2814
J . Phys. Chem. 1991,95, 2814-2817
Comparison to Suyenic Acid. Smardzewski and L i d produced sulfenic acid, HSOH (hydrogen thioperoxide), by the mercury arc photolysis of mixtures of O3and H2Sin solid argon. The 0-H stretching mode was observed at 3425 cm-I, the S-O-H bend at 1177 cm-I, the S-OH stretch at 763 cm-I, and a torsional mode at 449 cm-I. Except for the 1.4wavenumber oxygen-18 shift of the S-0-H bending vibration, the deuterium and 0-18isotopic shifts of the sulfenic acid absorptions were comparable to the observed sulfinic acid isotopic shifts reported here. The correlation between the vibrational frequencies and isotopic shifts for these two related molecules adds further support to the identification of sulfinic acid. Conclusion
The photolysis of argon matrices containing hydrogen sulfide and sulfur dioxide produced sulfinic acid by in situ hydrogen atom
addition to SO2. Eight infrared absorptions of this reaction product were observed. Deuterium and oxygen- 18 isotopic substitution studies, normal-coordinate calculations, and comparison to the previously observed sulfenic acid molecule all support the identification of sulfinic acid. This is the first time the infrared spectrum of this reactive molecule has been reported. Acknowledgment. The authors thank the National Science Foundation and the Cottrell Research Corporation for financial support. We thank Mr. Daniel R. Lorey I1 for his assistance with many parts of this work, including modification of the microcomputer software and preparation of the figures. The authors also thank Mr. Norris Bingham for his work on the H I experiments. Registry NO. HSOZH, 131865-77-5;D2,7782-39-0;lsO, 14797-71-8; Ar, 7440-37-1; SO2, 7446-09-5;HIS, 7783-06-4.
Photolysls of Nitric Acid In Solid Argon: The Infrared Absorption of Peroxynitrous Acid (HOONO)~ Bing-Ming Cheng,* Jeng-Wen Lee, and Yuan-Pern Lee* Department of Chemistry, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu, Taiwan 30043, R.O.C. (Received: October 1. 1990)
Nitric acid (HON02)in solid argon at 12 K was irradiated with ultraviolet light from various sources. Recombination of the fragments OH and NO2 from photolysis within the argon lattice site has led to the formation of peroxynitrous acid (HOONO). IR absorption lines at 3545.5, 1703.6, 1364.4,952.0,and 772.8 cm-I have been assigned to this molecule on the basis of isotopic shifts. Under certain conditions the lines at 3563.3,1708.3,1372.7,957.4,and 782.9 cm-l were also observed, and they have been attributed to HOONO in a less stable matrix site. The observed vibrational frequencies are in agreement with recent theoretical calculations on HOONO. The implication of the formation of HOONO from HONOz to atmospheric chemistry is also discussed.
Introduction
The reaction of OH and NO2 to form nitric acid OH
+ NO2 + M
+
HONO2
+M
(la)
is important in the atmosphere because in this reaction the reactive HO, and NO, species are converted to stable nitric acid, an important component of acid precipitation. The rate coefficient for the reaction has been studied e~tensively.~-’In flash photolysis experiments, Robertshaw and Smith found that the rate coefficient of reaction 1 a has not yet reached its high-pressure limit under 6.5 X IO3 Torr of CF4 at 300 K.’ The possibility of a second reaction channel OH
+ NO2 + M + H O O N O + M
(1b)
was proposed to explain the observed results. Later, Atkinson et al. reported the formation of alkyl nitrate (RONOJ from the A mechanism reaction of alkyl peroxyl radicals (RO,)and consisting the rearrangement of an excited peroxynitrite (ROONO*) intermediate has been proposed to account for the production of RON02. The peroxynitrous acid (HOONO) is expected to be photolyzed more rapidly in the UV region than its isomer nitric acid (HONO2), and the photodissociation products of HOONO are likely to be H 0 2 and NO; accordingly the formation of HOONO via reaction I b would affect the catalytic ‘Dedicated to Professor George Pimentel. Y.P.L. was a student of Pimente1 and received his Ph.D. in 1979. *Research associate at Synchrotron Radiation Research Center, Taiwan, R.O.C. To whom correspondence should be addressed. Also affiliated with the Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan, R.O.C.
