Infrared spectra of cis- and trans-peroxynitrite anion, oono-, in solid argon

Article abstract of DOI:10.1021/ja0114299

The peroxynitrite anion, of vast importance in biochemistry, is formed in vivo from the reaction of NO and O₂^-. Laser ablation of 10 different metal targets with concurrent 7 K codeposition of NO/Ar and O₂/Ar mixtures give

Full text of DOI:10.1021/ja0114299

9848
J. Am. Chem. Soc. 2001, 123, 9848-9854
Infrared Spectra of cis- and trans-Peroxynitrite Anion, OONO-, in
Solid Argon
Binyong Liang and Lester Andrews*
Contribution from the Department of Chemistry, UniVersity of Virginia, P.O. Box 400319,
CharlottesVille, Virginia 22904-4319
ReceiVed June 11, 2001
Abstract: The peroxynitrite anion, of vast importance in biochemistry, is formed in vivo from the reaction of
NO and O2-. Laser ablation of 10 different metal targets with concurrent 7 K codeposition of NO/Ar and
O2/Ar mixtures gives new metal-independent infrared bands at 1458.3 and 806.1 cm-1, and at 1433.3 and
-
-
983.2 cm-1, in addition to known O4 and (NO)2 absorptions. The new bands are not observed with CCl4
added to capture electrons or in O2 and NO experiments without laser ablation to produce electrons, which
identifies new product anions. Based on 15NO and 18O2 isotopic shifts, splitting patterns in mixed isotopic
experiments, and comparison with DFT isotopic frequency calculations, the former absorptions are assigned
to cis-OONO-, and the latter pair to trans-OONO-, which are isolated from metal cations trapped elsewhere
in the matrix. The cis- and trans-peroxynitrite anion isomers are probably formed via the ion-molecule reaction
between O2- and NO: the O2- anion, made by the capture of ablated electrons, is attested by the observation
of O4-. cis- and trans-OONO- are reversibly photoisomerized by visible and near-UV radiation. Collisional
stabilization of the OONO- ion-molecule dimer complex during formation of the solid argon matrix appears
to be crucial.
Introduction
carbon dioxide reaction, mechanistic studies, nitration reactions,
oxidation of C-H bonds, nitrosation reactions, and peroxynitrite
isomerization by myoglobin, to name some examples.10-16
Nitrogen oxides are among the most studied and fascinating
molecules in chemistry because they are not only byproducts
of fuel combustion but, more importantly, are involved in a large
number of biological processes, either as benefactors1 or as
unwanted toxic species.2 In biological systems, it is believed
that nitric oxide (NO), which is formed enzymatically, can react
with superoxide (O2-) to form an exceptional oxidant, per-
oxynitrite anion (OONO-).3-5 Peroxynitrite formed by ultra-
violet irradiation of nitrate in the Martian soil may account for
the initially promising signs of life during the Viking mission.6
Peroxynitrous acid (HOONO) is also a highly toxic acid and
acts as a mediator of free-radical toxicity. It oxidizes proteins
and nonprotein sulfhydryls, membrane phospholipids, and
DNA.7,8 As a matter of fact, thousands of papers can be found
using “peroxynitrite” as the keyword in the literature search,
and most of them are in medical journals.9
The great importance of OONO- has inspired a recent flurry
of theoretical and spectroscopic studies. Peroxynitrite anion is
expected to exist in two geometric forms, the cis and trans
conformers. Hartree-Fock quantum chemical calculations
predicted that the trans form is more stable,17 and later
multiconfiguration self-consistent-field (MCSCF) computations
predicted that the cis isomer is slightly more stable, but the trans
conformer lies only 0.5 kcal/mol higher.18 A detailed theoretical
study of peroxynitrite reported Hartree-Fock, second-order
perturbation theory (MP2), coupled-cluster singles and doubles
(CCSD), and density functional theory (DFT) calculations on
both cis and trans conformers.19 This study finds that cis-
OONO- is more stable than trans-OONO- by 3-4 kcal/mol.
A recent X-ray structure analysis concludes that tetramethyl-
ammonium peroxynitrite crystallizes in the cis form.20
The chemistry of peroxynitrite has also attracted considerable
interest as over thirty papers in this journal since 1994 focus
on peroxynitrite. These papers include the role of conformation,
(10) Tsai, J. H. M.; Harrison, J. G.; Martin, J. C.; Hamilton, T. P.; van
der Woerd, M.; Jablonsky, M. J.; Beckman, J. S. J. Am. Chem. Soc. 1994,
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Chem. Soc. 2000, 122, 6911.
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1993, 20, 1.
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Chem. 1996, 100, 15087.
