IR spectrum of molecular complexes of sulfuric acid with N and NO
2
trapped in solid argon
Aharon Givan,a Aharon Loewenschuss*a 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
Recei¿ed 24th August 1998, Accepted 30th October 1998
The IR spectra of vapours above neat sulfuric acid trapped, mixed with varying amounts of N and NO gases
2
in argon matrices are reported. Absorptions due to 1 : 1 (N ) É (H O), (N ) É (H SO ), (NO) É (H O),
2
2
2
2 4
2
(NO) É (H SO ) and (N O ) É (H SO ) dimeric complexes were identiÐed in all relevant spectral regions. The
2 4
2 2
2 4
interactions of nitrogen and nitrogen oxides with SO do not indicate complex formation. A T-shaped
3
attachment of N to H SO , and an O-end interaction of the nitrogen oxide monomer and dimer with the OH
2
2 4
groups of H SO is suggested. The formation of the complex species is by surface di†usion during deposition
2 4
and not a result of vapour phase equilibria.
Introduction
Sulfuric acid is an important atmospheric species formed via
the oxidation of SO to SO and a subsequent reaction with
2
3
one or two H O molecules.1h5 It is found especially in polar
2
stratospheric clouds, together with nitric acid, and in low temperature sub-lm aerosol drops.6 In two recent studies 7,8 we
reported the IR spectra of pure H SO vapours (also contain2 4
ing SO and H O in equilibrium concentrations) trapped in
3
2
matrices of solid argon and carbon monoxide, as well as of
gaseous H SO ÈCO mixtures frozen in argon matrices. Bands
2 4
were assigned to vibrational modes of monomeric and dimeric
H SO and SO species. In addition, various molecular com2 4
3
plexes such as (H O) É (H SO ) , (OC) É (H SO ),
2 m
2 4n
1h2
2 4
(OC É H O) É (H SO ) and (OC) É (SO ) were spectroscopically
2
2 4
3
identiÐed and their possible structures discussed. It was indicated that the preferred attachment of both H O and CO to
2
the H SO moiety occurs through the OH subunit of the
2 4
latter, whereas the OCÈSO interaction proceeds via the
3
central S atom. No evidence was found for the stabilization of
(H O) É (SO ) molecular complexes in either solid argon or
2 1h2
3
solid CO, in contrast to previous studies in nitrogen9 and
oxygen10 matrices.
Here, we extend our investigations to argon-matrix-isolated
mixtures of H SO vapours with the atmospheric constituent
2 4
N and the trace component NO, aiming at a characterization
2
of their complexes with H SO , H O and SO . No previous
2 4
2
3
spectroscopic evidence for the existence of such complexes,
either in the gas phase or in solid matrices has been reported.
Experimental
Sulfuric acid was supplied by Prolabo (p.A.), the Ar (5.7), N
2
(5.2), and NO (3.0) gases by AGA.
Most experimental details were given previously.7,8 BrieÑy,
vapours for deposition were taken from a drop of sulfuric acid
placed in a quartz tube ending in a nozzle, wrapped by a
heating coil. Gaseous ArÈN and ArÈNO mixtures (typically
2
of 1 : 10È1 : 1000 ratios) were either passed through the
H SO -containing nozzle (warmed to a maximum of 36 ¡C) or
2 4
the two components were independently sprayed onto a 5 K
CsI window. Cooling was achieved 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 1È4 h, depending upon
sample dilutions, with deposition rates of several mmol h~1.
Temperature cycling up to 38 K of the deposited samples was
conducted by a slow warming (1 K min~1), 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.
Results and discussion
The samples investigated contain a variety of matrix-isolated
molecules, their dimers and higher polymers as well as mixed
molecular complexes. Since bands belonging to H O, SO ,
2
3
H SO , (H O) É (H SO ) , and (H O) É (H SO ) É (SO )
2 4
2 m
2 4n
2 m
2 4n
3p
were previously discussed in detail,7 only bands belonging to
N or NO-containing species will be considered here. The rel2
evant band frequencies are summarized in Tables 1 and 2.
H O has the highest di†usion capability of all components in
2
the sample and its presence is inevitable due to both desorption from the vacuum system walls and H SO decomposition
2 4
in the vapour phase. It is, therefore, expected that water molecules will be involved in most complex formations during
deposition and, even more so, upon temperature cycling.
Spectral e†ects due to temperature cycling, which deÐnitely go
beyond the changes due to H O di†usion reported for pure
2
H SO Èargon samples,7 were, however, attributed to the pres2 4
ence of the new guest species in the matrix.
Assignments
In the following text, bandwidths are given in parentheses,
next to peak positions in cm~1. Whenever possible, these data
are followed by band intensities, speciÐed in absorbance units
(a.u.)
