J. Phys. Chem. 1995,99, 10824-10829
10824
Photochemistry of Nitric Acid in Low-Temperature Matrices Thomas G. Koch and John R. Sodeau* School of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, United Kingdom Received: February 22, 1995; In Final Form: May 2, 1995@
A combination of matrix isolation and Fourier transform infrared (FTIR) spectroscopy has been employed to investigate the low-temperature photochemistry (4.2 K) of nitric acid (HON02). UV photolysis of the matrixisolated precursor in argon (MR = 1/4000) at 184.9 and 253.7 nm led to the formation of peroxynitrous acid (HOONO), presumably via cage recombination of OH NO2. HOONO appeared to undergo secondary photolysis to yield NO and HO2. A second primary photolysis channel produced the novel species C2”HN02, a thermodynamically less stable isomer of nitrous acid; deuterated studies combined with group frequency arguments and theoretical studies support this assignment. In nitrogen matrices, N20, cis-HONO, and truns-HONO were observed as additional products due to efficient scavenging of O(’D2) by the matrix material. Irradiation of HONO2 in oxygen-doped argon matrices resulted in photooxidation and produced primarily peroxynitric acid (H02N02) as well as ozone.
+
Introduction The W l v i s absorption spectra and photochemistry of nitric acid have been the subject of continuing research in the past due to the key role of HONO2 in the balance of catalytically active HO, and NOx radicals in the stratosphere.’-’ Nitric acid absorbs light at wavelengths shorter than 330 nm and can be readily photolyzed. Processes l a and lb are thought to be the two major photodissociation channels: HONO,
+ hv - OH + NO, - HONO + 0
(la) (1b)
It is generally accepted that OH and NO2 are formed with a quantum yield (‘3) near unity upon HON02 photolysis between 200 and 300 nm.334However, in a recent publication Tumipseed et aL5 pointed out that other channels, in particular channel lb, the formation of HONO 0, are likely to be favored at wavelengths shorter than 200 nm. They reported a drop in the OH yield to 0.33 at 193 nm and an O(3P+’D2) yield of 0.81. It has been argued that photoexcited N02* and HONO* may be formed as primary products rather than their ground state counterparts because at short wavelengths OH radicals and 0 atoms are produced simultaneously. The excited molecules then decay as follows:
+
-
+0 HONO* - OH + NO NO2*
NO
(2)
(3)
Electronically excited N02* has been identified by “photolysis induced fluorescence” (PIF),6 and the formation of an excited triplet HONO* molecule has been rationalized on the basis of spin conservation rules and energy consideration^.^ The formation and fate of these products are of pazticular importance for the upper stratosphere since at altitudes above 25 km oxygen concentrations are sufficiently low to cause a spectroscopic window to appear at ca. 200 nm between the more intense absorptions of the Schumann Runge series and the onset of the Hartley bands of ozone. Previous photochemical studies of matrix-isolated nitric acid have largely employed laser photolysis at 248 nm or longer @
Abstract published in Advance ACS Abstracts, June 15, 1995.
0022-365419512099- 10824$09.00/0
wavelengths. Peroxynitrous acid (HOONO) was found to be the major product in solid argon, formed via the recombination of the primary products OH and NO2 (la) in the matrix cage:*%g OH
+ NO2 -HOONO
(4)
In nitrogen matrices, nitrous acid and nitrous oxide were observed as additional products, which was attributed to the photodecomposition of a cyclic, six-membered HONOyN2 complex. In this study we have investigated the matrix photochemistry of HONO2 at 184.9 nm since primary processes other than (la) are thought to become important at A < 200 nm. We report a repartitioning of the photoproducts in the matrix caused by the generation of a photoexcited intermediate and provide evidence for the secondary photolysis of HOONO. The photooxidation of matrix-isolated nitric acid has also been studied.