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10.1021/ja0114299 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/14/2001

IR Spectra of cis- and trans-Peroxynitrite Anion
J. Am. Chem. Soc., Vol. 123, No. 40, 2001 9849
Experimental and Theoretical Methods
Despite numerous kinetic experiments with reactions involv-
ing peroxynitrite anion in aqueous solution,21 spectral identifica-
tion of these conformers is not well established. In aqueous
solution, a broad absorption at 302 nm is ascribed to OONO-.22,23
When solid nitrates are irradiated at 254 nm, OONO- is
reportedly produced,24 but no information was deduced about
conformation. The Raman spectrum of OONO- in aqueous
alkaline solutions reveals bands at 375, 642, 791, 931, 999, and
1564 cm-1, which are assigned to cis-OONO- based on the
15N isotopic substitution and CCSD calculation.10 In an impor-
tant spectroscopic study, 193-nm irradiation of potassium nitrate
(KNO3) molecules isolated in solid argon at 13 K produced both
cis- and trans-potassium peroxynitrite species (K+)(OONO-).25
The cis-KOONO isomer absorbs at 1444.5, 952.3, 831.6, and
749.1 cm-1, whereas trans-KOONO absorbs at 1528.4, 987.4,
and 602.2 cm-1. Similar conformers have also been reported
for the lithium and sodium counterparts, with as much as 52-
cm-1 difference in their infrared absorptions.25
The laser ablation, matrix isolation methods and apparatus have been
described in detail previously,45-47 For the current experiment, separate
spray-on lines were used to minimize the reaction of NO and O2 and
formation of unwanted NO2. Such a co-condensation at 7 K gave only
a small amount of extra NO2. The Nd:YAG laser fundamental (1064
nm, 10-Hz repetition rate with 10-ns pulse width) was focused onto
rotating metal targets (Al, Fe, In, Li, Ni, Pd, Ru, Tl, Th, Y). Typically,
low laser energy (3-5 mJ/pulse) was used, which favored the
stabilization of ionic species. Laser-ablated metal atoms, cations, and
electrons were codeposited with O2 (0.2%-1%) and NO (0.5%) in
excess argon onto a 7 K CsI window for 1-2 h at 3-4 mmol/h. Nitric
oxide (Matheson) and 15NO (MSD Isotopes, 99% 15N) samples were
prepared after fractional distillation from a coldfinger. Oxygen (Mathe-
son), 18O2 (Yeda), and 16O2/16O18O/18O2 (Yeda) were used as received.
Infrared spectra were recorded at 0.5-cm-1 resolution on a Nicolet 550
spectrometer with 0.1-cm-1 accuracy using a mercury-cadmium-
telluride (MCTB) detector down to 400 cm-1. Matrix samples were
annealed to allow diffusion and selected samples subjected to irradiation
by filtered light from a medium-pressure mercury lamp (Philips, 175
W, λ > 240 nm) with the globe removed or a tungsten lamp (Wiko,
360 W operated at 25% power, visible, near-IR).
In addition to the extensive biological and chemical impor-
tance of OONO-, there is considerable interest in gas-phase
ion-molecule reactions involving NO- and O2
extensive investigations of O4 and (NO)2 have been
performed,29-35 we have found no evidence for the gaseous
OONO- ion-molecule complex.
.
- 26-35 Although
Density functional theory calculations were performed for both cis-
and trans-peroxynitrite anions using the Gaussian 98 program48 and
the hybrid B3LYP functional.49 The 6-311+G(d) basis set was used
for both nitrogen and oxygen atoms.50 Geometries were fully optimized
and the vibrational frequencies computed by using analytical second
derivatives. The transition-state optimization employed synchronous
transit-guided quasi-Newton methods.51,52
-
-
Laser ablation of metal targets is known to produce electrons
and cations.36-41 This technique coupled with matrix isolation
has been used to produce and characterize small anions in this
laboratory, notably O4- and (NO)2- 36,37,42-44 Here we present
.
Results and Discussion
a spectroscopic characterization of the isolated cis- and trans-
peroxynitrite anions, which are prepared in the straightforward
reaction of NO and O2 in excess argon.
Systematic matrix infrared spectroscopic investigations of
laser-ablated metal reactions with O2/Ar and NO/Ar mixtures
and DFT calculations of potential new products will be
described.
-
(20) Worle, M.; Latal, P.; Kissner, R.; Nesper, R.; Koppenol, W. H.
Chem. Res. Toxicol. 1999, 12, 305.
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Goldstein, S.; Meyerstein, D.; van Eldik, R.; Czapski, G. J. Phys. Chem. A
2000, 104, 9712.