N complexes. Bands with intensities dependent upon nitro2
gen concentrations, recorded by us at 3730 cm~1 (l ), and at
3
1600 cm~1 (l ), were previously assigned to the 1 : 1
2
(N ) É (H O) complex.11 These frequencies di†er, however,
2
2
from those reported by Devlin et al.12 for N attached to dan2
gling OH units on microporous ice surfaces. Sadlej et al.13
Phys. Chem. Chem. Phys., 1999, 1, 37È43
37
Table 1 Band positions (cm~1) of N complexes with H SO , SO
2
2 4
3
and H O formed by low temperature depositions of ArÈN ÈH SO
2
2 2 4
mixtures
Frequency
Assignment
3730 (m)
3547 (m)
3542 (w)
3519 (m)
2278 (w)
1600 (w)
1450.9 (m)
l mode of H O in (N ) É (H O)
3
2
Bonded-OH
stretch
of 2H SO2 in (N ) É (H SO )
2 4
2
2 4
Bonded-OH stretch of (N ) É (H O) É (H SO )
2x
2 y
2 4
l mode of H O in (N ) É (H O)
3
2
2x
2 y
NwN stretch in (N ) É (H O)
l mode of H O in 2(N ) É2(H O)
2
2
2
2
Antisymmetric OxSxO stretch of H SO in
2 4
(N ) É (H SO )
2
2 4
l (e@) of SO in N -rich matrix environment
l3(e@) of SO3 with 2single N next neighbour
3
3
2
Antisymmetric Sw(OH) bend of H SO in
2
2 4
(N ) É (H SO )
2
2 4
Antisymmetric Sw(OH) bend of H SO in
2 4
(N ) É (H O) É (H SO 2)
2x
2 y
2 4
Antisymmetric Sw(OH) bend of H SO in
2
2 4
(N ) É (H O) É (H SO )
2
2
2 4 stretch of H SO in
Antisymmetric
Sw(OH)
2
2 4
(N ) É (H SO )
2
2 4
symmetric Sw(OH) stretch of H SO in
2
2 4
(N ) É (H SO )
2
2rock4of H SO in (N ) É (H SO )
OxSxO
2 4
2x
2 4
OxSxO rock of H SO in (N ) É (H SO )
2 4
2
2 4
OxSxO bend of H SO in (N ) É (H SO )
2 4
2x
2 4
OxSxO bend of H SO in (N ) É (H SO )
2 N
4 next2 neighbour
2 4
l (e@) of SO with single
2
l4(e@) of SO3 in N -rich matrix
environment
4
3
2
l (aA) of SO in N -rich matrix environment
2 2
3
2
1398 (w)
1393 (w)
1178 (w)
1177.3 (w)
1173 (w)
892 (m)
839.8 (w)
560.3
559.5
550.6
549.8
530.1
528.7
489.6
(w)
(w)
(w)
(w)
(w)
(w)
(w)
Table 2 Band positions (cm~1) of NO complexes with H SO and
H O formed by low temperature depositions of ArÈNOÈH 2SO 4 mix2
2 4
tures
Frequency
Assignment
3725.5 (m)
3474 (w)
l mode of H O in (NO) É (H O)
3
2
Bonded-OH
stretch
of H SO2 in
2 ) 4
(NO) É (N O ) É (H SO
x
2
2
y
2
4
Bonded-OH stretch of H SO
in
(NO) É (H O) É (H SO ) 2 4
2
2
4
Bonded-OH stretch of H SO in
2 4
(NO) É (H SO )
4
Bonded-OH2 stretch
of H SO in
2 4
(N O ) É (H SO )
2 stretch
2
2of (NO)
4
NwO
É (H O) É (H SO )
NwO stretch of (NO) É (H2SO ) 2 4
NwO stretch of (NO) É (H2SO4) É (NO)
NwO stretch of (NO) É (H2O) 4
NwO stretch of (NO) É (N2 O ) É (H SO )
x stretch
2 2 of
y
2 4
l antisymmetric NwO
5 (N O ) É (H SO )
2 2
2 NwO
4
l antisymmetric
stretch of
5 (N O ) É ( H SO )
2
2
x
2
4
Antisymmetric OxSxO stretch of
H SO in (N O ) É (H SO )
2 4 OxSxO
2 2 stretch
2 4of H SO
Symmetric
2 4
in (N O ) É (H SO )
2 2
2 4
Antisymmetric
Sw(OH)
stretch of
2 É (H SO )
H SO in (NO) É (H O)
2
4
2
2 of
4
Antisymmetric Sw(OH) stretch
2 )
H SO in (NO) É (H SO
2 4
2 stretch
4
Antisymmetric
Sw(OH)
of
H SO in (N O ) É (H2SO )
2 4 Sw(OH)
2 2 stretch
2 4of
Symmetric
H SO in (N O )2É (H SO )
2 4 Sw(OH)
2 2 stretch
2 4of
Symmetric
2 SO )
H SO in (NO) É (H
2 4 rock of H SO
2 4in (NO) É (H SO )
OxSxO
2
2 SO
4 )
OxSxO rock of H SO4 in (N O ) É (H
2 2É (H SO
2 )4
OxSxO bend of H2 SO4 in (NO)
2 SO
4 )
OxSxO bend of H 2SO4 in (N O ) É (H
2 4
2 2
2 4
3464 (w)
3452.3 (w)
3446.5 (m)
1892.3 (w)
1890.2 (w)
1886.6 (w)
1879.6 (m)
1835 (m)
1807 (w)
1790.8 (w)
1445.7 (m)
1214.4 (m)
898 (w)
893.3 (m)
888.1 (m)
838.9 (m)
834.5 (w)
560 (w)
555.3 (w)
552.2 (w)
545 (w)
38
Phys. Chem. Chem. Phys., 1999, 1, 37È43
recently calculated the structure (almost linear NNwHO) and
stabilization energy (4.77 kJ mol~1) of this complex.
The OH stretching modes region (3800È3300 cm~1) of a
reference nitrogen-free sample of argon-matrix-isolated
H SO [Fig. 1(a), trace A] shows peaks of H O vib-rotors,14
2 4
2
the sharp, strong (2.2 a.u.) band at 3566.7 (1.5) cm~1 of monomeric H SO with two satellites of (H O) É (H SO ) com2 4
2 1h2
2 4
plexes,7 water dimer absorptions at 3708 cm~1 (0.12 a.u.) and
the H O trimer feature at 3527.8 (sh) cm~1 (0.1 a.u.) and at
2
3516.2 cm~1 (0.15 a.u.).15
In the low nitrogen concentration (1%) sample [Fig. 1(a),
trace B], the 3527.8 cm~1 shoulder disappears and four bands,
three of them new, are observed. Next to the water trimer
band at 3516.5 (2.5) cm~1 (0.15 a.u.), a new shoulder is found
at 3519 (9) cm~1 (0.07 a.u.). Also, a rather sharp band at 3547
(2.5) cm~1 (0.15 a.u.) with a broad shoulder at 3542 (6.5) cm~1
(0.01 a.u.) is recorded.
