The Synthesis of 15N- and 18O-Isotopically-Enriched Nitryl Bromide, IR Matrix Spectra, and Force Fields of BrNO2, cis-BrONO, and trans-BrONO

Article abstract of DOI:10.1021/ic9803006

The gas phase reaction between BrNO and O₃ at low pressure (<10 mbar) results in the formation of BrNO₂ in about 60% yield. In this manner ¹⁵N- and ¹⁸O-labeled BrNO₂ are prepared. In addition, it is shown that BrNO₂ also forms by the heterogeneous low-temperature reaction between gaseous BrNO and solid sulfuric acid, doped with H₂O₂. It can be assumed that BrNO₂ may be formed in a similar way under stratospheric conditions. The isotope scrambling in the reaction between BrNO and ¹⁸O₃ as well as in the gas phase equilibrium BrNO + NO₂ ? BrNO₂ + NO are investigated. For the equilibrium constant a lower limit of K₂₉₈ ≥ 1 × 10^-3 is deduced from infrared measurements. A detailed IR and Raman study on BrNO₂ is performed. Photolysis of matrix-isolated BrNO₂ at different wavelengths leads to a mixture of cis- and trans-BrONO. The vibrational data of BrNO₂ (6 fundamentals, 7 combinations), cis-BrONO (5 fundamentals, 1 overtone), and trans-BrONO (4 fundamentals, 8 combinations) as well as the calculated force fields are in excellent agreement with the ab initio values, predicted by T. J. Lee (J. Phys. Chem. 1996, 100, 19847).

Full text of DOI:10.1021/ic9803006

4500
Inorg. Chem. 1998, 37, 4500-4506
15
18
The Synthesis of N- and O-Isotopically-Enriched Nitryl Bromide, IR Matrix Spectra, and
Force Fields of BrNO2, cis-BrONO, and trans-BrONO
Dieter Scheffler and Helge Willner*
Institut fu¨r Anorganische Chemie der Universita¨t Hannover, Callinstrasse 9,
D-30167 Hannover, Germany
ReceiVed March 17, 1998
The gas phase reaction between BrNO and O3 at low pressure (<10 mbar) results in the formation of BrNO2 in
about 60% yield. In this manner 15N- and 18O-labeled BrNO2 are prepared. In addition, it is shown that BrNO2
also forms by the heterogeneous low-temperature reaction between gaseous BrNO and solid sulfuric acid, doped
with H2O2. It can be assumed that BrNO2 may be formed in a similar way under stratospheric conditions. The
isotope scrambling in the reaction between BrNO and 18O3 as well as in the gas phase equilibrium BrNO + NO2
h BrNO2 + NO are investigated. For the equilibrium constant a lower limit of K298 g 1 × 10-3 is deduced from
infrared measurements. A detailed IR and Raman study on BrNO2 is performed. Photolysis of matrix-isolated
BrNO2 at different wavelengths leads to a mixture of cis- and trans-BrONO. The vibrational data of BrNO2 (6
fundamentals, 7 combinations), cis-BrONO (5 fundamentals, 1 overtone), and trans-BrONO (4 fundamentals, 8
combinations) as well as the calculated force fields are in excellent agreement with the ab initio values, predicted
by T. J. Lee (J. Phys. Chem. 1996, 100, 19847).
Introduction
Br and NO2 was assumed. The impact of bromine atoms on
the Arctic tropospheric ozone concentration has been confirmed
by field measurements.8,9 Under stratospheric conditions BrNO2
is possibly formed via the reaction of N2O5 with HBr on nitric
acid trihydrate particles.10 In recent laboratory experiments it
was found that BrNO2 was formed in a continuous-flow system
by the reaction of either diluted gaseous ClNO2 or Br2 with
aqueous solutions of NaBr or NaNO2, respectively.11,12 This
synthetic approach11 has enabled us to isolate pure BrNO2 for
the first time and to study its properties in detail.13 It was
possible to record 5 fundamental and 4 combination bands in
the infrared region and to study the UV/vis spectrum between
180 and 600 nm. Photolysis of matrix-isolated BrNO2 resulted
in trans-BrONO, and some infrared bands of this isomer were
assigned. BrNO2 was found to be surprisingly stable in the
gas phase at room temperature, where it exhibits a lifetime of
about 1 h in our vacuum system. It was hence possible for us
to record a high-resolution (<0.003 cm-1) infrared spectrum
and to determine rotational and centrifugal distortion constants
for the ground and ν4 ) 1 states of 79BrNO2 and 81BrNO2.14
The lifetime of BrNO2 appears to be limited by heterogeneous
decomposition on the walls of our vaccum system. Using the
fitted parameters for unimolecular decay, a gas phase lifetime
of about 7 h at 298 K was found.12 This is clearly inconsistent
The first attempts to synthesize nitryl bromide were reported
in the early 1960s.1,2 Kuhn and Olah tried to use nitryl bromide
in Friedel-Crafts type nitration reactions of aromatics. They
explored a number of possible formation reactions: the halogen
exchange of nitryl chloride with KBr in liquid SO2, the reaction
of BBr3 with either anhydrous nitric acid or N2O5, the reaction
of bromosulfonic acid with nitric acid, and the ozonolysis of
nitrosyl bromide. But in all cases the isolation of BrNO2 was
not accomplished.1 Martin et al. conducted gas phase studies
of the system BrNO/ClO2 and, under photolytic conditions, the
system Br2/NO2. They concluded that, while BrNO2 cannot
be formed at room temperature, there is evidence for its
formation in the phase diagram of Br2/N2O4 at low temperature.2
IR spectra of the reaction products of bromine atoms with NO2
isolated in an argon matrix, have been assigned to BrNO2 and
BrONO.3,4 No reference was made to the cis or trans isomer.