Th + O2/Ar and Th + NO/Ar Experiments. Experiments
with thorium and O2 were performed to develop a background
since a high yield of O4- was observed in recent laser ablation
investigations.36,53 The reaction product infrared absorptions
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1969, 73, 3445.
included ThO at 876.4 cm-1, ThO2 at 787.2 and 735.1 cm-1
,
(23) Barat, F.; Gilles, L.; Hickel, B.; Sutton, J. J. Chem. Soc. A 1970,
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and metal-independent bands also present in other metal-
oxygen experiments, namely, O3 at 1039.5 cm-1, O4- at 953.8
cm-1, and O3- at 803.9 cm-1 36,54-56 A new band at 653.2 cm-1
.
shifted to 619.6 cm-1 with 18O2 and gave an oxygen 16/18
isotopic ratio of 1.0543. This isotopic ratio is lower than the
(45) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697.
(46) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992, 96, 9177.
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(48) Gaussian 98, ReVision A.1. Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, J. A.;
Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam,
J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Cioslowki, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.;
Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M.
W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian,
Inc., Pittsburgh, PA, 1998.
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Panas, I. J. Chem. Phys. 1998, 109, 177 and references therein.
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9450.
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Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. Hay, P. J. J. Chem.
Phys. 1977, 66, 4377. K. Raghavachari, K.; Trucks, G. W. J. Chem. Phys.
1989, 91, 1062.
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9850 J. Am. Chem. Soc., Vol. 123, No. 40, 2001
Liang and Andrews
Table 1. Infrared Absorptions (cm-1) Observed for Reactions of
Laser-Ablated Thorium with NO and O2 in Excess Argon
as listed in the Table 1. Besides those absorptions common to
the reaction between thorium and O2 or NO, several new bands
were observed. Figure 1 shows the 1480-1400-, 1000-940-,
and 810-770-cm-1 regions from the infrared spectrum of a
sample formed by laser-ablated thorium codeposited with 0.5%
NO/Ar and 0.5% O2/Ar at 7 K using an O2 sample flow rate
50% higher than the NO sample. The bands at 1458.3 and
1433.3 cm-1 denoted as “c” and “t”, respectively, were observed
on deposition. A 25 K annealing slightly decreased the c-band,
but did not change the t-band. Tungsten lamp irradiation
increased the c-band by half, but decreased the t-band by half
14N16O + 16O2 15N16O + 16O2 14N16O + 18O2
identity
+
1589.3
1458.3
1433.3
1243.7
1222.7
1221.0
1039.5
983.2
981.1
978.3
953.8
879.0
876.5
806.1
805.2
803.9
795.7
792.0
787.2
760.5
735.2
697.4
653.2
1561.9
1432.9
1408.0
1218.3
1198.6
1199.9
1039.5
971.8
981.1
978.3
953.8
879.0
1561.9
1452.6
1426.2
1218.3
1198.6
1199.9
982.2
958.9
926.6
924.7
901.7
832.0
829.8
779.2
(NO)2
cis-OONO-
trans-OONO-
-
NO2
-
cis-(NO)2
trans-(NO)2
-
O3
trans-OONO-
(O2)x(O4-) ?
(O2)x(O4-) ?
-
-
(Figure 1c) while O4 decreased 15%, cis-(NO)2 decreased
-
O4
-
25%, and trans-(NO)2 increased 35%. Both new bands
ThO site
ThO
decreased on the following annealing to 35 K. Finally the c-band
was totally eliminated, whereas the t-band was greatly decreased
by 45 K annealing. A comparatively weaker band at 983.2 cm-1,
also denoted “t”, tracks with the 1433.3-cm-1 band throughout
the annealing and irradiation cycles and accounts for 5/8 relative
absorption intensity. Weak nearby bands at 981.1 and 978.3
cm-1 (denoted “*”) are common to laser ablation metal reactions
with oxygen and are probably due to a higher order dioxygen
anion cluster. Another new band at 806.1 (denoted “c”) also
tracks with the 1458.3-cm-1 band and is one-third of the relative
absorption intensity.