Fig. 1(a), trace C, shows the inÑuence of temperature cycling
on this sample. The e†ects are all irreversible, but the various
peaks are di†erently a†ected. The 3516.5 cm~1 trimeric water
band shows a typical low H O polymer behaviour, by being
2
signiÐcantly enhanced (up to 0.6 a.u) by the Ðrst annealing
stage and then decreasing at higher temperatures. The 3519
cm~1 shoulder gains intensity (measured by peak integration)
with temperature, in a non-linear mode : the growth rate being
smaller at the lower temperatures, increasing rapidly at the
higher ones. This is taken to indicate a dependence of the
product, not only on H O mobility, but also on the setting in,
2
at higher temperatures, of di†usion of the heavier N mol2
ecules to produce (N ) É (H O) (x,y [ 1) species. At higher N
2x
2 y
2
concentrations (trace D) these two bands merge to a single
broader (10 cm~1) absorption at 3518.4 cm~1.
The 3547 cm~1 band is little a†ected in either position, line
width or intensity by up to 22 K. Upon cycling to higher temperatures, the spectral parameter most a†ected is its intensity,
which increases more rapidly. This thermal behaviour indicates a water-free, but nitrogen-containing species, with the
little a†ected band sharpness pointing to a geometrically and
compositionally well deÐned species. Accordingly, this band is
attributed to the 1 : 1 (N ) É (H SO ) complex. Its small (0.6%)
2
2 4
red shift from monomeric H SO , indicates a weak
2 4
N wH SO bonding interaction. The disappearance of this
2
2 4
band, along with the 3566.7 cm~1 l(OH) stretch of argonisolated H SO , at still higher nitrogen concentrations [Fig.
2 4
1(a), trace D] is in support of this assignment. The broader
3542 (7) cm~1 shoulder shifts to 3540 cm~1 at ca. 26 K and
the gradual growth with increasing temperature is typical of
water dependence. Its increasing width at both higher temperature and higher nitrogen concentrations [Fig. 1(a), trace
D] suggests an assignment to less well deÐned absorbers of
the (N ) É (H O) É (H SO ) (x,y P 1) type.
2x
2 y
2 4
In the SwOwH bending region, a comparison of the
spectra in Fig. 1(b), traces AÈD, and considerations analogous
to the above, lead to the following assignments : the strong
1156.9 (2.8) cm~1 (0.225 a.u.) and 1135.9 (3.7) cm~1 (0.16 a.u.)
are the l and l modes of H SO in an argon matrix.7 The
11
9
2 4
narrower 1178 (2) cm~1 and weaker (0.02 a.u.) band, blue
shifted (1.8%) from the monomeric H SO value, is attributed
2 4
to the well deÐned 1 : 1 (N ) É (H SO ) complex. The broader
2
2 4
1173 (5) cm~1 (0.2 a.u.) is assigned to (N ) É (H O) É (H SO )
2
2
2 4
absorbers. The 1177.3 (4) cm~1 (0.05 a.u.) band in trace D
(high N doping) is due to polymeric (N ) É (H SO ) (x [ 1)
2
2x
2 4
and (N ) É (H O) É (H SO ) (x,y P 1).
2x
2 y
2 4
In the SwOH stretching mode region [Fig. 1(c), traces
AÈD], similar arguments lead to the assignment of the two
bands at 892 (4) cm~1 (0.125 a.u.) and 839.8 (2) cm~1 (0.06
a.u.) to the antisymmetric and symmetric SwOH stretches of
the 1 : 1 (N ) É (H SO ) complex.
2
2 4
The SxO vibrations are far less a†ected by the presence of
2
N . A new shoulder at 1450.9 cm~1 on the monomeric H SO
2
2 4
Fig. 1 N concentration and temperature cycling e†ects on IR vibrational bands of ArÈN ÈH SO samples as deposited at 5 K. (a) H SO
2
2 2-stretch
4
2 4
l(OH)-stretching
mode region. (b) H SO Sw(OH) -bending mode region. (c) H SO Sw(OH)
modes region. (d) H SO SxO -rock
2 antisymmetric
4
2 stretch region. (f) SO l (e@)
2 antisymmetric
4
2 bending mode region. A, ArÈN
2 4 ÈH SO
2 \
and bend mode region. (e) SO l (e@)
3 3: 1 : 1 C, Sample B temperature cycled to 24
3 K.
4 D, ArÈN ÈH SO \ 200 : 10 : 1
2 2 4
200 : 0 : 1. B, ArÈN ÈH SO \ 200
2 2 4
2 2 4
1452.4 cm~1 band,7 is attributed to the antisymmetric stretch
1 : 1 (N ) É (H SO ) complex. In the low wavenumber region,
2
2 4
[Fig. 1(d), traces AÈD], new bands emerge at 559.5 and 549.8
cm~1, as shoulders on the pure H SO absorptions, upon
2 4
raising the N concentration and as a result of temperature
2
cycling. They are assigned to the 1 : 1 (N ) É (H SO ) rocking
2
2 4
and bending modes, respectively. A higher nitrogen concentration (trace D) broadens and shifts them to 560.3 and 550.6
cm~1, respectively, indicating the production of less deÐned
(N ) É (H SO ) species.