Due to increasing interest in bromine compounds and their
possible importance in atmospheric chemistry, recently several
experimental and theoretical studies of the title compounds were
undertaken. Gaseous BrNO2 was detected for the first time5 in
a long-path gas cell and identified by the strongest IR bands at
787, 1292, and 1660 cm-1 following the reaction of N2O5 with
BrNO or solid NaBr. Subsequently it was shown that BrNO2
can be formed in the troposphere, by the reaction of N2O5 with
sea salt aerosol.6,7 After formation, photolysis of BrNO2 into
(7) Behnke, W.; Scheer, V.; Zetzsch, C. J. Aerosol Sci. 1994, 25 (Suppl.
I), 277.
(8) Jobson, B. T.; Niki, H.; Jokouchi, Y.; Bottenheim, J.; Hopper, F.;
Leaitch, R. J. J. Geophys. Res. 1994, 99, 25355.
(9) Hausmann, M.; Platt U. J. Geophys. Res. 1994, 99, 25399.
(10) Hanson, D. R.; Ravishankara, A. R. J. Phys. Chem. 1992, 96, 9441.
(11) Frenzel, A.; Scheer, V.; Behnke, W.; Zetzsch, C. J. Phys. Chem. 1996,
100, 16447.
(12) Frenzel, A.; Scheer, V.; Sikorski, R.; George, Ch.; Behnke, W.;
Zetzsch, C. J. Phys. Chem. A 1998, 102 (8), 1329.
(13) Scheffler, D.; Grothe, H.; Willner, H.; Frenzel, A.; Zetzsch, C. Inorg.
Chem. 1997, 36, 335.
* Author to whom correspondence should be addressed.
(1) Kuhn, S. J., Olah, G. A. J. Am. Chem. Soc. 1961, 83, 4564.
(2) Martin, H.; Seidel, W.; Cnotka, H.-G.; Hellmayer, W. Z. Anorg. Allg.
Chem. 1964, 331, 333.
(3) Tevault, D. E. J. Phys. Chem. 1979, 83, 2217.
(4) Feuerhahn, M.; Minkwitz, R.; Engelhardt, U. J. Mol. Spectrosc. 1979,
77, 429.
(5) Finlayson-Pitts, B. J.; Livingston, F. E.; Berko, H. N. J. Phys. Chem.
1989, 93, 4397.
(6) Finlayson-Pitts, B. J.; Livingston, F. E.; Berko, H. N. Nature 1990,
343, 622.
(14) Orphal, J.; Frenzel, A.; Grothe, H.; Redlich, B.; Scheffler, D.; Willner,
H.; Zetzsch, C. J. Mol. Spectrosc., in press.
S0020-1669(98)00300-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/08/1998

The Synthesis of Isotopically-Enriched BrNO2
Inorganic Chemistry, Vol. 37, No. 18, 1998 4501
with an earlier value of 2.3 s reported by Kreuter and Wine.15,16
They observed the decay of Br atoms in the reaction with NO2,
and it may be assumed that BrONO was formed in their
experiments, in analogy to the reaction of Cl atoms with NO2,
where ClONO is formed.17,18 ClONO isomerizes heteroge-
neously to ClNO2.19 Hence BrONO seemed to be less stable
than BrNO2, in agreement with a recent ab initio study.20
This study suggests a Br-N bond energy of 94 kJ mol-1
,
which is comparable to the experimental activation energy of
89 ( 9 kJ mol-1 for unimolecular decay.12 In addition,
molecular structures, relative enthalpies (∆H(cis-BrONO-
BrNO2) ) 26.8, ∆H(trans-BrONO-BrNO2) ) 43.1 kJ mol-1
)
and complete vibrational spectra for the title compounds were
predicted.20 This prompted us to complete the experimental
spectrum of BrNO2 by Raman measurements and to reinvesti-
gate the photoisomerization of matrix-isolated BrNO2 in order
to detect both cis- and trans-BrONO. Additional vibrational
data were needed for the calculation of reliable force fields.
For this purpose a convenient synthesis of isotopically-enriched
BrNO2 was needed.
Figure 1. Glass ozonizer for the synthesis of isotopically-enriched
18O3: (a) connection to the vacuum line; (b) electrical heater; (c)
electrical field between concentrical tubes; (d) U-trap.
In this paper a new synthesis and a possible further route to
BrNO2 in the stratosphere are communicated.
Isotopically-enriched 18O3 was synthesized from 18O2 (99.5 atom %
18O, Chemotrade, Du¨sseldorf, Germany) in a small homemade ozonizer
with an internal volume of 60 mL (see Figure 1), attached to the vacuum
line (internal volume ca. 100 mL). In the ozonizer, 18O2 at a pressure
of about 500 mbar was circulated by an electrical heater. After oxygen
was passed through the electrical field of 20 kV cm-1 in the concentric
glass tubes, the produced 18O3 was trapped at -183 °C. The conversion
of 18O2 into 18O3 was followed by monitoring the pressure. After about
3 h, 2 mmol of 18O3 was formed. The excess 18O2 was recovered by
cryopumping the oxygen into a vessel containing molecular sieves (5
Å) held at -196 °C.
Experimental Section
CAUTION! Ozone and hydrogen peroxide are potentially explo-
sive, especially in the presence of organic materials or of catalytic traces
of transition metal ions adsorbed on glass walls. It is important to
take safety precautions when these compounds are handled in the pure
liquid state. Reactions involving either one of them should be carried
out only with millimolar quantities.