876.5
797.2
cis-OONO-
O3- site
-
O3
795.7
792.0
782.0
759.1
735.2
676.5
653.2
752.2
748.2
743.9
760.5
697.2
697.4
619.6
ThO2 site
ThO2 site
ThO2
NThO
ThO2
NThO
-
ThO2
diatomic ThO oxygen 16/18 ratio, 1.0563, but close to the ratio
1.0545 for the antisymmetric mode in the ThO2 molecule. This
band split into a 1:2:1 triplet in the 16O2 + 16O18O + 18O2
isotopic experiment but only a doublet feature in the 16O2 +
18O2 experiment. The band is reduced to <10% on the addition
of 0.05% CCl4 to the sample,37-40,57 which eliminated O4- and
reduced ThO2 by only 35%. We tentatively assign this band to
Figure 2 shows spectra from laser-ablated thorium codepos-
ited with 0.5% 15NO/Ar and 0.5% O2/Ar at 7 K. The 1458.3-
cm-1 c-band shifted to 1432.9 cm-1, with a nitrogen 14/15
isotopic ratio of 1.0177. The 1433.3-cm-1 t-band shifted to
1408.0 cm-1, with a nitrogen 14/15 ratio of 1.0180. Both ratios
are very close to the diatomic NO nitrogen 14/15 ratio of 1.0179,
signifying that both absorptions arise from N-O stretching
modes. The other t-band at 983.2 cm-1 shifted to 971.8 cm-1
in this experiment, and the other c-band absorption at 806.1
red-shifted to 797.2 cm-1. The two lower bands have much
smaller nitrogen 14/15 isotopic ratios, and are mixed vibrational
modes. In the mixed 14NO/15NO + O2 experiment, only three
bands at 1458.4, 1433.1, and 1408.2 cm-1 were observed in
the 1400-1500-cm-1 region (Figure 3f). However, the middle
band is broader than both the 1433.3-cm-1 t-band in the 14NO
+ O2 experiment and the 1432.9-cm-1 c-band in the 15NO +
O2 experiment, and clearly two absorptions contribute to this
-
the antisymmetric stretching mode in the ThO2 anion.
The second experiment involved the reaction between thorium
and NO. Products bands include NThO at 760.5 and 697.4
cm-1,58 and the metal-independent ionic products, cis- and trans-
(NO)2- at 1222.7 and 1220.9 cm-1, NO2- at 1243.4 cm-1, and
(NO)2 at 1589.2 cm-1 37,43
.
+
Th + O2/Ar + NO/Ar Experiments. In the reaction between
laser-ablated thorium with O2 and NO, metal-related bands,
ThO, ThO2, ThO2-, and NThO, and metal-independent bands,
O3, O4-, O3-, NO2-, (NO)2-, and (NO)2+, were again observed,
Figure 1. Infrared spectra in the 1480-1400-, 1000-940-, and 810-770-cm-1 regions for laser-ablated Th codeposited with 0.5% 14NO in argon
and 0.5% O2 in argon at 7 K: (a) sample deposited for 70 min, (b) after 25 K annealing, (c) after 1-h W lamp irradiation, (d) after 30 K annealing,
and (e) after 40 K annealing. The bands labeled “c” and “t” are identified in the text.

IR Spectra of cis- and trans-Peroxynitrite Anion
J. Am. Chem. Soc., Vol. 123, No. 40, 2001 9851
Figure 2. Infrared spectra in the 1460-1380-, 1000-940-, and 810-770-cm-1 regions for laser-ablated Th codeposited with 0.5% 15NO in argon
and 0.5% O2 in argon at 7 K: (a) sample deposited for 70 min, (b) after 30 K annealing, (c) after 1-h W lamp irradiation, (d) after 35 K annealing,
and (e) after 40 K annealing. The bands labeled “c” and “t” are identified in the text.
1428.6 cm-1. The lower t-band also split to a quartet with new
bands at 976.9 and 963.2 cm-1. The 1458.3-cm-1 c-band split
into a triplet at 1458.1, 1454.8 and 1452.7 cm-1, with ap-
proximate relative intensity of 4:2:1. Compared to the relative
intensities of the 1458.3- and 1452.6-cm-1 bands in the
previously mentioned NO + 16O2/18O2 reaction (Figure 3c), this
1458.1-cm-1 band clearly contains more than one absorption.
The overall splitting pattern of this 1458.3-cm-1 c-band in the
scrambled oxygen experiment should also be a quartet. The
isotopic splitting pattern of the lower c-band (806.1-cm-1
)
cannot be resolved. The 1458.3- and 1433.3-cm-1 bands showed
small 18O2 isotopic shifts (5.7 and 7.1 cm-1, respectively),
indicating that both N-O stretching modes are slightly coupled
with the oxygen molecule. The doublet feature in the 16O2/18O2
mixture, and the quartet feature in the 16O2/16O18O/18O2 mixture,
indicate that there is one oxygen molecule involved in the
responsible molecules, and the two oxygen atoms in this oxygen
molecule are not equally involved in the 1458.3- and 1433.3-
cm-1 modes.