cycles of ozone destruction and the H N 0 3 / N 0 2 ratio in the atmosphere. HOONO has been variously proposed as a transient intermediate in aqueous solution in the reaction of nitrite ion and H2O2,*I2 in the photolysis of nitrate and in the reaction of OH and NO2.Is No spectroscopic information has been reported for HOONO except that a UV absorption a t -345 nm was tentatively assigned to HOONO in a continuous aqueous flow mixture of H N 0 2 , H202,and HC104. Recently, Burkholder et al. have studied the products of reaction 1 by means of a high-resolution FTIR spectrometer coupled with a rapid-flow multipass absorption ce11.I6 They failed to observe ( I ) Robertshaw, J. S.; Smith, I. W. M. J . Phys. Chem. 1982, 86,785. (2)Anderson, J. G.; Margitan, J. J.; Kaufman, F. J . Chem. Phys. 1974, 60,3310. (3)Howard, C. J.; Evenson, K. M. J . Chem. Phys. 1974,61, 1943. (4)Anastasi, C.; Smith, 1. W. M. J. Chem. SOC.,Faraday Trans. 2 1976, 72, 1459. ( 5 ) Atkinson, R.; Perry, R. A,; Pitts, J. N., Jr. J . Chem. Phys. 1976, 65, 306. (6)Wine, P. H.; Kreutter, N. M.; Ravishankara, A. R. J . Phys. Chem. 1979. 83. 3139. (7) Burrows, J. P.; Wallington, T. J.; Wayne, R. P. J . Chem. Soc., Faraday Trans. 2 1983, 79, 11 1. ( 8 ) Atkinson, R.; Carter, W. P. L.; Winer, A. M. J. Phys. Chem. 1983,
87. 2012. . ,_.._
(9)Gleu, K.; Hubold, P. Z . Anorg. ANg. Chem. 1935, 223, 305. (IO) Halfpenny, E.;Robinson, P. L. J . Chem. Soc. 1952, 928. ( 1 1 ) Anbar, M.; Taube, H. J . Am. Chem. Soe. 1954, 76.6243. (12)Benton, D.J.; Moore. P. J . Chem. SOC.A 1970, 3179. (13)Bart, F.;Hickel, B.; Sutton, J. Chem. Commun. 1969, 125. (14)Shuali, U.;Ottolenghi, M.; Rabani, J.; Yelin, Z . J . Phys. Chem. 1969, 73, 3445. (15) Grltzel, M.; Henglein, A.; Taniguchi, S.Eer. Bunsen-Ges. Phys. Chem. 1970, 74, 292. (16)Burkholder, J. B.; Hammer, P. D.; Howard, C. J. J . Phys. Chem. 1987, 91, 2136.