General Procedures and Reagents. Volatile materials were
manipulated in a glass vacuum line equipped with a capacitance pressure
gauge (221 AHS-1000 MKS Baratron, Burlington, MA), three U-traps,
and valves with PTFE stems (Young, London, U.K.). The vacuum
line was connected to an IR cell (optical path length 200 mm, Si
windows 0.5 mm thick) contained in the sample compartment of the
FTIR instrument. This allows one to observe the purification processes
and to follow the course of reactions. Glass reactors (250 mL bulbs)
fitted with 10 mm valves with PTFE stems were carefully cleaned with
hot concentrated sulfuric acid and distilled water prior to use. The
following chemicals were obtained from commercial sources: bromine
(p.a. quality, Merck), nitric oxide (99.8% Messer Griesheim, Du¨sseldorf,
Germany), and nitrogen dioxide (99% Baker, Philipsburg, NY). They
were purified by trap-to-trap condensation prior to use. Sulfuric acid
(96%, p.a. quality, Merck), hydrogen peroxide (80%, Solvay, Interox
GmbH, Hannover, Germany), and mercury (Riedel de Haen AG, Seelze,
Germany) were used without further purification.
Thermally-labile products were vacuum transferred in glass ampules,
flame-sealed, and stored under liquid nitrogen in a long-term Dewar
vessel. The ampules were opened and resealed using an ampule key.21
Synthesis of O3. Ozone was made in an ozonizer (model 301,
Sander, Eltze, Germany) and trapped using liquid oxygen as coolant
in order to prevent the condensation of oxygen. Oxygen dissolved in
ozone was removed in a vacuum at -196 °C.
Synthesis of BrNO. For the synthesis of BrNO and Br15NO a dry,
evacuated reaction vessel with an internal volume of 250 mL was filled
with a mixture of 150 mbar of Br2 (1.5 mmol) and 320 mbar of NO
(3.2 mmol). After a reaction time of 30 min at 20 °C, the products
were passed in a vacuum through three U-traps held at -65, -120,
and -196 °C. The trap held at -120 °C contained about 2.4 mmol of
pure BrNO (≈80% yield).
15NO was made by the reaction of sulfuric acid, mercury, and
Na15NO3 (>99 atom % 15N, Isotec Inc., Miamisburg). A 250 mL bulb
equipped with a 10 mm Young valve was charged with 688 mg of
Na15NO3 (8 mmol) and 15 g of mercury (75 mmol) and then evacuated.
About 10 g of H2SO4 (containing 20 wt % H2O) was slowly introduced
through the valve into the evacuated reaction vessel. The contents were
shaken for 1 h at room temperature, and the gaseous reaction products
were passed in a vacuum through U-traps held at -100 and -196 °C.
The trap at -196 °C contained 15NO with a few percent 15N2O as
impurity.
Synthesis of BrNO2. In the course of this study, it was found that
BrNO can be oxidized to BrNO2 by O3 or H2SO4/H2O2 under
appropriate conditions.
The dry evacuated 250 mL reaction vessel was charged with 10 mbar
of ozone at 20 °C (0.1 mmol). Into this vessel containing the ozone
was slowly introduced 0.07 mmol of BrNO within a few minutes.
CAUTION! This slow filling process was very important, because
otherwise the contents did explode. After a reaction time of about 5
min the products were slowly passed in a vacuum through three U-traps
held at -90, -125, and -196°. In the trap at -125 °C, yellow solid
nitryl bromide was collected (yield ca. 60%). An excess of ozone
during the synthesis was necessary in order to remove NOx as the less
volatile side product N2O5. Br15NO2 was formed in the same manner
from Br15NO and O3. Treatment of BrNO with 18O3 resulted in an
equimolar mixture of BrN16O2, BrN16O18O, and BrN18O2 as well as
N2O5 containing 18O.
(15) Kreutter, K. D.; Nicovich, J. M.; Wine, P. H. J. Phys. Chem. 1991,
95, 4020.
(16) Wine, P. H.; Nicovich, J. M.; Stickel, R. E.; Zhao, Z.; Shackelford,
C. J.; Kreutter, K. D.; Daykin, E. P.; Wang, S. In Halogen and sulfur
reactions releVant to polar chemistry. The Tropospheric Chemistry
of Ozone in the Polar Regions; Niki, H., Becker, K. H., Eds.; Springer-
Verlag: Berlin, 1993.
(17) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys.
Lett. 1978, 59, 78.
(18) Leu, M.-T. Int. J. Chem. Kinet. 1984, 16, 1311.
(19) Janowski, B.; Knauth; H.-D.; Martin, H. Ber. Bunsen-Ges. Phys. Chem.
1977, 81, 1262.
Another route to BrNO2 was the heterogeneous reaction of BrNO
on a cold H2SO4/H2O2 surface. First, 3.5 g of a solution of hydrogen
(20) Lee, T. J. J. Phys. Chem. 1996, 100, 19847.
(21) Gombler, W.; Willner, H. J. Phys. E: Sci. Instrum. 1987, 20, 1286.

4502 Inorganic Chemistry, Vol. 37, No. 18, 1998
Scheffler and Willner
peroxide (80%) in concentrated sulfuric acid (96%) at a mass ratio of
1:20 was distributed as a thin film over the inner surface of the 250
mL reaction vessel. Then, after being cooled to -20 or -80 °C, the
vessel was evacuated and charged with 0.25 mmol of BrNO. The
course of the reaction was followed by removal of small samples (0.05
mmol) for infrared analysis. At -20 °C the BrNO was converted into
BrNO2, some Br2, N2O5, and HNO3 within 1 min. At -80 °C the same
reaction required several hours.
