple structure of the l (aA) negates the possibility of
2 2
complex formation with H O and as in argon matrices,1 there
2
is no evidence for any mixed (H O) É (SO ) complexes,
2 1h2
3
neither in the SO nor in the water vibrational modes. Even
3
2282
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
the e†ects of neighbouring H O molecules, as observed for the
2
l mode, are not resolved here. In comparison to the argon
3
matrix [which allows H O rotation while not stabilizing any
2
mixed (H O) É (SO ) complexes] and to N matrix (which
2 1h2
3
2
suppress H O monomer rotation but stabilize the complex),
2
the solid CO matrix represents an intermediate case : it suppresses H O rotation but cannot stabilize (H O) É (SO )
2
2 1h2
3
complexes. On the other hand, the red shift relative to the
vapor phase is larger for this than for any other SO mode.
3
This implies an OC É É É SO interaction between CO and the
3
sulfur atom of SO .
3
3.1.5 (H O) Æ (H SO )
species. The existence of
2 n
2 4m
(H O) É (H SO ) adducts already in the vapor phase under
2 n
2 4m
our experimental conditions is unlikely.1 Band attribution to
(H O) É (H SO ) species is based upon the following : (1) The
2 n
2 4m
1 : 1 complex bands are already present at deposition at 5 K
due to the di†usion capability of water molecules. They do
not change signiÐcantly upon temperature cycling due to
competition with formation of higher pure or mixed water
polymers. (2) The broader 2 : 1 complex bands appear at
higher annealing temperatures and increase up to the
warming limit (33 K). (3) The broadest bands to appear at still
higher annealing temperatures are of the higher
(H O) É (H SO ) species.
2 n
2 4m
The 3551.5 and 1604.4 cm~1 weak bands were thus assigned to the l (H O) and l É (H O) modes of the 1 : 1
1 2
2
2
(H O) É (H SO ) complex, respectively.
2
2 4
Upon warming to 30 K, the higher wavenumber band of
the 3320.8, 3293.5 cm~1 doublet leaves a residual band at
3309.5 cm~1 [Fig. 2(e)], assigned to the bonded OH stretch of
H SO in (H O) É (H SO ). Other bands [Fig. 1(b), 2(c) and (d)]
2 4
2
2 4
having the attributes of the 1 : 1 complex are : 1419.2 cm~1,
assigned to the SxO antisymmetric stretch ; 1220.5 cm~1,
2
the SxO symmetric stretch and 897.1 cm~1, the Sw(OH)
2
2
antisymmetric stretch.
Bands which can be assigned to the 2 : 1 complex are :
1193.6 cm~1 (symmetric SxO stretch), 941.7 cm~1
2
[antisymmetric Sw(OH) stretch] and 861 cm~1 [symmetric
2
Sw(OH) stretch].
2
The broad bands at 1047.7, 974.3, 577.6 and 503.5 cm~1 are
attributed to (H O) É (H SO ) or (H O) É (H SO ) É (SO )
2 n
2 4m
2 n
2 4m
3p
polymeric species. The latter assignment is supported by the
sharp band at 1060 cm~1, the IR inactive l (a) mode of SO .
1
3
3.2 IR spectra of (OC) Æ (H SO ), (OC) Æ (H O) Æ (H SO ),
2 4
2
2 4
(OC) Æ (H SO ) and (OC) Æ (SO ) complexes in argon
2
2 4
3
CO containing species (Table 3) were characterized by CO
concentration variations, (Fig. 4), by changing the deposition
temperatures and by temperature cycling (Fig. 5).
CO competes with H O for the free OH bonds of other
2
water molecules and of H SO . Higher CO concentrations
2 4
augment
the
formation
of
(OC) É (H O)
and
n
2 m
(OC) É (H SO ) adducts but reduce the amount of
1h2
2 4
(H O) É (H SO ) species. This is reÑected in the H O and
2 n
2 4m
2
H SO modes, especially in their OH stretching bands.
2 4
Trace A of Fig. 4(a) shows the l (H O) antisymmetric
3 2
stretch region of a CO free matrix.1 Traces B and C reveal
new bands at 3723.4 and 3690 cm~1 , assigned to l (H O) of
3 2
(OC) É (H O) and (OC) É (H O) species, respectively.3 A new
2
m
2 n
band at 3720 cm~1 is attributable to a ternary
(OC) É (H O) É (H SO ) complex. The signiÐcant red shift from the
2
2 4
corresponding (H O) É (H SO ) mode at 3745.1 cm~1,1 on the
2
2 4
one hand and the similarity to the respective (OC) É (H O)
2
band at 3723.4 cm~1 on the other, indicate that the OH of
H O is bonded in an OC É É É H O É É É HOwSO wOH
2
2
2
sequence rather than an OC É É É HOwSO wOH É É É H O
2
2
structure. Increasing the CO concentration gradually changes
the H O spectrum from vib-rotors in solid argon (trace A) to
2
non-rotating H O species in a CO matrix (trace D). l (H O)
2
3 2
of the monomer at 3701 cm~1 in a concentrated 4 : 1 Ar : CO
matrix (trace C), is close to that in an unstable CO matrix site
(Tables 1 and 2), indicating the number of nearest CO neighbours in the unstable site to be much lower than in the stable
one. The higher symmetry of this unstable site does not a†ect
a splitting of the degenerate SO modes. With twelve nearest
3
neighbours in fcc solid argon and a 4 : 1 Ar : CO ratio, a
(CO) É (H O) species would thus Ðt the observations.