Experimental Section The experimental arrangement consists of a closed cycle helium system optically coupled to an FlTR spectrometer.I0The three-stage refrigeration unit (Heliplex, Model CS-308) was routinely operated at 4.2 K. Temperature measurements were made with a helium bulb pressure gauge. The deposition cold window, a highly polished cesium iodide disk seated on indium gaskets to provide good thermal contact with a nickel-plated copper holder, was housed in a stainless steel vacuum shroud fitted with a removable extension sheath. A pulsing nozzle, a Spectrosil-B window, and two KBr windows were mounted on ports along the axes of the rotatable sheath to allow for matrix deposition, UV photolysis, and infrared detection, respectively. Infrared absorption spectra of matrices were recorded with a Digilab FIX-40 spectrometer using a liquid nitrogen cooled HgCdTe detector. A Perkin-Elmer globar served as an extemal infrared source. It was housed together with the rotatable part of the cold head and associated optics in an airtight Perspex box, linked to the emission port of the spectrometer. To minimize the absorption of atmospheric water vapor and carbon dioxide, the spectrometer and the Perspex box were routinely flushed with dry,C02-free air. Typically, single beam spectra were taken at 1 cm-I resolution, boxcar apodized, and computed from the coaddition of 1024 interferograms. 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 27, 1995 10825
Photochemistry of Nitric Acid
TABLE 1: Absorption Bands Observed after 60 min Photolysis at 184.9 nm of an HONOdAr (MR = l/4OoO), HONOzflYz (MR = l/4000),and HONOdOdAr (MR = l/200/4000)Matrix Ar matrix/cm-I N2 matridcm-I OdAr matridcm-' assignment
I
0,024
3556.8 m 3540.3 m
3402.1 w 3296.1 w 3279.2 w 2710.6 w
I
-o.oe{ 3600
1
I
1
3400
3200
3000
I
I
1
2000 2600 YavenumbePS
1 2200
2400
1
1
2000
1800
1873.8 m 1705.0 vs 1700.8 vs 1621.6 s 1607.5 w 1597.5 vs 1392.5 s 1369.9 m 1361.7 s
3542.3 s 3535.1 m 3510.9 w 3404.0 w 3398.6 w 3288.6 m 2237.4 m 2232.6 s 1874.8 m 1698.3 vs 1677.6 m 1629.9 w 1612.9 s 1599.9 vs 1392.0 s 1392.0 s
3550.0 w 3540.0 m
3402.5 m 3296.4 vw 3281.4 vw 2708.4 vvw 1873.4 w 1726.2 vs 1703.4 vs 1701.3 vs 1684.7 w 1629.6 w 1621.0 w 1607.7 w 1597.5 vs 1389.9 m 1362.2 m
1293.7 w 1292.7 m -0,204
I
1098.8 vvw
I
I
I
I
I
1800
1700
1600
1500
1400
I
I
,
1300
1200
1100
,
1000
,
900
,
800
1277.9 w 1109.2 vvw
,
700
Uavenumoers
Figure 1. Absorbance difference spectrum of an HONOdAr matrix (MR = 1/4000) after 60 min photolysis at 184.9 nm.
Anhydrous nitric acid and deuterated nitric acid were prepared by vacuum distillation of a mixture of dry sodium nitrate (Fisons, stated purity 98%) and concentrated sulfuric acid (Fisons, 98% in H20) or deuterated sulfuric acid (Aldrich, 98% in D20) according to the method of Johnston et aL3 The matrix gases argon (99.999%), nitrogen (99.999%), and oxygen (99.998%) were purchased from Messer Griesheim. Using standard manometric procedures, gas mixtures were prepared in 2 dm3 darkened glass bulbs. Typically, matrix ratios (MR)of less than 1/1OOO were required to ensure the absence of nitric acid dimers in the low-temperature solid.' To prevent decomposition of the reactive guest molecule during deposition, a specially designed, all-glass pulsing manifold was used and the CsI window routinely coated with a prelayer of the inert matrix gas. The gas mixtures were pulsed on manually by consecutively opening and closing two Young's taps on the manifold. For in situ matrix photolysis at 184.9 and 253.7 nm a low-pressure mercury arc lamp (Phillips 91307) was tightly placed against the Spectrosil-B window and flushed with oxygen-free nitrogen. RWultS
The absorption difference spectrum recorded after 60 min photolysis of an HON02/Ar matrix (MR = 1/4000) at 184.9 and 253.7 nm is shown in Figure 1. Negative bands denote the loss of the precursor and positive absorptions the formation of photolysis products. The assignment of matrix-isolated nitric acid is detailed e l ~ e w h e r e . ~Peak ~ ~ positions ~ ~ ' ~ ~ ~and relative intensities of the products are summarized in Table 1. Peroxynitrous acid, which exhibits a characteristic band splitting due to two distinctly different trapping sites, A (3356.8, 1705.0, 1369.9,955.7, and 780.1 cm-I) and B (3540.3, 1700.8, 1361.9, 950.4, and 771.5 cm-I), was assigned according to previous data? Characteristic absorptions due to nitric oxide (1873.8