In reference to the reaction
BrNO + O3 f BrNO2 + O2
(1)
which results in the formation of BrNO2 in about 60% yield,
we note that the analogous reaction
ClNO + O3 f ClNO2 + O2
(2)
Determination of the Equilibrium BrNO + NO2 h BrNO2 +
NO. Into the IR cell, containing NO2 with a pressure of about 3 mbar,
was introduced BrNO within a few seconds to achieve a total pressure
of about 6 mbar. Simultaneously the IR measurement was started and
repeated every 10 s. The maximal partial pressure of BrNO2 was
calculated from the observed absorbance at 1301 cm-1 with the help
of the known absorption cross section.13 For the calculation of the
equilibrium constant, the partial pressure of NO was set equal to the
partial pressure of BrNO2, because NO is formed together with BrNO2
simultaneously. The molar ratio NO:BrNO2 is slightly affected by the
decomposition of BrNO2, which is slow in comparison to the BrNO +
NO2 reaction.
Preparation of the Matrices. Small amounts of pure BrNO2
samples (ca. 0.1 mmol) were transferred in a vacuum into a small
U-trap kept in liquid nitrogen. This U-trap was mounted in front of
the matrix support and allowed to reach a temperature of -125
°C. A gas stream (3 mmol h-1) of argon was directed over the
solid BrNO2, and the resulting gas mixture was immediately
quenched on the matrix support at 12 K. For each BrNO2 sample two
different amounts of matrix material (1 and 4 mmol, respectively) were
deposited. Photolysis experiments were undertaken in the visible
(>350, >450 nm) and UV regions (254 nm) by using a 250 W tungsten
halogen lamp (Osram) and a high-pressure mercury lamp (TQ 150,
Heraeus), respectively, in combination with cutoff and interference
filters (Schott). Details of the matrix apparatus have been described
elsewere.22
was reported in 1929 by Schumacher and Sprenger.23 This
reaction is kinetically hindered and is catalyzed by traces of
NOx, presumably NO3.24 Reaction 1 goes to completion in a
few minutes at room temperature, and this indicates the
participation of the same catalysis. In contrast to ClNO, BrNO
easily decomposes according to
2BrNO h Br2 + 2NO
(3)
and NO as a catalyst is always present in situ.
The large heat of reaction 1 needs to be dissipated in the
reaction mixture. It is therefore important to introduce BrNO
slowly into the reaction vessel containing ozone. In addition,
it is observed that BrNO catalyzes the decomposition of BrNO2.
In order for this to be prevented, BrNO must be introduced into
the excess of ozone and not in the opposite way, ozone into an
excess of BrNO.
As described in the experimental part, BrNO reacts with 18O3
in an equilibrium mixture of BrNO2, BrNO18O, and BrN18O2.
This result may be rationalized by a fast Br atom exchange
between BrNO2 and the decomposition products of the byprod-
uct N2O5 according to
N2O5 f NO2 + NO3
(4)
Instruments. Matrix infrared spectra were recorded on a Bruker
IFS 66v FT spectrometer in the reflectance mode using a transfer optic.
A DTGS detector together with a KBr/Ge beam splitter operated in
the region 5000-400 cm-1. In this region 64 scans were co-added for
each spectrum using apodized resolutions of 1.2 or 0.3 cm-1. A far-
IR DTGS detector together with a 6 µm Mylar beam splitter was used
in the region 440-80 cm-1. In this region 128 or 64 scans were co-
followed by
BrN*O2 + NO2 f BrNO2 + N*O2
N*O2 + NO3 f N2*O5
(5)
(6)
added for each spectrum using apodized resolutions of 1.2 or 0.3 cm-1
respectively.
,
By introducing 15NO2 into an IR cell containing BrNO2 we have
been able to demonstrate that the exchange in reaction 5 is fast
compared to the formation rate of BrNO2 according to reaction
1. Very fast isotope scrambling reactions between nitrogen
oxides as formulated in reactions 4 and 6 are well-known.25,26
The investigation of the reaction
Gas phase infrared spectra were recorded on a Nicolet Impact 400
D FTIR spectrometer operating between 5000 and 400 cm-1 at a
resolution of 2 cm-1
.
Raman spectra of solid BrNO2 at -196 °C were recorded with a
resolution of 2 cm-1 on a Bruker FRA 106 FT Raman spectrometer
using the 1064 nm exciting line of a narrow-band Nd:YAG laser (DPY
301, ADLAS, Lu¨beck, Germany).
BrNO + NO2 h BrNO2 + NO
(7)
Results and Discussion
shows that an equilibrium can be achieved very fast from either
side at 3 mbar partial pressure of each component at 25 °C (<5
s, the time-resolving limit of our spectrometer). It is possible
by IR spectroscopy to determine the equilibrium pressure of
BrNO2 by mixing NO2 with BrNO in the IR cell. This approach
is preferred over the reverse reaction between BrNO2 and NO,
because BrNO is more easily prepared and purified than BrNO2.
After introducing BrNO into the NO2-containing gas cell, BrNO2
appears in the IR spectrum in less than 5 s, but it decays
subsequently into Br2 and NO2, because BrNO catalyzes the
decomposition of BrNO2. Hence the highest BrNO2 concentra-
tion, observed in several experiments, determines the lower limit
Syntheses. All syntheses of BrNO2 described previous5,11,12
are inefficient, have low product yields, and are inappropriate
for the preparation of isotopically-enriched Br15NO2 and
BrN18O2. In a previous study on the properties of BrNO2 we
were able to demonstrate that its decomposition into Br2 and
NO2 is strongly affected by heterogeneous wall reactions.13 In
dry glass vacuum systems, pretreated with NO2 and O3, the
lifetime of BrNO2 at partial pressures of a few millibars at room
temperature is about 1 h, which is sufficient for all manipula-
tions.