3
2
The l (H O) mode is also seen in Fig. 4(a). Bands, not due
1 2
to (H O) species34 are assigned to mixed (OC) É (H O) com2 n
m
2 n
plexes : (OC) É (H O) at 3627.7 cm~1, (OC) É (H O) at 3622.5
2
2 2
cm~1, and (OC) É (H O) at 3618 cm~1.3 The weak peak at
2
2
3616 cm~1 is ascribed to the (OC) É (H O) É (H SO ) species,
2
2 4
mentioned above. Again, its red shift from the corresponding
mode of (H O) É (H SO ) at 3640 cm~1 (ref. 1) and the proximity
2
2 4
to l (H O) of (OC) É (H O) at 3627.7 cm~1 points to an
1 2
2
OC É É É H O É É É HOwSO wOH structure. In 4 : 1 Ar : CO
2
2
ratio matrix [Fig. 4(a), trace C], the argon matrix peaks
diminish and converge into a single band at 3613.2 cm~1, the
position of l (H O) in an unstable site CO matrix in line with
1 2
a smaller number of CO nearest neighbours. In analogy to the
l band at 3701 cm~1, the peak is attributed to (CO) É (H O).
3
3
2
The l (H O) region [Fig. 4(b), Fig. 5(b)] shows several
2 2
bands of complexes : 1600 cm~1 of (H O) É (H SO ),1 1595.5
2
2 4
cm~1 of (OC) É (H O) and the 1605.5 and 1615.5 cm~1 doublet
2
of (OC) É (H O) .3 In a CO free layer [Fig. 4(b), trace A] the
2 2
two l (H O) bands, 1593 cm~1 (acceptor), and 1610.5 cm~1
2 2 2
(donor)17 are of almost equal intensity. With CO in the matrix
(trace B), the 1610.5 cm~1 component gains relative intensity,
even more so at a higher deposition temperature [Fig. 5(b),
trace B]. Part of this band can then be assigned as due to
(OC) É (H O) É (H SO ). Its blue shift from the (H O) É (H SO )
2
2 4
2
2 4
band1 and its proximity to l [(OC) É (H O) ], again support the
2
2 2
OC É É É H O É É É HOwSO wOH structure.
2
2
We turn to H SO spectral features. In the l(OH) regions
2 4
[Fig. 4(a) and (c), and Fig. 5(c)] the band at 3566.7 cm~1 is
due to monomeric H SO , the peak at 3572.6 cm~1 stems
2 4
from (H O) É (H SO ) and the 3582 cm~1 feature is from
2
2 4
(H O) É H SO .1 None of these bands are observed in a pure
2 2 2 4
CO matrix [Fig. 4(b), trace D].
For a 5 K deposited, low CO concentration sample [Fig.
4(c), trace B] a triplet of weak bands at 3400, 3380, and 3371.7
cm~1 is attributed to (OC) É H SO , (OC) É (H O) É (H SO ) and
2 2 4
2
2 4
(OC) É (H SO ), respectively. For a higher CO concentration
2 4
[Fig. 4(c), trace C], the relative intensities change accordingly :
the 3400 cm~1 band increases, the 3371.7 cm~1 band signiÐcantly decreases and the 3380 cm~1 band remains unchanged.
Even higher CO concentrations [Fig. 4(c), trace D] change
these absorptions into the features of H SO in solid CO [Fig.
2 4
4(c), trace E]. The (OC) É (H O) É (H SO ) band (3380 cm~1)
2
2 4
grows signiÐcantly by raising the deposition temperature
[Fig. 5(c), traces A and B]. Its large red shift (187 cm~1)
relative to that of (H O) É (H SO ) supports the
2
2 4
OC É H O É HOwSO wOH bonding scheme. These wave2
2
number values are similar to those of (OC) É (HNO ) comn
3
plexes in solid argon matrices.29
For the l(CO) spectral region [Fig. 5(d)], the (CO) and
n
(OC) É (H O) bands were discussed previously.3 Of the addim
2 n
tional absorptions at 2158.6, 2163 and 2167.1 cm~1, the latter,
the only to appear in a 5 K deposition of an
Ar : H SO : CO \ 500 : 2 : 5 mixture, is assigned to
2 4
(OC) É (H SO ). The other two emerge only in an 18 K deposi2 4
tion and are assigned to (OC) É (H SO ) (2163 cm~1) and
2
2 4
(OC) É (H O) É (H SO ) (2158.6 cm~1).