Figure 3. Infrared spectra in the 1480-1400-cm-1 region for laser-
ablated Th codeposited with (a) 0.5% NO in argon and 0.5% O2 in
argon for 70 min, (b) 0.5% NO in argon and 0.5% 18O2 in argon for 70
min, (c) 0.5% NO in argon and 0.4% 16O2 + 0.4% 18O2 in argon for
90 min, (d) 0.5% NO in argon and 0.13% 16O2 + 0.26% 16O18O +
0.13% 18O2 in argon for 90 min, (e) 0.5% 15NO in argon and 0.5% O2
in argon for 70 min, and (f) 0.25% 14NO + 0.25% 15NO in argon and
0.5% O2 in argon for 80 min. The bands labeled “c” and “t” are
identified in the text.
CCl4 is well known for its electron capture ability, and it has
been used in this laboratory to help identify numerous ionic
species.37-40,57 The 0.5% O2 argon sample was doped with
0.05% CCl4, the experiment with laser-ablated thorium and 0.5%
-
NO/Ar mixture was repeated, and the c-, t-, O4-, and (NO)2
absorptions were not observed. This result suggests that both
c- and t-absorbers are anionic species. Finally, codepositing
0.5% O2 and 0.5% NO in argon onto a 7 K CsI substrate gave
no product bands except for a small amount of NO2, which is
similar to a previous study.59
Other Metals + O2/Ar + NO/Ar. Nine other metals, Al,
Fe, In, Li, Ni, Pd, Ru, Tl, and Y, were ablated with various
isotopic O2 + NO samples to demonstrate the metal-independent
character of the c- and t-bands. Except for thallium, where a
very strong TlNO absorption at 1454.6 cm-1 masked this
region,60 all other metals showed absorptions at 1458.3 ( 0.1
middle band. The isotopic splitting patterns for both c- and
t-modes are doublets with two pure isotopic bands. The lower
t- and c-counterparts also showed 1:1 doublet features at 983.2
and 971.8 cm-1 and at 806.1 and 797.2 cm-1. These results
reveal that there is only one NO subunit involved in the c- and
t-absorbers.
In the reaction between thorium and NO/Ar + 18O2/Ar, the
1458.3-cm-1 c-band red-shifted to 1452.6 cm-1 (Figure 3b);
however, the lower c-band cannot be identified in this experi-
ment, because of strong ThO2 absorption. The t-bands red-
shifted to 1426.2 and 958.9 cm-1, respectively. In the NO +
16O2/18O2 experiments, doublets were observed for both bands
(Figure 3c). The c-doublet showed a relative intensity of 5:2,
although equimolar concentrations of 16O2 and 18O2 were used.
The lower t-band also showed a doublet at 983.2 and 958.9
cm-1. In the NO + 16O2/16O18O/18O2 (molar ratio 1:2:1)
experiment (Figure 3d), the 1433.3-cm-1 t-band showed a 1:1:
1:1 quartet splitting with new intermediate bands at 1431.4 and
(55) Andrews, L.; Spiker, R. C., Jr. J. Phys. Chem. 1972, 76, 3208.
(56) Andrews, L.; Ault, B. S.; Grzybowski, J. M.; Allen, R. O. J. Chem.
Phys. 1975, 62, 2461.
(57) Zhou, M. F.; Andrews, L. J. Am. Chem. Soc. 1998, 120, 11499.
(58) Kushto, G. P.; Andrews, L. J. Phys. Chem. A 1999, 103, 4836.
(59) Andrews, L.; Hassanzadeh, P.; Brabson, G. D.; Citra, A.; Neurock,
M. J. Phys. Chem. 1996, 100, 8273.
(60) Zhou, M. F.; Andrews, L. J. Phys. Chem. A 2000, 104, 8475.

9852 J. Am. Chem. Soc., Vol. 123, No. 40, 2001
Liang and Andrews
Figure 4. Infrared spectra in the 1480-1400-, 975-895-, 810-760-cm-1 regions for laser-ablated Pd codeposited with 0.5% NO in argon and
0.2% 18O2 in argon at 7 K: (a) sample deposited for 90 min, (b) after 1-h λ > 290-nm irradiation, (c) after 1-h W lamp irradiation, (d) after 40-min
λ > 290-nm irradiation, (e) after 30 K annealing, and (f) after 40-min λ > 240-nm irradiation.
and 1433.3 ( 0.1 cm-1. Although the lower t- and c-bands are
weaker, they were observed in all but the nickel experiment,
where strong oxide bands are dominant. All c- and t-bands in
different metal and isotopic experiments showed no metal
dependence as their absorption wavenumbers differ from the
values in Table 1 by no more than 0.1 cm-1
.