The formation of BrNO2 in equilibrium with Br2 and N2O4
in the liquid or solid state, as assumed earlier,2 must be ruled
out, because by Raman measurements we are unable to see the
bands of BrNO2 although the detection limit is as low as 0.1%.
of the equilibrium constant K(7) g 1 × 10-3
.
(23) Schumacher, H. J.; Sprenger, G. Z. Anorg. Chem. 1929, 182, 140.
(24) Dust, H.-P. Dissertation, University of Kiel, 1953.
(25) Scharma, H. D.; Jervis, R. E.; Wong, K. Y. J. Phys. Chem. 1970, 74,
923.
(22) Argu¨ello, G. A.; Grothe, H.; Kronberg, M.; Willner, H.; Mack, H.-G.
J. Phys. Chem. 1995, 99, 17525.
(26) Ogg, R. A., Jr. J. Chem. Phys. 1953, 21, 2079.

The Synthesis of Isotopically-Enriched BrNO2
Inorganic Chemistry, Vol. 37, No. 18, 1998 4503
Table 1. Vibrational Band Positions (cm-1) of Natural and Isotopically-Enriched BrNO2
ab initioa
CCSD(T)/TZ2P
IR (Ar matrix)
Br15NO2
Ramanc
BrNO2
assign.
according C2V sym
Ib
BrNO2
Ib
BrN18OO
BrN18O2
3294.61
2931.01
2840.39
2574.45
2498.34
1659.62
1564.87
1290.98
1195.88
782.68
2
6
0.3
2
0.5
92
0.2
100
16
54
2
3224.45
2881.35
2776.08
2537.41
2453.97
1623.84
1543.59
1277.16
1167.11
772.59
3265.71
2893.55
2816.70
2528.46
2464.13
1645.15
1538.17
1266.65
1186.60
769.47
3233.04
2856.41
2790.50
2492.55
2428.75
1628.77
1510.44
1245.03
1176.54
755.43
2ν4
ν1 + ν4
ν4 + 2ν6
2ν1
ν1 + 2ν6
ν4 (b1)
2ν2
ν1 (a1)
2ν6
ν2 (a1)
ν6 (b2)
ν5 (b1)
ν3 (a1), 79Br
ν3 (a1), 81Br
1663
1288
86
1584, m
100
1273, m
1210, w
785, s
612, w
366, s
799
613
349
58
1
0.03
605.72
590.05
602.14
598.56
281.83
280.67
279.85
278.56
278.05
276.65
272.69
271.43
281
8
8
314, vs
}
a Reference 20, harmonic wavenumbers, no scaling factor used. b Relative integrated absorbance. c Solid sample at -196 °C.
For the analogous reaction
ClNO + NO2 h ClNO2 + NO
(8)
the equilibrium constant is determined from kinetic27 and IR28
measurements to be 1.01 × 10-4 and 1.23 × 10-4, respectively.
For this reaction a chlorine atom transfer mechanism is
established.27
It is now well documented, on the basis of several laboratory
experiments, that BrNO2 can be formed in the troposphere by
heterogeneous reactions between N2O5 or ClNO2 with sea salt
aerosol.6,7 However, possible routes to BrNO2 in the strato-
sphere, suggested by laboratory experiments, are yet unknown.
The stratospheric background aerosol, called the Junge layer,
consists mainly in 40-80 wt % sulfuric acid doped with
different compounds.29 If NO3 enters the aerosol and H2O2 is
formed,30 the aerosol particles should exhibit strong oxidizing
power. This oxidizing power is demonstrated by heating
sulfuric acid containing H2O2, which results in ozone in up to
a 30% yield.31,32 In order to check for the possible formation
of BrNO2 in the stratosphere, we have studied the heterogeneous
reaction between BrNO and sulfuric acid, doped with H2O2.
BrNO at a pressure of ca. 1 mbar over thin layers of H2SO4/
H2O2 at -20 °C is converted within seconds into BrNO2, Br2,
N2O5, and HNO3. At -80 °C this reaction requires several
hours for completion. This heterogeneous reaction has to be
studied in detail, to establish its impact on the stratospheric
chemistry.
Vibrational Spectra. The molecules BrNO2, cis-BrONO,
and trans-BrONO have been studied recently at the CCSD(T)/
TZ2P level of theory.20 Reliable energies, molecular geometry,
dipole moments, harmonic vibrational wavenumbers, and IR
intensities were predicted at a time when our experimental study
on the properties of BrNO2 and its photoisomerization into trans-
BrONO was in press.13 With all predicted vibrational data now
at hand, some of our previously communicated band positions
of trans-BrONO13 must be assigned to cis-BrONO. In order
to complete our previous experimental study on the vibrational
spectra of BrNO2 and trans-BrONO,13 we have recorded a
Figure 2. Vibrational spectra of BrNO2. Upper trace: IR spectrum,
argon matrix at 15 K. Lower trace: Raman spectrum, solid at 77 K.
Impurities: (O) NO2, (*) N2O4, (b) Br2.
Raman spectrum of BrNO2 and have reinvestigated the pho-
toisomerization of matrix-isolated BrNO2 and its isotopomers
into cis- and trans-BrONO.