2
2 4
Despite its triple bond and low dipole moment,35 CO is a
ligand in many complexes with metals and metal halides,
acting either as an electron donating base or as an acid. The
bonding occurs between the metal and the CO carbon
atom,36 even though it is less electronegative.35 Electron donation depletes the weakly antibonding 5p É (CO) orbital, centered on the carbon atom,37 thus blue shifting the l(CO)
frequency. “ n backdonation Ï has the opposite e†ect, as metal
electrons occupy the n É (CO) antibonding orbitals. In metal
carbonyls the net e†ect is to lower l(CO) from its vapor phase
position at 2143 cm~1 38 to 1800È2100 cm~1. In metal halide
carbonyls, due to the positive charge on the metal atom, n
backdonation is insigniÐcant and l(CO) is raised39h41 to
values even beyond l(CO`) at 2184 cm~1.39 The l(CO) shift is
thus a good criterion of bonding strength and character.
The bonding between CO and H O is described as an
2
almost linear hydrogen bond with the C atom of CO.42h45
Here the bond is due to p donation, (no n backdonation from
the H atom) a†ecting an increase of l(CO) above its “ free Ï
molecule value (2138.5 cm~1 for an argon matrix3). This e†ect
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
2283
Fig. 5 Deposition temperature e†ect on an Ar : H SO : CO \ 500 : 2 : 5 sample. All spectra recorded at 5 K. A, Deposition at 5 K. B, Deposi4 stretching modes. (b) The region of the l (H O) bending mode. (c) The region of the
tion at 18 K. (a) The region of the l (H O) and l 2(H O)
3
2
1
2
2 2stretch of H SO . (f ) The region SxO
H SO wOH stretch. (d) The region of the l(CO) stretch. (e) The region of the SxO antisymmetric
2 4
2
2
symmetric
stretch of H SO . (g) The region of the Sw(OH) symmetric and antisymmetric
stretches of H SO 2. (h)4 The region of the low
2 of 4the l (e@) antisymmetric stretch of SO
2 .
2 4
frequencies. (i) The region
3
3
will be the more pronounced, the higher the acidity of the
complexants : 2149.3, 2155, 2164 and 2167.1 cm~1 observed
for (OC) É (H O),3 (OC) É (HCl),39 (OC) É (HNO )29 and
2
3
(OC) É (H SO ), respectively. The 2163 cm~1 peak of
2 4
(OC) É (H SO ) falls within this trend, since H-bonding to one
2
2 4
of the H SO hydrogens reduces the acidity and the p dona2 4
tion of the other to the CO molecule. Also within this trend,
when an H O molecule separates H SO from CO, the e†ect
2
2 4
is reduced to position the l(CO) at 2158.6 cm~1.
In the SxO antisymmetric stretching mode region, at
2
lower CO dopings [Fig. 4(d), traces A and B], two weak
bands, additional to the monomer peaks, are found at 1447
and 1442 cm~1 [of (H O) É (H SO ), (ref. 1)]. An 18 K depo2 2
2 4
sition, [Fig. 5(e), trace B] shows their signiÐcant enhancement,
at 1448 and 1442.7 cm~1, with a shoulder at 1437 cm~1.
2284
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
Increasing the CO : Ar ratio [Fig. 4(d), trace C], broadens the
doublet and shifts it to 1436 and 1438.5 cm~1, very similar to
H SO monomer isolated in a CO matrix. We assign the 1448
2 4
cm~1 band to (OC) É (H SO ), the 1442.7 cm~1 band to
2 4
(OC) É (H O) É (H SO ) and the weaker 1437 cm~1 shoulder to
2
2 4
the (OC) É (H SO ).
2
2 4
An analogous spectral behaviour was observed for the
SxO symmetric stretch bands [Fig. 4(e) and Fig. 5(f )],
2
leading to the assignment of 1209 cm~1 to the (OC) É (H SO )
2 4
1 : 1 dimer, its weak broad shoulder at 1204 cm~1 to the
(OC) É (H SO ) trimer and the 1214.1 cm~1 to the
2
2 4
(OC) É (H O) É (H SO ) mixed trimer.