2x
2 4
Turning our attention to the nitrogen moiety, one notes a
very weak band at 2278 cm~1, which increases slightly with
sample warming and shifts to higher wavenumbers with
increasing nitrogen concentrations. We attribute it to the
l(N ) stretching vibration of the 1 : 1 (N ) É (H SO ) complex.
2
2
2 4
This band position is red shifted from the frequency of argon
matrix isolated N at 2325. 9 cm~1.16
2
All samples investigated also contain certain amounts of
argon matrix trapped SO , as a result of its presence in a gas
3
phase equilibrium with H SO . In Fig. 1(e), both trace A, the
2 4
““ reference ÏÏ spectrum, and trace B of a sample doped with 1%
N , show the two prominent bands at 1385.1 (2.5) cm~1 (0.8
2
a.u.) and 1389.4 (3) cm~1 (0.2 a.u.) of l of argon-isolated SO
3
3
and (SO ) species.7 Upon warming the sample (trace C),
32
these two lines lose intensity, and new bands at 1393 (3) cm~1
(0.3 a.u.) and 1396 (2.5) cm~1 (0.15 a.u.) grow in. As bands at
similar frequencies were shown to belong to SO species in
3
H O enriched argon cages,7 their nitrogen dependence should
2
be examined. The spectrum with increased nitrogen content
(trace D), shows a relative enhancement of the 1393 cm~1
band intensity compared to the 1385.1 and 1389.4 cm~1
peaks, while the 1396 cm~1 absorption remains unchanged. A
new weak band emerges at 1398 cm~1 (0.1 a.u.). Thus, only
the 1393 cm~1 absorption may contain a component associated with an interaction with N next-neighbours in the
2
matrix. The nitrogen-dependent 1398 cm~1 band resembles
l (e@) SO monomeric position in a nitrogen matrix9 and rep3
3
resents SO in a nitrogen-rich environment.
3
The l (e@) SO bending mode region [Fig. 1(f)], is the most
4
3
a†ected by the presence of N . The addition of ca. 1% N to
2
2
the sample causes emergence of a shoulder at 528.7 cm~1
(0.085 a.u.) on the 527.2 cm~1 band of argon-isolated SO 7
3
(traces A and B). By temperature cycling (trace C), the 528.7
cm~1 shoulder is further enhanced (0.12 a.u.), with an additional broader shoulder of the same intensity appearing at
530.1 cm~1, on the blue side of the main band. At higher
nitrogen concentrations (trace D) the absorption appears as a
broad band at 530.1 cm~1 with a prominent shoulder at 529
cm~1. For nitrogen-matrix-trapped SO monomer, the l (e@)
3
4
mode is a doublet at 531.5 and 532.1 cm~1,9 similar to our
result for the high N concentration. In analogy to the assign2
ments for l , the 528.7 cm~1 band is attributed to the e†ect of
3
an N next neighbour in the matrix and the 530.1 cm~1
2
absorption is due to SO in a nitrogen-rich matrix environ3
ment.
The l (aA) SO mode is the least a†ected by temperature
2 2
3
cycling and N concentration. A shoulder at 489.6 cm~1,
2
emerging on the main 490.3 cm~1 band is attributed to e†ects
of a nitrogen-rich environment on this mode.
NO complexes. The spectrum of NO isolated in argon is
complicated by the presence of various polymeric (NO)
x
species in solid trapping sites of di†erent stabilities and
varying rotational freedom17h21. In our experiments, the band
Phys. Chem. Chem. Phys., 1999, 1, 37È43
39
structure is further complicated by interactions of the NO
species with H SO , SO and H O . In the assignment, we are
2 4
3
2
helped by the fact that the formation of such complexes
depends upon deposition and trapping conditions of the NO
species. Band positions of only the relevant complexes are
summarized in Table 2.
The NO-stretching mode region of an ArÈNOÈH SO \
2 4
300 : 3 : 1 sample, deposited at 5 K [Fig. 2, trace A] is comprised of three sharp (1.5È1.8 cm~1) bands. The monomeric
NO stretch is seen at 1871.9 cm~1, and the l and l modes of
1
5
the stable cis-isomer of N O are found at 1863.3 and 1776.4
2 2
cm~1, respectively.17h21 These intense lines represent NO and
cis-N O species in stable argon matrix sites.
2 2
Increasing the deposition temperature to 15 K (Fig. 2, trace
B) caused the emergence of two new groups of absorptions.
The Ðrst includes absorptions of cis-N O trapped in less
2 2
stable matrix sites at 1779.8 (1.1) cm~1 (0.06 a.u.), 1865.3 cm~1
(sh), 1867.2 (1.5) cm~1 (0.008 a.u.)18,19,21, of cis-N O
2 2
aggregates18,19,21 at 1770.9 (1.6) cm~1 (0.1 a.u.) and of monomeric NO trapped in less stable sites at 1874.6 (1) cm~1 (0.03
a.u.)19 and 1876.9 (1.5) cm~1 (0.005 a.u.).21
The second group includes new bands absent in pure
NOÈAr samples. These absorptions at 1790.8 (3) cm~1 (0.03
a.u.), 1807 (2.2) cm~1 (0.02 a.u.), 1879.6 (1.1) cm~1 (0.075 a.u.),
1886.6 (2) cm~1 (0.025 a.u.), 1890.2 (1.1) cm~1 (0.04 a.u.),
1892.3 (1.1) cm~1 (0.03 a.u.) are therefore assigned to H SO 2 4
and H O-containing complexes. High temperature deposi2
tions (Fig. 2, traces B and C) increase the intensity of the Ðrst
three bands, along with the two cis-N O peaks, listed above.