0022-3654/91/2095-2814%02.50/00 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2815
Infrared Absorption of HOONO any IR absorption that they could ascribe to HOONO and concluded that either HOONO is not formed in significant proportions in reaction 1 or that HOONO absorbs only weakly in the spectral region searched (1850-3850 cm-I). Recent theoretical calculations by McGrath et al. showed that HOONO is 147 kJ mol-' less stable than H N 0 3 and supported reaction 1b to be a minor channel;17 the equilibrium geometry and the vibrational frequencies of HOONO have also been predicted. The product analysis of a similar gaseous CIO NO2 M CION02 M (2a) CIOONO M (2b) also provided no evidence for the formation of chlorine peroxynitrite (CIOONO). However, the products of both channels of the reaction C1+ NO2 M CIN02 M (3a) CION0 M (3b) have been observed by means of IR ~ p e c t r o m e t r y . ~ ' - ~ ~ The matrix isolation technique is known to be an excellent method to trap and preserve unstable species.24 An in situ photolysis technique has been extensively used as a means to produce the unstable species in the matrix. If the fragments formed from photolysis cannot diffuse through the noble-gas matrix at low temperature due to the cage effect, they may recombine to regenerate the precursor, producing an apparent inhibition of In some cases the recombination may proceed via different reaction paths and form other products, possibly isomers of the precursor. An early study of the photolysis of nitromethane (CH3N02)in solid argon by Brown and Pimentel in 1958 served as a good example of this technique;26 several photolysis products including C H 3 0 N 0 were identified by their IR absorption. Analogously, we have studied the products of photolysis of nitric acid in solid argon and observed the IR absorption spectrum of its isomer, peroxynitrous acid.
+
- + +
-
+
+
+
+
+
Experimental Section' The experimental setup is a typical matrix isolation/FTIR arrangement. Briefly, the sample was deposited onto a CsBr window cooled to 12 K in a closed-cycle cryogenic system (Air Products, CSW-202). The cold matrix window was rotatable and could face the deposition inlet, the photolysis source, or the detection IR beam. Windows of KBr and fused silica were mounted on the vacuum shroud along the axes of the IR and the photolysis beams, respectively. Various light sources have been employed for photolysis: a low-pressure Hg lamp, a medium-pressure Hg lamp, a Nd:YAG laser (266 and 355 nm, 10 Hz, Spectra Physics DCR-ll), a XeCl excimer laser (308 nm, 10-50 Hz, Lambda Physik, LPXIOS), and a KrF excimer laser (248 nm, 10-200 Hz, Lambda Physik, LPX12Oi). The photolysis radiation could be projected from either the same side or the opposite side of the deposition surface. When the photolysis radiation was projected from the opposite side of the deposition surface, the CsBr target window served as a filter. IR absorption spectra of the matrix were recorded before and after irradiation by means of a FTIR spectrometer (Bomem ~
(17) McGrath, M. P.; Francl, M. M.; Rowland, F. S.;Hehre, W. J. J . Phys. Chem. 1988,92, 5352. (18) Griffith, D. W. T.; Tyndall, G.S.;Burrows, J. P.; Moortgat, G.K. Chem. Phys. Lett. 1984, 107, 341. (19) Bhatia, S.C.; George-Taylor, M.; Meredith, C. W.; Hall, J. H. J . Phys. Chem. 1983.87, 1091. (20) Burrows, J. P.; Griffith, D. W. T.; Moortgat, G.K.; Tyndall, G.S. J . Phys. Chem. 1985,89, 266. (21) Tevault, D. E.; Smardzewski, R. R. J . Chem. Phys. 1977,67,3777. (22) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Letr. 1978. 59, 18. (23) Leu, M.-T. Int. J. Chem. Kiner. 1984, 16, 1311. (24) See,for example: Chemistry and Physics of Mairix-Isolated Species; Andrews, L., Moskovits, M., Eds.; Elsevier: Amsterdam, 1989. (25) See, for example: Moore, C. B.; Pimentel, G.C. J. Chem. Phys. 1964, 41. 4504. (26) Brown, H. W.; Pimentel, G. C. J . Chem. Phys. 1958, 29, 883. (27) Johnston, H.S.;Chang, S.-G.; Whitten, G.J. Phys. Chem. 1974, 78, I.
WAVENUMBERS / CM-'
Figure 1. IR absorption spectra of matrix-isolated H O N 0 2 in Ar (1/ 5000) before and after irradiation with a medium-pressure Hg lamp: 1, before photolysis; 2, after photolysis (with spectrum shifted downward by 0.1 transmittance, and the new features labeled A and B, see text).