Figure 4 shows the 1480-1400-, 975-895-, and 810-760-
cm-1 region for laser-ablated palladium with NO and 18O2 in
argon. The strong absorption at 966.0 cm-1 is due to Pd(18O2).61
The intensities of both t- and c-bands are not as strong as in
the thorium experiment (Figure 3b), and a reduced yield was
also observed for the 18O4- band at 901.7 cm-1 36
. One advantage
of this experiment is the clean region around 800 cm-1, in
contrast to strong dioxide bands in the thorium experiment. In
this palladium experiment, we can identify the 18O-substituted
lower c-band at 779.2 cm-1. Figure 4a shows the spectrum after
sample deposition; λ > 290 nm irradiation (Figure 4b) increased
both c- and t-bands, and a following tungsten lamp irradiation
increased c by a third, and eliminated t. Another λ > 290-nm
irradiation almost restored both bands to the level before the
tungsten lamp irradiation. A subsequent 30 K annealing
decreased c-bands, and increased t-bands slightly, and a full-
arc irradiation eliminated all t- and c-bands.
Figure 5. Infrared spectra in the 1480-1400-cm-1 region for laser-
ablated Fe codeposited with 0.5% NO in argon and 0.15% 16O2 + 0.25%
18O2 in argon at 7 K: (a) sample deposited for 90 min, (b) after 25 K
annealing, (c) after 1-h W lamp irradiation, (d) after 1-h λ > 290-nm
irradiation, (e) after 30-min λ > 240-nm irradiation, and (f) after 35 K
annealing.
-
290-nm exposure decreases the c, cis-(NO)2-, and O4 bands
and increases the t absorptions (Figure 6d), but final exposure
to the full arc decreases all of these features (Figure 6e).
Calculations. DFT calculations using the B3LYP functional
and 6-311+G(d) basis set for both cis- and trans-OONO- and
the transition state have been reported previously.19 These results
were reproduced, and in addition, infrared intensities and
isotopic frequencies were computed as listed in the Table 2. In
short, cis-OONO- is more stable than trans-OONO- by 3.0
kcal/mol, and each isomer has several frequencies with sufficient
infrared intensity to be observed. For comparison, calculations
using the BPW91 functional gave lower frequencies closer to
the observed values as expected.
Identification of c- and t-Absorbers. Doublet splitting
patterns were observed for all c- and t-bands in the mixed 14NO/
15NO experiment, and hence, it is obvious that only one NO
unit is involved in both c- and t-products. In the mixed 16O2/
18O2 experiment, doublet splitting features were observed,
whereas in the 16O2/16O18O/18O2 experiment, quartets were
observed for 1458.3- and 1433.3-cm-1 bands. This demonstrates
Figure 5 illustrates the 1480-1400-cm-1 region of laser-
ablated iron codeposited with 0.5% NO in argon + 0.15% 16O2
and 0.25% 18O2 in argon to explore different photochemistry.
First, irradiation using a tungsten lamp increased the c-bands
by half and almost destroyed the t-bands (Figure 5c) and
decreased (NO)2- bands by a third. Then a following λ > 290-
nm mercury-arc irradiation decreased c-bands by a third, but
recovered almost half of t-bands from the last annealing. A full
arc λ > 240-nm irradiation decreased both c- and t-bands, and
the last annealing to 35 K almost eliminated these bands.
Finally, Figure 6 illustrates a Ru experiment with NO and
O2 to contrast the photochemistry of NO2-, (NO)2-, O4-, and
the new product species. Annealing to 25 K has little effect on
the product features. Visible-near-infrared photolysis clearly
-
increases the c-feature and decreases the t-absorption, but O4
and trans (NO)2- also decrease (Figure 6c). A subsequent λ >
(61) Bare, W. D.; Citra, A.; Chertihin, G. V.; Andrews, L. J. Phys. Chem.
A 1999, 103, 5456 and references therein.

IR Spectra of cis- and trans-Peroxynitrite Anion
J. Am. Chem. Soc., Vol. 123, No. 40, 2001 9853
Figure 6. Infrared spectra in the 1470-1425-, 1250-1210-, 970-945-cm-1 regions for laser-ablated Ru codeposited with 0.5% NO in argon and
0.5% O2 in argon at 7 K: (a) sample deposited for 70 min, (b) after 25 K annealing, (c) after 1-h W lamp irradiation, (d) after 1-h λ > 290-nm
irradiation, and (e) after 40-min λ > 240-nm Hg lamp irradiation.