2
2 4
On the basis of similar considerations we assign the
Sw(OH) antisymmetric stretch and symmetric stretch bands
2
[Fig. 4(f ) and Fig. 5(g)]. The 898.6 and 844.8 cm~1 bands are
the antisymmetric and symmetric Sw(OH) stretch modes of
2
(OC) É (H SO ), the 892 and 835.5 cm~1 are the analog bands of
2 4
(OC) É (H O) É (H SO ) (their positions being the closest to the
2
2 4
(H O) É (H SO ) features1) and the 907 cm~1 and 841.2 cm~1
2
2 4
broad shoulders are these modes due to the (OC) É (H SO )
2
2 4
trimer.
In the lowest frequency range, the new appropriate rocking
mode bands are found at 562 cm~1 with a shoulder at 560
cm~1 and the SxO bends at 555.5 and 553 cm~1, for low
2
CO concentrations [Fig. 4(g), trace B]. Again, these features
grow and appear as respective triplets in the 18 K deposition
[Fig. 5(h), trace B]. The assignments are : the 560 cm~1 and
550.7 cm~1 bands belong to the SxO rock and SxO bend
2
2
of the (OC) É (H O) É (H SO ) species, respectively. The 562
2
2 4
and 553.2 cm~1 features are due to those of the
(OC) É (H SO ) dimer, while the shoulders at 563.7 and 555.5
2 4
cm~1 are ascribed to the (OC) É (H SO ) trimer.
2
2 4
We now consider the e†ects of CO presence in the matrix
on the SO bands [Fig. 4(h) and (i), and Fig. 5(h) and (i)]. The
3
1385.1 cm~1 band of the l (e@) antisymmetric stretch of mono3
meric SO (ref. 1) persists up to the highest CO concentrations
3
[Fig. 4(h), trace C]. With CO as matrix [Fig. 4(h), trace D], it
is replaced by a triplet at 1399.1, 1396.6, and 1394.9 cm~1,
assigned above to monomeric SO in stable and unstable sites
3
(Section 3.1.4). With low CO doping of the matrix [Fig. 4(h),
trace B], the Ðrst two bands of the 1396.7, 1394.5, and 1391.4
cm~1 triplet [Fig. 4(h), trace C] correlate with the CO presence. Their position resembling the SO frequencies in solid
3
CO and their being signiÐcantly enhanced at an 18 K deposition [Fig. 5(i), traces A and B], lead to their assignment to a
split l (e@) mode of the (OC) É (SO ) complex. A similarly
3
3
assigned doublet was found in solid O (1393.4 and 1392
2
cm~1).7
Analogous arguments lead to the assignment of the broader
doublet at 532.6 and 530.6 cm~1 (ref. 1), next to the 527.2
cm~1 l (e@) bending mode of monomeric SO in solid argon
4
3
[Fig. 4(i)], to a splitting of this degenerate mode in the
(OC) É (SO ) dimer. In solid oxygen, the analogous feature was
3
observed at 530.5 cm~1.7
In the l (aA) out of plane bending mode region, one Ðnds a
2 2
band at 474 cm~1, additional to the 490.3 cm~1 feature of
monomeric SO and to a weak broad band of (SO ) at 480
3
3n
cm~1 (ref. 1) [Fig. 4(i), trace B]. This additional band,
enhanced upon deposition at 18 K [Fig. 5(h), trace B], is
ascribed to the l (aA) mode of the (OC) É (SO ) complex in
2 2
3
solid argon, similar to the 478.2 cm~1 feature in oxygen
matrices.7 At higher CO concentrations [Fig. 4(i), trace C]
this (OC) É (SO ) band red shifts to 473 cm~1, also its position
3
in a CO matrix (Table 1).
When comparing the modes of the SO subunit of the
3
(OC) É (SO ) complex with the vapor phase values (Table 2),
3
the largest red shift is found for the l (aA) mode (4.6%), indi2 2
cating a bonding interaction between the central sulfur atom
and CO. Similar to our previous study,1 where there was no
evidence for a binary (H O) É (SO ) complex, here we Ðnd no
2
3
spectral features assignable to a ternary (OC) É (H O) É (SO )
2
3
complex.
between the moieties of the mixed complexes. No spectral features assignable to either a binary (H O) É (SO ) or a ternary
2
3
(OC) É (H O) É (SO ) complex were found. Site e†ects were
2
3
observed as band shifts and as splittings of degenerate mode
bands. In CO matrices, the less stable site provides fewer
nearest neighbours and is of higher symmetry. Similar to
argon matrices1 no proton transfer takes place between the
H SO and H O below 30 K1 in either pure CO or CO con2 4
2
taining argon matrices.
A. L. acknowledges a visiting scientist grant from the Norwegian Research Council.
References
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2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
4 Conclusion
In this paper we found spectral evidence in all relevant spectral regions for the existence of (OC) É (H SO ),
2 4
(OC) É (H O) É (H SO ), (OC) É (H SO ) and (OC) É (SO )
2
2 4
2
2 4
3
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2 4
3
conclusions from spectral assignments indicate the bonding
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