TABLE I: Wavenumbers of Absorption Lines of a Sample of HN03 in Solid Ar (1/5000) before and after Photolysis with a Medium-Pressure Hg Lamp lines before photolysis cm-l assignment 3967.0 H N 0 3 , vI v9 H2O 3756.6' HNO3, V I 3522.3 HNO3, V I 3519.3 H N 0 3 , u2 vq 2990.4 H N 0 3 , 2v3 2635.5 2588.8 HNO3, v2 Y6 2345.1b COz, v3 1786.5 HN03, v3 + u9 1699.4 HN03, u2 1696.2 HNO3, Y 2 1623.7' H20 1610.8 N02, v3 1380.3 HN03, v4 + v, 1321.4 HNO3, V3 1318.7 HNOp, v3 1304.4 HN03, Vq 1 199.6 H N 0 3 , us + v9 896.9 HNOI, u6 889.5 HNQ, u6 873.3 H N 0 3 , 2v9 763.6 HNO3, Vg
+ +
+
new lines after photolysis cm-' assignment 3563.3 A OH 3548.1 3545.5 B 2715.4 HNO NO 1871.7 1708.3 A 1703.6 B 1562.2 HNO 1372.7 A 1364.4 B 957.4 A 952.0 B 789.8 ? 782.9 A 772.8 B
'Multiplet. bDoublet: 2345.1 and 2339.0 cm-I.
DA3.002) equipped with a KBr beam splitter. Usually a narrow-range Hg/Cd/Te detector cooled with liquid N 2 was used to cover the spectral range 700-4000 cm-I; sometimes a wide-range Hg/Cd/Te detector was used to extend the range to 500 cm-I, although its detectivity is relatively poor. Typically 400 scans were taken to yield spectra with a resolution of 0.5 cm-I. The molar fraction of nitric acid in Ar or N2 was varied from 1/250 to l/lOOOO. Typically approximately 5-10 mmol of gas mixture was deposited with a flow rate of (2-8) X lo-) mol h-' during 25-90 min. In a few experiments, special care was taken to eliminate trace H 2 0 from the system. However, the experimental results of the photolysis of HON02/Ar matrices with trace H20and without H 2 0 showed no differences. The various anhydrous isotopic species of HONO, were prepared by mixing concentrated (98%) sulfuric acid (or deuterated sulfuric acid) with sodium nitrate (14N and "N species). The product was vacuum distilled and stored at 195 K in the dark. The gas mixtures were prepared by standard manometric procedures. H O N 0 2 was warmed to its melting point and degassed before use. Ar (99.9995%) and N2 (99.999%) were used without further purification. The nominal purity of sodium nitrate was 99.5%; the isotopic purity of D2SO4 and NaISNO3 was 99%.
2816 The Journal of Physical Chemistry, Vol. 95, No. 7. 1991
Cheng et al.
TABLE II: Fundamental Vibrational Modes (in cm-') of Various Isotopic HOONO in Matrices
HOONO assignments HO str
3541.7
N=O str
1701.4
HOO bend
N2
1394.9
00 str
960.5
ON=O bend
793.6
ON str c c
HOO torsion
Ar 3545.5 (B) 3563.3 (A) 1703.6 (B) 1708.3 (A) 1364.4 (B) 1372.7 (A) 952.0 (B) 957.4 (A) 772.8 (B) 782.9 (A)
cab 3677 1692 1398 946 814
Ar 3545.7 (B) 3563.4 (A) 1673.8 (B) 1678.3 (A) 1364.1 (B) 1372.5 (A) 947.7 (B) 953.3 (A) 764.0 (B) 774.0 (A)
439 384 331 21 1
HOO'SNO isotopic shift exp cak* +0.2 +O.l -29.8 -30.0 -0.3 -0.2 -4.3 -4.1 -8.8 -8.9
0.0
-30.1 0.0
-2.9 -10.4 -5.3 -5.2 -0.2 -0.5
DOONO Ar 2615.4 (B) 2633.5 (A) 1703.7 (B) 1708.3 (A) 1089.7 (B) 1091.7 (A) 950.3 (B) 955.7 (A) 772.1 (B) 782.0 (A)
isotopic shift exP cah? -930.1 -929.8
-997.8
+o. 1
0.0 -274.7 -281.0 -1.7 -1.7 -0.7 -0.9
-0.2 -354.4 -7.2 -1.9 -6.3 -13.8 -0.6 -52.2
"Ab initio calculation at RMP2/6-31G* level, from ref 17. *Ab initio calculation at RMP2/6-31GS level, from ref 36. 'According to ref 17, the ON=O torsion and OON bending modes are coupled and cannot be exclusively assigned.