Table 2. Ground Electronic States, Equilibrium Geometries, Relative Energies, and Isotopic Frequencies Calculated for cis-OONO-,
trans-OONO-, and Their Transition State (TS)a
frequencies, cm-1 (infrared intensities)
isotopic
molecule
OON
bend
OONO
tors
O-N
str
O-O
str
ONO
bend
NdO
str
species
geometries (Å, deg)
c-OONO-
O-O, 1.388; O-N, 1.367;
NdO, 1.213; OON, 118.3;
ONO, 116.3; D(OONO), 0.0
16-16-14-16
336.4
(3)
336.3
325.4
496.9
(0)
489.5
484.7
718.1
(34)
711.3
688.9
833.9
(160)
822.2
808.4
969.2
(19)
964.1
933.6
1517.0
(395)
1489.1
1516.7
1A′
16-16-15-16
18-18-14-16
t-OONO-b
O-O, 1.408; O-N, 1.338;
NdO, 1.228; OON, 114.1;
ONO, 111.2; D(OONO), 180.0
16-16-14-16
422.6
(12)
419.6
407.2
256.1
(7)
253.7
249.6
852.0
(0)
846.8
811.1
627.1
(25)
623.8
608.6
1024.4
(324)
1011.3
998.9
1489.2
(360)
1461.1
1486.1
1A′
16-16-15-16
18-18-14-16
TSb
1A
O-O, 1.501; O-N, 1.417;
NdO, 1.191; OON, 114.1;
ONO, 101.8; D(OONO), 77.3
16-16-14-16
270.1
(15)
620.5i
(8)
574.5
(42)
756.0
(31)
822.6
(132)
1609.3
(345)
a DFT/B3LYP calculation, 6-311+G(d) on N, O. b Relative energy (including zero-point energy) compared to cis-OONO- is 3.0 and 25.8 kcal/
mol for trans-OONO- and the transition state, respectively. The higher energy triplet state is a weak complex of NO and O2-; see ref 19.
the involvement of one oxygen molecule in both c- and
t-absorbers, with the two oxygen atoms not participating equally
in the vibrational modes. Hence, both c- and t-absorbers involve
single O2 and NO molecules. The c- and t-bands were not
observed in the experiment without laser ablation or in the CCl4
doped experiment, which suggests the anion identification for
both c- and t-absorbers.
experimental data, except for overestimation of the 15N shift
and underestimation of O2 involvement in the terminal NdO
stretching mode. For example, the B3LYP computation predicts
a
14N16O/15N16O frequency ratio 1.0187 for cis and 1.0192 for
trans and the observed ratios are 1.0177 and 1.0180, respectively.
The calculation predicts a 0.3-cm-1 18O2 shift for cis and 3.1
cm-1 for trans, but 5.7 and 7.1 cm-1, respectively, are observed.
The DFT/B3LYP calculation predicted strong infrared bands
for cis-OONO- at 1517.0 and 833.9 cm-1, with intensities of
395 and 160 km/mol, respectively, and for trans-OONO- at
1489.2 and 1024.4 cm-1, with intensities of 360 and 324 km/
mol, respectively. Excellent agreement with the experimental
result identifies the c-absorber as cis-OONO-; the two observed
modes at 1458.3 and 806.1 cm-1 require scale factors of 0.961
and 0.967, which are expected values for DFT/B3LYP calcula-
tions.62 The t-absorber is assigned as trans-OONO-, and the
two observed modes at 1433.3 and 983.2 cm-1 require scale
factors of 0.962 and 0.960. The calculated intensities of two
modes in trans-OONO- do not fit the experimental data as well
as in cis-OONO-, but the predicted trends are correct. The
vibrational frequencies for isotopic species agree with the
The stable cis-OONO- and trans-OONO- anions separated
by a substantial 25.8 kcal/mol energy barrier are both observed
isolated in the matrix independent of the metal cation that
provided the anion electron. Likewise, both alkali metal-
dependent (M+)(OONO-) conformers are produced by pho-
tolysis of MNO3 “molecules” in solid argon;25 however, the
normal modes in the (M+)(OONO-) isomers involve the alkali
cation to a minor degree. In contrast, interactions in the
tetramethylammonium peroxynitrite crystalline solid apparently
favor the cis form.20
-
We have found no evidence for the isolated NO3 anion,
observed at 1356 cm-1 in solid neon,63 or the nitrate radical,
observed at 1492 cm-1 in the gas phase.64 Apparently, ultraviolet
photolysis dissociates or detaches isolated OONO- before
(62) Bytheway, I.; Wong, M. W. Chem. Phys. Lett. 1998, 282, 219.