2 2
On the basis of the assignments of the cis-N O peaks, the
2 2
1807 cm~1 band is assigned to the l antisymmetric stretch of
5
the 1 : 1 (N O ) É (H SO ) complex. Similar wavenumber
2 2
2 4
values were reported for 1 : 1 cis-N O complexes with
2 2
HNO ,22 with HF 21 and with HCl and HBr.19 Frequency
3
values and shifts are summarized in Table 3. Similarly to
Saxce et al.,19 we note that the formation of the 1 : 1
(N O ) É (H SO ) complex is related to the ““ stable site ÏÏ
2 2
2 4
trapped cis-N O and not to the dimer in less stable sites. The
2 2
second broader band at 1790.8 cm~1 follows the concentration of cis-N O aggregates and is assigned to the less deÐned
2 2
(N O ) É (H SO ) species.
2 2x
2 4
The 1879.6 cm~1 band [close to the l(NO) stretch at 1879.1
cm~1] is the complex band most a†ected by water concentra-
Fig. 2 NO concentration and deposition temperature e†ects in the
l(NO) region of ArÈNOÈH SO samples. All spectra recorded at 5 K.
4 deposited at 5 K. B, ArÈNOÈ
A, ArÈNOÈH SO \ 300 2: 1 : 1,
2
4
H SO \ 300 : 3 : 1, deposited at 15 K. C, ArÈNOÈH SO
2 4
\23004: 1 : 1, deposited at 15 K.
40
Phys. Chem. Chem. Phys., 1999, 1, 37È43
Table 3 E†ects of complexation on the l(NO) band of argon-matrixisolated NO
Complexing
species
Band position/
cm~1
Shift/cm~1
Relative shift
(%)
(Argon matrix)
HBr
(HBr)
HCl 2
1871.9
1877.6
1879.5
1880.4
1882
1887.3
1889
1890
5.7
7.6
8.5
10.1
15.4
17.1
18.1
0.3
0.4
0.45
0.54
0.82
0.91
0.97
(HCl)
HF 2
HNO
3
H SO
2 4
Ref.
19
21
22
This work
tion and deposition temperature. It has a parallel in the
l (H O) stretching mode region at 3725.5 (2) cm~1 (0.2 a.u.)
3 2
absorption. The two latter bands are thus assigned to the
l(NO) and l (H O) of the 1 : 1 (NO) É (H O) complex, respec3 2
2
tively.
The triplet of absorptions at 1890.2 cm~1 and the two
weaker bands at 1886.6 and 1892.3 cm~1 are located close to
the l(NO) stretch and attributed to NO complexes with
H SO . Increasing the deposition temperature a†ects the
2 4
1892.3 cm~1 band more than the 1886.6 cm~1 peak and has
only a minor inÑuence on the 1890.2 cm~1 absorption (Fig. 2).
Temperature cycling has an analogous e†ect and these
changes are thus attributed to the inÑuence of H O, the most
2
mobile matrix guest. Taking into account the low di†usion
coefficient of NO in solid argon,23 we conclude that the 1890.2
cm~1 band originates in the 1 : 1 (NO) É (H SO ) complex. The
2 4
1892.3 cm~1 band is assigned to the (NO) É (H O) É (H SO )
2
2 4
species and the 1886.6 cm~1 band is tentatively assigned to
(NO) É (H SO ) É (NO). The intensity of the 1886.6 cm~1 line
2 4
and, to a lesser extent (because of its H O dependence), that of
2
the 1892.3 band demonstrate a limited reversibility upon temperature cycling, similar to that of monomeric NO, indicating
some NO rotational freedom and, thus, a less deÐned structure. The band position of the (NO) É (H SO ) complex and its
2 4
shift [relative to l(NO)], is compared in Table 3 to other NO
complexes :
(NO) É (HF),21
(NO) É (HCl),
(NO) É (HCl) ,
2
(NO) É (HBr), (NO) É (HBr)19 and (NO) É (HNO ).22
2
3
In the OH stretching mode region, a complex band of
medium intensity, only found in NO-containing samples, is
centred at 3446.5 (11) cm~1 (0.1 a.u.) and red shifted (3.8%)
from the l(OH) band of H SO at 3566.7 cm~1. This band is
2 4
extremely weak for 5 K depositions (Fig. 3, trace A), but gains
intensity at higher deposition temperatures (Fig. 3, traces B
and C). Curve Ðtting of the band of the lowest (0.3%) NO
concentration sample, deposited at 15 K (Fig. 3, trace C), suggests four components : a main, strong band at 3446.5 (7)
cm~1 (0.1 a.u.) and three weaker ones, at 3452.3 (3) cm~1 (0.25
a.u., shoulder in the experimental trace), 3464 (4) cm~1 (0.01
a.u.), and 3474 (8) cm~1 (0.005 a.u.). Temperature cycling of
the sample between 15 and 5 K, reversibly changes the relative
intensity ratios, with the 3452.3 and 3464 cm~1 band intensities increasing upon cooling. Such behaviour resembles that
of monomeric NO species trapped in solid argon,19 and suggests the assignment of the latter two peaks to weak NO complexes, retaining some rotational freedom, in analogy to bands
in the l(NO) region. The 3464 cm~1 band shows H O concen2
tration dependence, paralleling that of the 1886.6 cm~1 in the
l(NO) region. The 3452.3 cm~1 shoulder is attributed to the
1 : 1 (NO) É (H SO ) species, and the 3464 cm~1 band to the
2 4
(NO) É (H O) É (H SO ) trimer. Both the l(OH) and l(NO) fre2
2 4
quencies attributed to (NO) É (H O) É (H SO ) resemble those
2
2 4
of (NO) É (H SO ) rather than those of (NO) É (H O), indicat2 4
2
ing an attachment of the H O moiety to the H SO molecule
2
2 4
rather than to NO, in favour of an H OwHOw
2
SO wOHwON conÐguration for this trimer.