Results and Discussion Although the efficiency of the photolysis was much smaller when the matrix was irradiated from the rear (through the CsBr target), its IR absorption spectrum was less complex than that irradiated without the CsBr filter. Portions of the IR absorption spectra of H O N 0 2in solid Ar before and after irradiation through the CsBr target with a medium-pressure Hg lamp are illustrated in Figure 1 . The observed peak positions in the range 700-4000 cm-' are listed in Table I. When the molar ratio of HON02/Ar exceeded 1/1000, absorption due to the dimers appeared; hence most of the experiments were carried out with a molar ratio of HON02/Ar less than 1/4000. The absorption spectrum of H O N 0 2 in solid Ar is somewhat different from that in solid N 2 reported previously by Guillory and Bernstein.28 The peak positions for HONOz in solid Ar are closer to the band centers in the gas phase. The much greater matrix shifts for v I (OH stretching) and v3 (HON bend) in N 2 matrix indicate a strong interaction between H O N 0 2 and N2. In this paper, we describe the identification of HOONO that was produced from UV irradiation of H O N 0 2 in solid Ar. Comparison of the IR spectra and the photochemistry of various isotopic HONO, in Ar and N2 matrices will be presented ~ e p a r a t e l y . ~ ~ Some weak peaks observed after irradiation are readily assigned to OH (3548.1 c ~ - I ) , ~HON O (2715.4 and 1562.2 ~ m - l ) , ~NO2 ' (1610.8 ~ m - l ) , 'and ~ N O (1871.7 ~ m - ' ) , In ~ ~a few cases with a slightly greater concentration of HON02, a weak peak due to t-HONO (1688.4 ~ m - was 9 ~also ~ identified. Several new peaks also appeared near the absorption region of the parent molecule; presumably they are due to H O N 0 2 produced from recombination of the photodissociation fragments OH and NO2 in a slightly different matrix environment. The other new features after photolysis have been divided into two groups (labeled A and B in Table I) based on the correlation of intensities in various experiments under different experimental conditions. The peaks in groups A and B have a one-to-one correspondence with peak wavenumbers of group A 5-18 cm-I greater than those of group B. Typically, when light from the Hg lamp was projected from the front side (so as not to pass through the CsBr target window), the peaks in group A dominated during the early stage of photolysis, and the peaks in group B increased in intensity gradually and sometimes became more intense than those in group A. When the matrix was irradiated by a Hg lamp from the rear, group B dominated. When the matrix was annealed to approx(28) Guillory, W. A.; Bernstein, M. L. J . Chem. Phys. 1975. 62, 1058. (29) Cheng, B.-M.; Lee, Y.-P. To be submitted to J . Chem. Phys. (30) Cheng, B.-M.; Lee, Y.-P.; Ogilvie, J. F. Chem. Phys. Lett. 1988,151, 109. (31) Jacox, M. E.; Milligan, D. E. J . Mol. Spedrosc. 1973, 48, 536. (32) St. Louis, R. V.;Crawford, B., Jr. J . Chem. Phys. 1965, 42, 857. (33) Frel, H.; Pimentel, G. C. J . Phys. Chem. 1981, 85, 3355. (34) Guillory, W. A.; Hunter, C. E. J . Chem. Phys. 1971, 54, 598.