9854 J. Am. Chem. Soc., Vol. 123, No. 40, 2001
Liang and Andrews
rearrangement can occur, in contrast to that observed for the
ion-pair (M+)(OONO-) species.25
tion from a reduced intensity tungsten lamp decreased trans and
increased cis absorptions (Figures 4-6) whereas a subsequent
exposure to ultraviolet-visible mercury arc light reduced the
cis and almost reproduced the trans isomer. This reversible
isomerization reaction proceeds through rotation of the middle
N- O bond. The transition state is 25.8 kcal/mol higher in
energy than the cis-OONO- isomer. This activation barrier
suggests significant conjugation of π symmetry orbitals across
the central N-O bond. An 800-nm photon provides enough
energy to overcome this barrier. A subsequent λ > 240-nm
irradiation reduced both isomers and failed to give evidence
Mechanistic Considerations. Two possible reaction channels
can participate in the matrix experiment to produce the isolated
cis-OONO- and trans-OONO- anions, namely, O2 reacting
-
with NO or NO- reacting with O2. Electrons produced in the
laser ablation process are captured by NO and O2 as attested
by observation of their dimer anions from reactions 1 and 2
NO- + NO f (NO)2
(1)
(2)
(3)
(4)
(5)
-
O2- + O2 f O4
-
-
for NO2 or NO3-. As has been observed in several matrix
photoisomerism processes including (S2)2 and (Te2)2, excitation
of one isomer into a dynamically equilibrating state allows the
isomer not absorbing light to be relaxed and stabilized by the
cold matrix.66,67 Similar results have been reported for the
(K+)(OONO-) isomers in solid argon.25 Irradiation at 308 nm
decreased cis and enhanced trans, but prolonged irradiation
reduced the trans isomer in favor of the original KNO3 form.
O2- + NO f OONO-
NO- + O2 f OONO-
NO- + O2 f O2- + NO
as demonstrated in several recent investigations,36,37,42,43 al-
though (NO)2- can also be formed through electron capture by
cis-OONO- {nveias,r-nUeVar,-IvRis} trans-OONO-
-
(NO)2. Accordingly, O2 can react with NO, and on the other
Conclusions
hand, NO- can combine with O2, to give OONO-, reactions 3
and 4, in both stable cis and trans forms. Since fast electron
transfer has been observed between NO- and O2, reaction 5,26-28
reaction 3 probably dominates the yield of OONO- anions in
these experiments. Owing to numerous argon collisions with
NO- and O2- during condensation, the more stable O2- anion65
is expected to have a higher survival rate since in the gas phase
reaction 5 dominates.26-28 Argon collisions with OONO- during
condensing the argon matrix appear to be crucial to stabilization
of the dimer anion complex product of reaction 3.
Laser ablation of 10 different metal targets with concurrent
7 K codeposition of NO/Ar and O2/Ar mixtures produces new
metal-independent bands at 1458.3 and 806.1 cm-1 and at
1433.3 and 983.2 cm-1. These absorptions require both O2 and
NO. Furthermore, the product bands were not observed in a
CCl4 doped sample due to electron capture by CCl4 or in an O2
and NO experiment without laser ablation to provide electrons,
which identifies anion products. Based on the isotopic shifts,
the splitting patterns in the mixed isotopic experiments, these
anions require one NO and one O2 molecule. On comparison
with DFT/B3LYP calculations, the former (1458.3 and 806.1
cm-1) bands are assigned to cis-OONO-, and the latter (1433.3
and 983.2 cm-1) bands are assigned to trans-OONO-. The
present metal-independent cis- and trans-OONO- absorptions
are near the cis- and trans-MOONO (M ) K, Na, Li)
absorptions,25 where the latter bands showed significant shifts
with different alkali metals.
Reactions 3 and 4 are elementary processes, and an increase
in either NO or O2 concentration should increase the reaction
rate and yield of product. However, note that NO undergoes a
dimerization reaction, but O2 does not, and the electron (and
anion) concentration is the limiting reagent as anion absorptions
in these experiments are typically a few percent as intense as
corresponding neutral absorptions. We find that doubling the
NO concentration from 0.25 to 0.5% while maintaining the other
experimental conditions has no effect on the OONO- absorp-
tions. However, doubling the O2 concentration increased the
cis and trans product bands by 75%, and doubling the flow rate
of the O2/Ar sample almost doubled the product absorptions.
This evidence, we believe, supports the hypothesis that the
formation of O2- is the rate-determining step and that reaction
3 is the dominant mechanism in these experiments.
Both cis- and trans-OONO- isomers observed in the current
experiments are probably formed by the ion-molecule reaction
-
between O2 and NO, which can access the stable minima on
each side of the barrier between them. A reversible photoisom-
erism has been found in the solid argon matrix with visible and
near-ultraviolet radiation, but λ > 240-nm light destroys both
isomers.
Photochemical isomerization was observed between the cis-
and trans-peroxynitrite anions. Visible and near-infrared radia-
Acknowledgment. The authors gratefully acknowledge
National Science Foundation support under grant CHE 00-78836.
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