2
Increasing the NO concentration (Fig. 3, trace B), irreversibly enhances the relative intensity of the 3446.5 cm~1 com-
Fig. 3 E†ects of NO concentration and of deposition temperature
on l(OH) of H SO . A, ArÈNOÈH SO \ 300 : 3 : 1 sample, depos2 4
2 4
ited and recorded at 5 K. B, ArÈNOÈH SO \ 300 : 3 : 1 sample,
2 4
deposited and recorded at 15 K. C, ArÈNOÈH SO \ 300 : 1 : 1
2 4
sample, cooled and recorded at 15 K.
ponent, revealing its N O -dependent nature. It is assigned to
2 2
the bonded OH stretch of the 1 : 1 (N O ) É (H SO ) species.
2 2
2 4
The 3474 cm~1 component is too weak and di†use to allow
a similar intensity determination or its deconvolution. As
several N O bands (e.g. produced via reduction of NO by
2
remnants of SO 24) were discerned in the sample (i.e. 2230,
2
2227, 2218.5, and 1283 cm~1 25), part of the 3474 cm~1 band
intensity originates in combination bands of argon-isolated
N O.
2
In the antisymmetric Sw(OH) stretching mode region,
2
three weak bands, not observed in NO free samples, emerge at
888.1, 893.3 and 898 cm~1 in NO-containing samples (Fig. 4,
traces A, B and C), blue shifted by 0.7È1.8% from the parallel
matrix-isolated pure H SO mode. By applying the above
2 4
considerations regarding concentration, deposition temperature and species mobility, we conclude that the 888.1 (1.7)
cm~1 (0.35 a.u.) relates to the 1 : 1 (N O ) É (H SO ) absorber
2 2
2 4
Fig. 4 NO concentration and deposition temperature e†ects on the
H SO Sw(OH) stretch. All spectra recorded at 5 K. A, ArÈNOÈ
H2SO4 \ 300 : 3 2: 1 sample, deposited at 5 K. B, ArÈNOÈH SO \
2 : 34: 1 sample, deposited at 15 K. C, ArÈNOÈH SO \ 300
2 : 41 : 1
300
2 4
sample deposited at 15 K.
and that the weaker 893.3 (2) cm~1 (0.1 a.u.) band represents
the parallel 1 : 1 (NO) É (H SO ). The weakest of these bands,
2 4
at 898 (3) cm~1 (0.05 a.u.), is assigned to the
(NO) É (H O) É (H SO ) species.
2
2 4
In the symmetric Sw(OH) -stretch region, two new absorp2
tions were discerned at 834.5 (1.1) cm~1 (0.2 a.u.) and 838.9
(2.4) cm~1 (0.4 a.u.), again blue shifted by 1% from the H SO
2 4
monomer values. Due to the presence in this region of bands
of sulfuric acidÈwater complexes,7 the assignments to the 1 : 1
(NO) É (H SO ) and the 1 : 1 (N O ) É (H SO ) species, respec2 4
2 2
2 4
tively, are more tentative.
The observed shift for the SxO modes due to the presence
2
of nitrogen oxide are negligible compared to those of the
l(OH) and the SwOH stretching modes. In addition to the
complicated H SO and (H O) É (H SO ) spectral picture,7
2 4
2 x
2 4
we Ðnd two extra shoulders, emerging at high N O concen2 2
tration, at 1445.7 and 1214.4 cm~1, and attribute them to the
1 : 1 (N O ) É (H SO ) complex.
2 2
2 4
In the \600 cm~1 region, the new bands are discerned at
560 (1) cm~1 (0.09 a.u.) and 555.3 (1) cm~1 (0.075 a.u.), near
the S(xO) -rock mode (558 cm~1) ;7 and at 552.2 (1) cm~1
2
(0.05 a.u.) and 545 (1.3) cm~1 (0.03 a.u.), close to the S(xO) 2
bending mode (548 cm~1).7 None reveal water concentration
dependence. The e†ects of varying the NO concentration and
the deposition temperature indicate that the 560 and 552.2
cm~1 bands belongs to the 1 : 1 (NO) É (H SO ) complex and
2 4
the 555.3 and the 545 cm~1 to the parallel 1 : 1
(N O ) É (H SO ) species. For higher NO concentration, the
2 2
2 4
new broad and strong bands at 561 and 551 cm~1, are
attributable to (NO) É (N O ) É (H SO ) (x,y [ 1) species.
x
2 2y
2 4
No absorptions could be attributed to the 1 : 1 (NO) É (SO )
3
and (N O ) É (SO ). For higher NO concentrations, new broad
2 2
3
bands emerge near the l (e@) SO stretch at 1395 (12) cm~1
3
3
(0.08 a.u.), near the l (e@) bend at 532 (6) cm~1 (0.14 a.u.) and
4
close to the l (aA) bend at 488 (3) cm~1 (0.1 a.u.). They are
2 2
assigned to (NO) É (N O ) É (SO ) (x,y [ 1) species.
x
2 2y
3
Structural considerations
1 : 1 (N ) Æ (H SO ). The shift in the l(OH) region of H SO
2
2 4
2 4
caused by N complexation, compared to the very small shifts
2
of the SxO vibrations, indicates a nitrogenÈOH group inter2
action rather than interaction with the double-bonded
oxygens.