imately 23 K, group A decreased in intensity whereas group B was enhanced. Hence, we deduce that these peaks belong to the same species in different matrix sites. Similar experiments have been performed in N2 matrices; only one group of lines having peak wavenumbers close to those of group B in the Ar matrix has been observed.29 This observation is also consistent with our postulate that the splitting was due to a matrix site effect. The site-splitting effect was also observed for HONOz in an Ar matrix (as listed in Table I) but not in a N2 matrix.28 The peaks in group B (3545.5, 1703.6, 1364.4, 952.0, and 772.8 cm-I) would thus correspond to the more stable site. When HO15N02was used, the peaks at 3545.5 and 1364.4 cm-l did not shift; the peak at 952.0 cm-I shifted only -4.3 cm-l, whereas the peaks at 1703.6 and 772.8 cm-' shifted -29.8 and -8.8 cm-I, respectively. When DON02 was photolyzed, the peaks at 1703.6,952.0, and 772.8 cm-' shifted slightly (0.1, -1.7, and -0.7 cm-I, respectively), whereas the peaks at 3545.5 and 1364.4 cm-' shifted -930.1 and -274.7 cm-', respectively. The peaks in group A also shifted similar to those in group B. The observed vibrational fundamentals of various isotopic species are listed in Table 11. Because the precursor contained only H, N, and 0 atoms, we consider first the vibrational modes containing these atoms. The peak at 3545.5 cm-' corresponds to O H stretching, based on the observed isotopic shift and its characteristic wavenumber. The 1703.6-cm-' band is assigned to N=O stretching because the ISN isotopic shift of nearly 30 cm-' indicates that it is a stretching mode containing N, and the region 1700 cm-l corresponds to N = O stretching. The peak at 772.8 cm-I is likely to correspond to a bending mode involving N (but not H), whereas the line at 1364.4 cm-' corresponds to a bending mode involving H (but not N), on the basis of their smaller shifts for the corresponding isotopes. The vibrational mode for the peak at 952.0 cm-I probably involves neither H nor N atoms because of the relatively small isotopic shifts but is characteristic of an 0-0 stretching mode. We therefore assign the observed new features to HOONO, with the 1364.4- and 772.8-cm-I peaks correspond to HOO bending and ON=O bending vibrations, respectively. The assignments of the vibrational modes are also listed in Table 11. The vibrational assignment is further supported by comparison with a similar molecule, pernitric acid (HOON02). Niki and co-workers observed HOON02 from the gaseous reaction of H 0 2 with NO2 by means of an infrared spectrometer in combination with a 40-m multipass absorption ~ e l 1 . j ~The lines a t 3540.1, 1396.5, 941.0, 922.1,919.2, 1728.3, 1303.9, and 802.7 cm-I have been ascribed to HOON02, with the latter three corresponding to the nitrate group. The 0-0stretching, 922.1 or 941 .O cm-', for H O O N 0 2 is close to 952.0 cm-' attributed to HOONO.
-
(35) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1977, 45, 564.
Infrared Absorption of HOONO
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2017 TABLE Ilk Comparison of tbe Products of Pbotolysis from Various Light Sourceso
J
HON02
species OH NO HOONO(A) HOONO( B) t-HONO
K O
line, cm-' 3548.1 1871.8 1708.3 1703.6 1688.4 1610.8 1562.5
photolysis efticiencyb
Dhotolvtic sources mediumXeCl Nd:YAG press. (308 nm) (266 nm) Hg lamp
8.6 10.4
0.32 0.35 3.4 5.1
35.4 1.4
4.1 0.20
0.28 0.30 1.7 5.4 0.18 2.0 0.8 I
0.09
0.36
0.03
KrF (248 nm) 0.3 1 1.s
0.69
0.8 0.21 3.5 0.2 1
trans-perp cis-perp HOONO HOONO Figure 2. Geometries and formation of cis-perp and trans-perp HOONO from photolysis of HON02.