1009.8 vw 955.7 s 950.4 s 925.3 vw
792.7 w 788.2 w 780.2 s 771.5 s
1016.3 w 958.6 s 924.4 w 869.4 w 816.4 w 792.3 ms 782.0 m
1098.9 mw 1037.0 m 1030.9 s 955.6 m 949.9 s 924.9 vw 920.5 vw 847.8 vw 822.8 vw 792.9 w 787.9 w 779.4 m 771.5 m 750.9 w
HOONO HOONO HOONO trans-HONO cis-HONO HOz
A HNO Nz0 Nz0 NO HOzNOz HOONO HOONO trans-HONO cis-HONO H20 NOz A HOz HOONO HOONO trans-HONO HO2NOz N20 HO2 0 3 0 3
A HOONO HOONO cis-HONO trans-HONO A HOONO HOONO NO2
cm-I) and the hydroperoxyl radical (3402.1, 1392.5, and 1098.8 cm-I) were also observed as well as a weak band due to NO2 (1607.7 cm-I).l3 The hydroxyl radical could not be assigned with certainty as it overlapped with the OH stretch of HOONO (3540.3 cm-I). In nitrogen matrices, the photolysis of nitric acid led to the additional formation of trans-HONO (3510.9, 1677.6, 1293.7, and 816.4 cm-I), cis-HONO (3404.1, 1629.9, and 869.4 cm-I), and N20 (2237.4,2232.6, and 1277.9 cm-I), in good agreement with data in the literature.8 Band assignments to the postphotolysis spectrum of an HON02/N2 matrix (MR = 1/4000), shown in Figure 2, are listed in Table 1. The peaks at 3542.3, 1698.3, 1392.0, 958.6, and 780.0 cm-' were attributed to peroxynitrous acid. NO (1874.8 cm-I) and NO2 (1612.9 cm-') were also easily identified. Weak bands at 3398.6 and 1109.2 cm-' were assigned to the OH and 00 stretch of the hydroperoxyl radical, respectively; its stronger bending mode overlapped with that of HOONO at 1392.0 cm-'. Nitric acid photolysis in argon matrices doped with 5% oxygen showed some remarkable differences to the experiments with pure argon. The results are illustrated in Figure 3, and product assignments are listed in Table 1. Absorptions due to HOONO (3540.0, 1701.3, 1362.2,949.9, and 771.5 cm-I), HO2 (3404.0, 1398.9, and 1098.9 cm-I), and NO2 (1607.7 cm-') were readily identified. In contrast to pure argon matrices, only
Koch and Sodeau
10826 J. Phys. Chem., Vol. 99, No. 27, 1995
"O61 0.04
n
-0.04 -0.06
-0.08
-0.10 I I 2800 2600 Wavenumbers
1
3600
3200
3400
3000
I
I
I
1
2400
2200
2000
MOO
n -0.04
-0.06
-0.08 ~
1800
l
1700
l
1600
1500
l
1400
l
1300 1200 Wavenumbers
l
1100
l
1000
l
900
800
l
700
Figure 2. Absorbance difference spectrum of an HON02/N2 matrix (MR = 1/4000) after 60 min photolysis at 184.9 nm.
small amounts of NO (1873.4 cm-l) were formed. The doublet at 1037.0 and 1030.7 cm-I was assigned to 0z0ne.I~ The new, strong absorption at 1726.2cm-l and the weaker band at 1292.7 cm-' were attributed to the asymmetric and symmetric NO2 stretch of peroxynitric acid (H02N02). These values agree well with gas phase data (1728.3 and 1303.9 cm-l) reported by Niki et ~ 1 . Absorptions '~ reported for the NO2 bending mode (802.7 cm-') and H02 group (3540.1, 1396.5,941.0,922.1, and 919.7 cm-') were considerably weaker and could not be assigned with certainty, as they also largely overlapped with HOONO bands. Weak absorptions due to cis-HONO (1629.6 and 847.8 cm-') and trans-HONO (1684.7 and 822.8 cm-') were also detected. In pure oxygen matrices, the rate of nitric acid photolysis was drastically reduced due to the efficient absorption of the 184.9 nm radiation by the matrix host material; even after several hours of photolysis only very weak absorptions due to H02N02, N02, and 0 3 could be detected. The products observed upon photolysis of HON02 at 184.9 nm agree well with the published results for photolysis at ;12 248 nm.*v9 HOONO appeared to be the major product in argon, accompanied by N02, N20, and HONO in solid nitrogen. However, considerably stronger absorptions due to NO and H02 were observed in the experiments reported here. More important, additional absorptions at 3289, 1600,1016, and 792 cm-l were detected in nitrogen matrices. The kinetic profiles of the individual bands, shown in Figure 4, suggest that they belong to the same compound A. In argon matrices or oxygen-doped argon matrices these bands appeared slightly shifted near 3280, 1598, 1010, and 793 cm-'. Upon deuteration in nitrogen the absorptions were observed at 2437, 1595, 890, and 789 cm-', respectively. The absorbance difference spectrum of an HONO2/ DON02/N2 matrix (MR = 1/1/8000) after 60 min photolysis at 184.9 and 253.7 nm is shown in Figure 5.