In Figure 2 are shown IR and Raman spectra of BrNO2, and
in Table 1, all experimental data of BrNO2 are compared with
the calculated wavenumbers and relative IR intensities of the 6
fundamentals. The fundamental ν5, which is not detectable by
IR, due to its low absorption cross section, is observed for the
first time in the Raman spectrum of solid BrNO2. Considering
that all calculated band positions are based on harmonic
vibrations (without use of a scaling factor) while all observed
band positions are affected by anharmonicity as well as by
matrix and solid state effects, the agreement between calculated
and observed band positions and intensities of IR bands is
excellent. The proposed assignment of the observed combina-
tions and overtones is strongly supported by the vibrational data
of 15N- and 18O-labeled BrNO2 molecules. The isotopic shifts
of the fundamentals are also used for force field calculations
(vide infra).
(27) Wilkins, R. A., Jr.; Dodge, M. C.; Hisatsune, I. C. J. Phys. Chem.
1974, 78, 2073.
(28) Ray, J. D.; Ogg, R. A., Jr. J. Chem. Phys. 1959, 31, 168.
(29) Steele, H. M.; Hamill, P.; McCormick, M. P.; Swissler, T. J. J. Atmos.
Sci. 1983, 40, 2055. Solomon, S. Nature 1990, 347, 347.
(30) Pedersen, T. Geophys. Res. Lett. 1995, 22, 1497.
(31) Staedel, W, Z. Anorg. Chem. 1902, 15, 642.
(32) Dimitrov, A.; Seppelt, K.; Scheffler, D.; Willner, H. J. Am. Chem.
Soc., in press.
The UV absorption spectrum of BrNO2 suggests that, at λmax
) 199, 247, and 372 nm, at least three electronically excited

4504 Inorganic Chemistry, Vol. 37, No. 18, 1998
Scheffler and Willner
Table 2. IR Band Positions (cm-1) of Natural and Isotopically-Enriched cis-BrONO Isolated in an Argon Matrix
ab initioa
CCSD(T)/TZ2P
IR (Ar matrix)
Br18ONO
assign.
according Cs sym
Ib
100
3
27
38
BrONO
Ib
BrO15NO
BrON18O
1608.63
Br18ON18O
1606.13
3262.4
1650.70
862.6
573.47
420.15
3
100
2
48
30
3204.49
1623.88
2ν1
1664
845
558
429
210
357
1650.7
ν1 (a′)
ν2 (a′)
ν3 (a′)
ν4 (a′)
ν5 (a′)
ν6 (a′′)
565.54
418.93
558.43
402.31
573.47
419.05
558.43
401.34
0.03
0.6
368.25
2
361.71
362.06
365.14
358.98
a Reference 20, harmonic wavenumbers, no scaling factor used. b Relative integrated absorbance.
Table 3. IR Band Positions (cm-1) of Natural and Isotopically-Enriched trans-BrONO Isolated in an Argon Matrix
ab initioa
CCSD(T)/TZ2P
IR (Ar matrix)
Br18ONO
assign.
according Cs sym
Ib
BrONO
Ib
BrO15NO
BrON18O
3330.1
Br18ON18O
3325.3
3413.0
2895.2
2560.7
2315.69
2119.78
1723.38
1414.3
1222.5
835.89
586.88
391.21
7
2
1
6
3
100
4
1
48
93
13
3354.27
2848.2
2520.1
2280.06
2087.06
1693.25
1399.10
1210.2
825.78
582.60
389.25
3409.7
2ν1
ν1 + 2ν3
ν1 + ν2
ν1 + ν3
ν1 + ν4
ν1 (a′)
ν2 + ν3
ν2 + ν4
ν2 (a′)
ν3 (a′)
ν4 (a′)
ν5 (a′)
2ν6
2293.25
2110.34
1721.95
1376.5
2271.69
2067.66
1680.34
1404.0
2249.17
2059.39
1678.98
1365.6
1740
100
835
601
385
226
44
82
20
817.89
565.77
382.95
826.16
585.22
382.95
807.55
564.23
375.12
0.01
299.3
2
294.6
158
0.001
ν6 (a′′)
a Reference 20, harmonic wavenumbers, no scaling factor used. b Relative integrated absorbance.
states are accessible,13 so that irradiation of matrix-isolated
BrNO2 with UV light of different energy should cause bond
rupture into various products with a variable amount of excess
energy:
Br + NO2
(9)
BrNO2 + hν
Br + NO + O
BrNO + O
Recombination of the fragments in the matrix cage can lead
in principle to trans-BrONO and cis-BrONO. If the O atom
escapes the matrix cage, BrNO and BrON may form as well.
During irradiation, the products can be photolyzed, too, so that
finally a photostationary product mixture is formed. Figure 3
presents typical IR spectra of the photolysis products, which
are assigned by comparison with predicted IR spectra of cis-
and trans-BrONO based on ab initio calculations20 and reference
spectra. The molar cis:trans ratios were deduced from the
observed integrated intensity ratios between ν3(cis-BrONO) near
573 cm-1 and ν3(trans-BrONO) near 587 cm-1 and the
respective calculated integrated band strength of 1:3.7 obtained
by ab initio calculations. By use of visible light of λ > 450
nm, the yield of cis-BrONO is the highest (the molar cis:trans
ratio is 3.1:1), and unfiltered light of a tungsten halogen lamp
forms a mixture of BrONO isomers with an increased trans-
BrONO content (the molar cis:trans ratio is 2.5:1). A similar
result is observed with the strong 254 nm emission of a high-
pressure mercury lamp (the molar cis:trans ratio is 1.2:1), but
in these experiments weak additional bands at 1799.1 cm-1
(BrNO), 1820.1 cm-1 (BrON),33 and 1858.2 cm-1(NO)34 are
observed. The assignment of the trans- and cis-BrONO bands
is based on the observed relative band intensities in different
Figure 3. Photolysis products of BrNO2 isolated in an argon matrix.