"The numbers listed are the ratios of the peak absorbance of the product at the listed wavenumbers to the decrease in peak absorbance of HON02 at 764.0 cm-I. bFraction of the decrease in peak absorbance of HON02 at 764.0 cm-I.
Similarly, the HOO bending (1 396.5 cm-I) of HOON02 is only 2.4% greater than the 1364.4-cm-' value ascribed to HOONO. Recently, McGrath et al. have performed ab initio calculations on the vibrational frequencies and the corresponding IR intensities of HOONO. The results obtained for various isotopic HOONO at RMP2/6-31G* level are also listed in Table II.17-36 Similar calculations on H O N 0 2 and related molecules indicate that the average error of the vibrational frequencies is probably within 7%. The theoretical predictions are consistent with our experimental data, with the greatest error of 5.4% for the O N 4 bending mode and an average error of 2.5% for the five vibrational modes observed. Predictions of four additional vibrational modes are also listed in Table 11; they are low-frequency modes lying beyond our detection limit. The weak lines observed at 789.8 cm-' may be due to the overtone or combination of these low-frequency modes. The relative absorption intensities of HOONO lines observed in this study agree qualitatively with theoretical predictions. The I5N isotopic shifts predicted by ab initio calculation are in excellent agreement with our experimental observation, but the predicted *H isotopic shifts are greater than the experimental value;36the deviation is probably due in part to the error in the predicted OH stretching and HOO bending frequencies and in part to an error in the predicted geometry of HOO group in HOONO. The structures and relative energies of six different conformers of peroxynitrous acid have also been investigated at RHF/6-3 1G* level by McGrath et al.I7 The two most stable conformers exhibit nearly perpendicular arrangements of the OH group with respect to the OON plane, as shown in Figure 2. The cis-perp conformer (referring to the rotation about the HOO-NO bond) is approximately 5 kJ mol-' more stable than the trans-perp conformer. The possibility of the assignment of the lines in group A and B to different conformers of HOONO should be considered. The product, HOONO, was presumably formed by the recombination reaction of the fragments OH and NO2 from photolysis within the argon lattice site. As illustrated in Figure 2, the formation of trans-perp HOONO from photolysis of H O N 0 2 requires less spatial rearrangement than that of cis-perp HOONO. Hence, the assignment of the lines in group A to trans-perp HOONO and those in group B to the more stable cis-perp HOONO is
consistent with the experimental observation of an initial growth of lines belonging to group A and the dominance of B at the later stage of photolysis. However, such assignments are not favored because the variations in wavenumbers for all corresponding lines in group A and B are much smaller than those normally expected for the cis and trans conformers. Several photolytic sources other than Hg lamp have also been tested; no additional features were observed. Table 111 summarizes the relative peak absorbance of various products of photolysis by different photolytic sources. The loss of HON02 from photolysis at 355 nm was much smaller, and the results are not listed. Although the efficiency of photolysis increases as the photolysis wavelength is decreased, the yield of HOONO relative to the loss of H O N 0 2 decreases. This observation suggests that HOONO was further photolyzed at shorter wavelength. The products of photolysis are likely to be H 0 2 and NO, although it is difficult to show a definitive proof from our experiments. NO was observed in all experiments except the ones employing laser photolysis at 308 nm. However, it may also be formed from photolysis of NO2. Weak H 0 2 absorption at 1392.1 cm-' was observed only in a few experiments; presumably further reaction or photolysis took place. It should be pointed out that all of the observed IR absorption lines of HOONO except the weakest one at 3545.5 cm-' (OH stretching) lie outside the spectral range 185Cb3850 cm-I investigated in the product analysis of reaction 1 by Burkholder et a1.I6 Therefore, the upper limit of the branching ratio of reaction 1b may be greater than they estimated (