l
I
l
~
1800
1700
l
l
1600
1500
l
1400
l
l
1300 1200 Wavenumbers
l
1100
l
l
1000
900
l
800
r
700
Figure 3. Absorbance difference spectrum of an HON02/02/Ar matrix (MR = 1/200/4000) after 60 min photolysis at 184.9 nm. 10.9
--
0.8
--
A 0.7
--
B
s 0.6
II
3 A
4
1016 1600
X 3288
i
--
0 0.5
--
A N 0.4 C
--
0
t I
I
I
I I
I
I I
I
10
20
30
40
50
60
I
TIME [MINI
Figure 4. Normalized kinetic profiles of the absorption bands (3289, 1600, 1016, and 792 cm-') attributedto compound A in solid nitrogen.
Discussion Vibrational Assignment of Compound A. The number of possibilities concerning the identity of the unknown A (3289, 1600, 1016, and 792 cm-') is rather limited, as it can only consist of a combination of 1 H, 1 N, and 3 0 atoms. A number of species have already been accounted for, and since an assignment to the nitrate or peroxynitrate radical can also be ruled out as they do not absorb in the 3000 cm-I region, the possibility should be considered that the set of absorption bands is due to C2,-HN02, a thermodynamically less stable isomer of
J. Phys. Chem., Vol. 99, No. 27, 1995 10827
Photochemistry of Nitric Acid
TABLE 2: Vibrational Assignment of CzY-XNO~ and XCOZ(X = H, D) HN02"/ HC02- b/ DC02- b/ cm-I cm-' cm-I
0.044
0.02-
3289 m 1600 vs C
b
2431 w 1595 s
2828 1590 1385 1355 1062 769
C
1376 w 1016 m 792 m
d 890 w 789 m
2130 1580 1010 1327 912 162
N2 matrix. * NaHC02 cry~tal.2~.Too weak to be observed. Overlaps with HONOz absorptions. 3600
3400
3200
3000
2800
2600
2400
Wavenumbers
I
I
2200
2000
TABLE 3: Vibrational Frequencies the Nitro Group of Some X-NO? Molecules normal mode CH3N02%m-I N02b/cm-' C1N02c/cm-1FNOf/cm-I
!SO0
BI, ~ ~ ( N 0 2 ) AI, Y,(NOZ) AI, 0 4 0 2 ) Ll
I
1600
I
1700
#
1600
,
1500
,
1400
,
,
1300 1200 wavenumows
1100
,
1000
,
900
~
800
l
700
Figure 5. Absorbance difference spectrum of an HON02/DONO& matrix (MR = 1/1/8000) after 60 min photolysis at 184.9 nm. nitrous acid (HONO). The valence structure of such a molecule is shown below: H
I
-
H
I
O/N\'O
Although there is no experimental evidence for its existence in the literature, related molecules such as FNOz, ClN02, and CH3NO2 are well characterized by vibrational spectroscopy.I6-l8 Published ab initio calculations at the MP2/6-31G** level indicate that Cz,-HN02 is only 25 kJ mol-' less stable than the linear HONO m o l e ~ u l e . ~ The ~ - ~ ~ON0 bond angle is an estimated 127.5', and the bond lengths for HN and NO are 1.013 and 1.201 A, respectively. The values of the nitro group fall between those of NO2 (134' and 1.19 A) and CH3N02 (125.3' and 1.224 A).'7922 However, calculations of the vibrational spectrum have not been reported. A vibrational assignment, based on isotopic shifts and comparison with the isoelectronic formate ion (HCOz-) and related XN02 molecules, is listed in Table 2. CZ,-HNOZis a tetraatomic, nonlinear molecule and therefore has 3N - 6 = 6 normal modes. Three of the normal modes are of A1 (the HN stretch, symmetric NO;? stretch, and NO2 bend) and two of Bl (the asymmetric NO2 stretch and HNO bend) symmetry. The nitrogen out-of-plane mode has B2 symmetry. Most evident is the assignment of the 3289 cm-I band to the NH stretch due to its large shift upon deuteration (A = 852 cm-I). In contrast, only small shifts were observed for the 1600 and 792 cm-I bands (5 and 3 cm-I, respectively), which indicate that they are solely related to vibrations of the nitro group. The two bands were therefore confidently assigned to the NO2 asymmetric
,
1583 1397 657
1618 1320 150
1685 1286 793
1792 1310 822
Reference 24. Reference 22. Reference 16.