For the otherwise IR-inactive l(N ) band of the 1 : 1
2
(N ) É (H SO ) complex, the very low intensity at which it is
2
2 4
observed, is induced by complexation. For the analogous 1 : 1
(N ) É (H O) and 1 : 1 (N ) É (HF) complexes, peaks of consider2
2
2
ably higher intensity were recorded.11 Another important difference is that its band position is red shifted from that of
argon-matrix isolated N . In contrast, the lines of the N
2
2
complexes with HF and H O,11 are blue shifted. These obser2
vations are in better accord with the more symmetrical Tshaped association of nitrogen to the H SO molecule as
2 4
depicted in Fig. 5(a), rather than with the alternative ““ headon ÏÏ complex. This statement is further supported by the fact
that, for the complexation of the isoelectronic, but polar, CO
with H SO , a ““ head-on ÏÏ conÐguration (OC) É (H SO ) was
2 4
2 4
suggested, and a blue shift for the l(CO) mode was observed.8
1 : 1 (NO) Æ (H SO ). The H SO vibration modes strongly
2 4
2 4
a†ected by complexation are those involving the wOH
groups rather than those related to the SxO double bonds.
In principle, the main bonding of H SO with NO can
2 4
involve either the nitrogen lone pair electrons (N-end
bonding) or the oxygen unpaired electron (O-bonding). The
induced polarization in the p electrons of the NO bond would
be an additional bonding contribution for both cases. Comparing the l(NO) band positions induced by the various complexants (Table 3), we note that Ðrst group of the H-bondingcapable and more polar acids (H SO , HNO , and HF) a†ect
2 4
3
Phys. Chem. Chem. Phys., 1999, 1, 37È43
41
Fig. 5 Models of complexes of H SO with N , NO and N O . (a) Suggested T-shaped conÐguration for the (N ) É (H SO ) complex. (b)
2 4
2
2 2
2
2 4
Suggested conÐguration for O-end interaction in the (NO) É (H SO ) complex. (c) Suggested conÐguration for O-end interaction in the
2 4
(N O ) É (H SO ) complex.
2 2
2 4
the larger shifts. For HBr and HCl as complexants, an interaction via the N atom of NO has been suggested.19 On the
other hand, for complexation with HF, a bonding via the
oxygen atom has been calculated to render the more stable
conÐguration.21 This comparison leads to the conclusion that,
for the present case of H SO , as well as for HNO ,22 the
2 4
3
more probable structure is the O-bonded one, as depicted in
Fig. 5(b). We also note that H SO interacts with both NO
2 4
and its dimer, (NO) . In contrast, for HCl and HBr19 only an
2
interaction with the NO monomer induces a wavenumber
shift, justifying a statement that a complex is formed (Table 3).
This fact is, again, in line with the complex structure
suggested19 for the 1 : 1 (NO) É (HX) complexes with X \ Cl,
Br, for which an interaction via the N atom of NO is argued.
In the latter case, when the interacting N atom is blocked by
bonding to form the dimer and has a net positive charge,20
the interaction of HCl and HBr with the N O molcules
2 2
becomes almost negligible.
Table 4 E†ects of complexation on the l band of argon-matrix-iso5
lated N O
2 2
Complexing
species
Band position/
cm~1
Shift/cm~1
Relative shift
(%)
(Argon matrix)
HCl
HBr
H SO
2 4
HF
HNO
3
1776.4
1776, 1777.7
1774.4, 1775.9
1807
1810.2
1813
0.45
[1.25
30.6
33.8
36.6
0.0.25
[0.07
1.7
1.9
2.06
Ref.
19
This work
21
22
1 : 1 (N O ) Æ (H SO ). The shifts induced on the l (cis2 2
2 4
5
N O ) band (Table 4) by the interaction with HCl or with
2 2
HBr are of the magnitude found by us for the SO ÈN inter3 2
action, which we preferred to describe as the e†ect of matrix
neighbours. On the other hand, the H-bonding capable complexants induce 20È30 times larger blue shifts on this mode.
As suggested above for the (NO) É (H SO ) complex, and pre2 4
viously for HF,21 and as is probably valid for HNO , the
3
complexation of H-bonding capable complexants with cisN O , occurs by their attachment to the oxygens of the nitro2 2
gen oxide dimer. Moreover, a comparison of the data in
Tables 3 and 4 shows that for the dimer, with one additional
interaction-capable oxygen, complexation with the H-bondcapable acids produces shifts in l which are larger, essentially
5
doubled, as compared to those of l(NO) of the monomer. In
the paragraph below, discussing the e†ects of complexation on
l(OH) of H SO and other acids, this di†erence in shifts is
2 4
suggested to be intrinsic to the antisymmetric nature of the l
5
dimer vibration.
E†ects on trapped SO species. The interaction between the
3
N and SO is very weak. It a†ects only minimal shifts
2
3
(\0.2%) from the argon matrix frequency values, for all SO
3
vibrational modes [as compared to the 5.3% shift in the l
2
bending mode of the (OC) É (SO )].8 Also, no lifting of the
3
degeneracy of the l (e@) and l (e@) modes is observed. The dis3
4
cerned frequency shifts indicate only a neighbouring inÑuence,
but not a stabilization of a deÐnable (N ) É (SO ) complex in
2
3
the matrix.
Table 5 E†ects of complexation on the l(HwX)/l(OH) band of argon-matrix-isolated acids and their [*G
[*G
Acid
H SO
2 4
HNO
3
HF
HBr (Q branch)
42
1264
1329.8
1520
HCl (Q branch)
acid
1372
1331
31,32
acid
(kJ mol~1)
Complexing
species
Band position/
cm~1
Shift/
cm~1
Relative shift
(%)
Ref.