Residual bands of BrNO2 are subtracted electronically; c, cis-BrONO;
t, trans-BrONO. Upper trace: after irradiation with light λ > 450 nm.
Lower trace: after irradiation with light λ ) 254 nm. Impurities: (O)
NO2, (*) N2O4.
photolysis experiments and supported by comparison with the
band positions obtained in the ab initio study.20 In Tables 2
and 3 the wavenumbers and relative IR intensities of the
fundamentals, obtained by ab initio calculations, are presented
together with experimental data for the cis- and trans-BrONO
isomers. With the exception of the very weak IR bands ν5 for
cis- and trans-BrONO and ν6 for trans-BrONO, experimental
(33) Lee, T. J. J. Phys. Chem. 1995, 99, 15074.
(34) Hedberg, L.; Mills, I. M. J. Mol. Spectrosc. 1993, 160, 117.

The Synthesis of Isotopically-Enriched BrNO2
Inorganic Chemistry, Vol. 37, No. 18, 1998 4505
Table 4. Input Dataa (cm-1) for the Force Field Calculation of
BrNO2
Table 6. Input Dataa (cm-1) for the Force Field Calculation of
trans-BrONOb
b
∆νid
∆νid
i
νic
Br15NO2
BrN18O2
81BrNO2
i
νic
BrO15NO Br18ONO BrON18O Br18ON18O
1
1277e
(1277)
782.68
11
(15)
10.2
(10.9)
2.00
(2.31)
36.1
(36.9)
52
(46)
27.5
(27.5)
9.14
(8.29)
31.2
(31.5)
0.0
(0.0)
0.00
(0.05)
1.16
(1.27)
0.00
1
1723.38
(1723.36)
835.89
(835.91)
586.88
(586.30)
391.21
(391.22)
30.4
(31.0)
10.2
(10.7)
4.32
(5.61)
1.98
(1.89)
1.44
(1.05)
18.2
(16.9)
21.3
(21.1)
8.34
(7.71)
43.5
(43.8)
9.82
(10.84)
1.68
(0.29)
8.34
(8.69)
44.8
(44.9)
28.6
(28.2)
22.9
(22.3)
16.3
(16.2)
2
3
4
5
6
2
3
4
5
6
(782.66)
281.83
(281.89)
1659.62
(1659.59)
366
(366)
605.72
(605.71)
(0.00)
(1.7)
15.7
(15.8)
(17)
7.16
(7.46)
(0.3)
0.00
(0.04)
(221)
(158)
(2.1)
(2.3)
(4.2)
(1.4)
(2.7)
(3.4)
(6.9)
(4.8)
a Uncertainties of values without a decimal point and with one digit
a Uncertainties of values without a decimal point and with one digit
and two digits behind the decimal point are (2, (0.2, and (0.05 cm-1
,
and two digits behind the decimal point are (2, (0.2, (0.05 cm-1
,
respectively. b Values in parentheses are calculated data with the final
force field. c Fundamentals according to Table 1. d ∆ν ) ν(BrNO2) -
ν(isotopomer). e Fermi-resonance corrected, see text.
respectively. b Values in parentheses are calculated data with the final
force field. c Fundamentals according to Table 3. d ∆ν ) ν(BrONO)
- ν(isotopomer).
Table 5. Input Dataa (cm-1) for the Force Field Calculation of
cis-BrONOb
Table 7. Quadratic Force Fieldsa for BrNO2, cis-BrONO, and
trans-BrONO
∆νid
BrNO2
cis-BrONO
trans-BrONO
i
νic
BrO15NO Br18ONO BrON18O Br18ON18O
ASYM20 ab initiob ASYM20 ab initiob ASYM20 ab initiob
1
1650.70
(1650.66)
862.6
(862.6)
573.47
(573.47)
420.15
(420.15)
27.1
(28.3)
0.0
(2.6)
42.5
(42.4)
45.0
(45.1)
1.481
0.500
11.424
1.371
0.761
12.091
2.445
0.868
2.323
2.475
0.768
1.760
0.004
2.021
12.830
0.258
0.391
2.682
0.422
1.694
0.751
1.889
13.364
0.070
0.313
0.264
0.944
0.232
0.208
0.436
0.324
1.962
0.109
2.996 F11
0.244 F21
1.608 F22
-0.070 F31
1.926 F32
13.641 F33
0.277 F41
0.172 F42
0.172 F43
0.937 F44
0.230 F51
0.210 F52
0.500 F53
0.327 F54
1.922 F55
0.109 F66
2
3
4
5
6
(12.3)
8.0
(8.1)
1.2
(11.5)
15.2
(17.0)
18.0
(15.9)
0.0
(0.2)
1.1
(27.6)
15.2
(17.4)
19.0
-0.132 -0.499 -0.559
0.724
1.655
0.798
2.022
2.934
12.573
0.451
0.502
0.228 -0.025
1.239 1.373
-0.086 -0.299
0.571
0.798
0.071 -0.018
2.436
0.224
(1.4)
(18.0)
(1.0)
(19.0)
(210)
368.25
(368.25)
(0.5)
6.5
(6.7)
(1.3)
6.2
(5.9)
(7.1)
3.1
(3.3)
(8.3)
9.3
(9.3)
9.113
9.181
0.614
0.674
a Uncertainties of values without a decimal point and with one digit
0.414
0.947
0.371
0.411
0.857
0.380
and two digits behind the decimal point are (2, (0.2, and (0.05 cm-1
,
2.230
0.211
respectively. b Values in parentheses are calculated data with the final
force field. c Fundamentals according to Table 2. d ∆ν ) ν(BrONO)
- ν(isotopomer).
a Force constants in 102 N m-1, normalized on 100 pm bond length.