(Argon matrix)
qNO
N O
2 2
rN
sCO2
3566.7
3452.3
3446.5
3547
3371.1
[114.4
[120.2
[19.7
[195.6
[3.21
[3.37
[0.55
[5.48
7
This work
This work
This work
7
(Argon matrix)
qNO
N O
2 2
rN
sCO2
3522
3404
3376, 3385
3488
3399
[118
[142
[34
[123
[3.35
[4.03
[0.97
[3.49
22
22
22
22
(Argon matrix)
qNO
N O
2 2
rN
sCO2
3919.5
3846.5
3839.1
3881.4
3789.3
[73
[80.5
[38
[130.2
[1.86
[2.05
[0.97
[3.32
21
21
11
28
(Argon matrix)
q NO
rN O
2 2
s CO
2870
2827
2815.1
2815.2
[43
[54.9
[54.8
[1.50
[1.91
[1.91
19
19
28
(Argon matrix)
q NO
rN O
2 2
s CO
2556
2517.3
2509.4
2520.1
[38.7
[46.6
[35.9
[1.51
[1.82
[1.40
19
19
28
Phys. Chem. Chem. Phys., 1999, 1, 37È43
We have no evidence of bands resulting from interactions of
NO or N O with SO . The e†ects of nitrogen oxide species
2 2
3
on SO may, therefore, be even weaker than those of N .
3
2
E†ects on m(HwX) stretch and m(OwH) stretch modes. In
Table 5 we list the wavenumber values of l(HwX) of several
matrix-isolated acids, together with the shifts induced by
interactions with NO, N O and N . It is quite obvious that
2 2
2
the N -induced shift is distinctly smaller than that of the other
2
two complexants, in strong support of the suggested di†erent
attachment character (T-shaped interaction). On the other
hand, the shifts induced by the nitrogen oxide monomer and
dimer are close in value, and are, consistently, somewhat
larger for the latter. This is, again, in line with the suggested
O-end interaction. We note that both monoprotic HNO 22
3
and diprotic H SO show similar shifts in their l(OwH) for
2 4
complexations with both NO and N O . Additionally, the
2 2
shifts in l(NO) of the monomer, induced by both acids, are
similar (Table 3) and the similarity remains for l (N O )
5 2 2
(Table 4). These similarities are all in accord only with an
interaction involving a single OwH bond of the acid and a
single OxN bond of the N O dimer, as illustrated in Fig.
2 2
5(c). The di†erences in complexation-induced shifts between
the NO monomer and the N O dimer must be attributed to
2 2
the nature of the vibrations involved. The l vibration is the
5
antisymmetric stretch of the dimer, and the larger sensitivity
of such modes to the molecular environment is already
known.26
The pattern of stronger complexation interactions with the
N O dimer is also evident in all the hydrogen halides, even
2 2
though an N-end interaction was suggested for NO with HCl
and HBr. Possibly, both kinds of (weak) interactions coexist
for the latter two. The weakness of the NxN bond in the
dimer may not allow a separate distinction of O-end interactions for monomer and dimer in the l(OwN) stretch, but it
is more evident in the l(HwX) modes. The case of HF as
complexant further supports the possibility of the coexistence
of both N-end and O-end interactions for hydrogen halides.
For its complexations with HCN 27 both possibilities have
been shown experimentally. For complexations with nitrogen
oxides, relevant to our present results, HF is an intermediate
case. As far as the e†ects on l(NO) positions is concerned it
clearly belongs to the group of H SO and HNO . In its
2 4
3
e†ects on the l(HwX) frequencies it is similar to the other
hydrogen halides, HCl and HBr.
The red shift of l(OH) is certainly a measure of hydrogenbond strength. In Table 5, where we summarize the relevant
shifts l(OH) for the H SO complexes of this study we also
2 4
include, for comparison, values for CO as ligand,7,22,28 and
for other acids : HNO ,22,29,30 HF,21 HCl and HBr.19 N
3
2
forms the weakest complex with all acids. Table 5 also
includes the relevant *G
values,31,32 where lower absolute
acid
values indicate higher acidity. The correlation of this thermodynamic quantity and the wavenumber shift is opposite for
the oxy-acids on the one hand and the hydrogen halides on
the other.
Conclusions
Absorptions due to argon-isolated 1 : 1 (N ) É (H O), 1 : 1
2
2
(N ) É (H SO ), 1 : 1 (NO) É (H O), 1 : 1 (NO) É (H SO ) and 1 : 1
2
2 4
2
2 4
(N O ) É (H SO ) dimeric complexes were identiÐed in all rele2 2
2 4
vant spectral regions. Several bands could also be assigned to
the
trimeric
species
(NO) É (H O) É (H SO )
and
2
2 4
(N ) É (H O) É (H SO ). Unlike the evidence for a speciÐc
2
2
2 4
(OC) É (SO ) species,7 the interactions of nitrogen and nitrogen
3
oxides with SO are insufficient to point to a speciÐc complex
3
formation. The spectroscopic evidence points towards a Tshaped attachment of N to H SO , and to an O-end inter2
2 4
action of the nitrogen oxide monomer and dimer with the OH
groups of H SO .
2 4
The relative band intensities of the complexes are found to
be independent from sample formation by either separate
depositions of (NO, N )ÈAr mixtures and pure H SO
2
2 4
vapours, or by Ñowing the (NO, N )ÈAr mixtures through the
2
H SO Ècontaining tube. We conclude that the formation of
2 4
the complex species is by surface di†usion during deposition
and is not a result of vapour phase equilibria.
A. L. acknowledges a visiting scientist grant from the Norwegian Research Council. We thank Dr. R. Fraenkel for her
help with the model Ðgures.
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23
24
25
26
27
28
29
30
31
32
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Paper 8/06635C
Phys. Chem. Chem. Phys., 1999, 1, 37È43
43