For BrNO2, symmetry internal coordinates (including order) are defined
as follows:
r
BrN, 2-0.5(rNO1 + rNO2), ONO, 2-0.5(rNO1 - rNO2),
-
values for all fundamentals are known. Again, the agreement
between calculated20 and observed values for both wavenumbers
and relative intensities of the fundamentals is excellent. In
addition the wavenumber of the band assigned to 2ν6 fits with
the wavenumber for ν6, predicted from ab initio calculation.20
With the exception of ν1, the assignment of the fundamentals
for the two BrONO isotopomere containing one 18O atom is
not straightforward. The fundamentals are assigned by calcu-
lated band position using the respective force fields (vide infra).
With the help of the available matrix data in Tables 1-3,
which are unambiguous, the results of early matrix experi-
ments3,4 on the cocondensation of NO2 with Br atoms can now
be reinterpreted. In the carefully conducted experiments by
Tevault,3 11 of the observed IR band positions coincide with
our data. These bands are attributed to a mixture of mainly
BrNO2 and cis-BrONO as well a small amount of trans-BrONO.
The spectral data obtained from similar experiments by Feuer-
hahn et al.4 are not in good agreement, because only two bands
coincide with our data. In conclusion, it is interesting to note
that by cocondensation of Br atoms with NO2 the more stable
cis-BrONO is formed in a thermodynamically-controlled manner
and the photoisomerization of BrNO2 in a matrix leads mainly
to the less stable trans-BrONO with increasing energy of the
light radiation.
2-0.5
(
BrNO1
BrNO2), δ oop. For the BrONO isomers, the
following internal coordinates have been used: rBrO, rON, rNO
,
BrON,
ONO, τ (torsional coordinate). b Reference 20.
General Valence Force Fields. The structural parameters
for BrNO2, cis-BrONO, and trans-BrONO from ab initio
calculations20 together with the wavenumbers of the fundamen-
tals and their isotopic shifts (Tables 4, 5, and 6, respectively)
are taken as input data for the calculation of the general valence
force fields with the program ASYM20.34 Strong Fermi
resonance between ν1 and 2ν6 is observed in the IR spectra of
BrNO2. Therefore the band position of ν1 for BrNO2 in Table
4 was corrected using the equation
νcorr ) (νa + νb)/2 ( (νa - νb)(s - 1)/[2(s + 1)], s ) Ia/Ib
(ref 35). Also the isotopic shifts ∆ν1 of BrNO2 are affected by
(35) Overend, J. In Infrared Spectra and Molecular Structure; Davies, M.,
Ed.; Elsevier: New York, 1963; p 352.
(36) Gatehouse, B.; Mu¨ller, H. S. P.; Heineking, N.; Gerry, M. C. L. J.
Chem. Soc., Faraday Trans. 1995, 91 (19), 3347 and references therein.
(37) Bernitt, D. L.; Miller, R. H.; Hisatsune, I. C. Spectrochim. Acta 1967,
23A, 237. Mirri, A. M.; Cazzoli, G.; Ferretti, L. J. Chem. Phys. 1968,
49, 2775.
(38) Orphal, J. Dissertation, University of Orsay, 1995.

4506 Inorganic Chemistry, Vol. 37, No. 18, 1998
Scheffler and Willner
Table 8. Valence Force Constants of XNO and XNO2 Molecules,
X ) F, Cl, Br
data from Tables 4-6 are weighted by their uncertainties. With
preliminary force fields for the cis- and trans-BrONO isoto-
pomers, the wavenumbers of the 18O-monosubstituted species
are calculated, allowing a correct assignment of the observed
bands (Tables 2 and 3). The iteration procedure is ended, after
all vibrational data are reproduced within their experimental
uncertainties as presented in Tables 4-6. The agreement
between the ab initio and the final experimental force fields of
the three BrNO2 isomers is excellent (see Table 7). This
demonstrates that high-level ab initio calculations are very
valuable for detailed analysis of experimental vibrational spectra.
In Table 8, the inner force constants f(NO) ) (F11 + F44)/2
and f(BrN) ) F22 of BrNO2 are compared with the respective
force constants of similar molecules. In both series the trends
in f(NO) and f(XN) as well f(NO) for ClNO2 and BrNO2 are
very similar. The weak BrN bonds are responsible for the
thermal instability of these molecules.
FNO
ClNO
BrNO
FNO2
ClNO2
BrNO2
f(NO)
f(XN)
15.912a
15.282a
15.254a
1.101a
2.133a
1.257a
f(NO)
f(XN)
11.5b
2.7b
10.61c
2.20c
10.27
1.48
a Reference 36. b Reference 37. c Reference 38.
Fermi resonance. They are corrected by the difference between
calculated 2∆ν6 and observed 2∆ν6 (-3.3 and + 5 cm-1 for
the 15N and 18O species, respectively). All isotopic shifts are
multiplied by 1.01 to correct for anharmonicity. The resulting
shifts for ∆ν1 14/15N and 16/18O are 11 and 52 cm-1, respectively.
All other isotopic shifts are reliable, because no combination
bands are in the vicinity of these fundamentals and uncertainties
arise solely from the precision of the measurements. As men-
tioned above, all measured isotope shifts were increased by 1%.
In the cases where fundamentals are not observed experimen-
tally, the respective wavenumbers from ab initio calculation20
are used. In the iteration procedure, the ab initio force constants
from Table 7 are used as preliminary force fields and the input
Acknowledgment. Financial support by the Deutsche For-
schungsgemeinschaft (DFG), the European Union (EU Contract
LAMOCS ENV-CT95-0046), and the Fonds der Chemischen
Industrie is gratefully acknowledged.
IC9803006