stretch and NO2 bending mode. Corresponding shifts (10 and 7 cm-I) have also been observed for the carboxyl group of the formate ion. The NO2 bending mode shows good agreement with the frequencies observed for other nitro compounds, such as NO2 (750 cm-I), ClNO;?(793 cm-I), and FNOz (822 cm-I), listed in Table 3. However, the NO2 asymmetric stretching frequency of an XNOz molecule is significantly more variable. It increases almost linearly with increasing electronegativity of the attached group X. This can be rationalized by considering the changing ionic character in the X-N bond with electronegativity, from X-N02+ at one extreme to X+N02- at the other. It is therefore unsurprising that the NO:! asymmetric stretching frequency of Cz,-HNOz is considerably lower than to that of ClNOz (1685 cm-I) and FN02 (1792 cm-I) and close to that of NO2 (1618 cm-I) and CH3NOz (1583 cm-I). The 1016 cm-' band of CZ,-HNOZand the 890 cm-' band of CZ,-DNOZwere assigned to the nitrogen out-of-plane bending modes, in excellent agreement with their HC02- and DCOi counterparts at 1062 and 912 cm-I, respectively. Although the two remaining normal modes, the NO2 symmetric stretch and the in-plane HNO bending mode, are infrared-active, they could not be assigned. The former is expected to absorb in the 1300-1400 cm-' region, which overlaps with HON02 absorptions. However, in the initial stages of photolysis a weak band at 1376 cm-' was observed. The HNO bending mode is expected to be a very weak IR absorber between 1400 and 1600 cm-' by comparison with the transient species HNO;I3 no absorptions were detected in this spectral region. Photochemical Mechanisms. Two likely mechanisms have to be considered for the formation of HOONO from nitric acid photolysis: cage recombination of the photofragments OH and NO2 as well as the photoisomerization via an excited HON02 intermediate. HONO,
+ hv - OH + NO, - HOONO
-
(HONO,)*
-
HOONO
(4)
(5)
In a previous study,*when the matrix material was altered from argon to nitrogen, a 10-fold decrease in the HOONO yield was measured upon 253.7 nm photolysis. The lower HOONO yield was rationalized by the hypothesis that the OH fragment moves less freely in nitrogen than in argon due to strong interactions of OH with the matrix material, which consequently led to stronger NO2 absorptions in nitrogen matrices. Our findings upon 184.9 nm photolysis are in good agreement with these
10828 J. Phys. Chem., Vol. 99, No. 27, I995
Koch and Sodeau
previous results. However, the decrease in the HOONO yield in nitrogen matrices was less pronounced. The two likely explanations for this observation are that (i) the larger excess energy of the OH fragment prevents complexation with the N2 matrix or (ii) photoisomerization ( 5 ) occurs at this short wavelength. HOONO is expected to undergo secondary photolysis in the ultraviolet region. Two channels may be considered in particular: HOONO
+ hv
-
H02
8
'T I 8 1701 8
0.8
A
0.7
B 0.6
0
+ NO
--
N
0.4
---
C E
0.3
--
0.5
A
+
--
-0.1 --
8
+ 8
+ 8
0.2
Nitric oxide and the hydroperoxyl radical were both observed as photoproducts, and it is most likely that they were produced via reaction 6a. However, some NO may also be formed from the photolysis of NO2. No evidence was detected for the peroxynitrate radicalsz2 The second major pathway in the photolysis of nitric acid is the formation of nitrous acid which, according to gas phase photolysis studies at 193 nm, is thought to become the dominant process at shorter wavelength^.^ Previous matrix studies8have reported the formation of HONO and O('D2) in solid nitrogen at 253.7 nm, which is well above the expected thermodynamic threshold (245 nm). Additional evidence from UV absorption experiments* suggests the existence of a conjugated, sixmembered HON02*N2 complex in solid nitrogen, which decreases the dissociation threshold for channel Ib': HONO, N,
+ hv
HONO
.-,
+ O('D,)
A < 245 nm
+ O('D2) - N 2 0
(lb')
(7)
In the present study irradiation with 184.9 nm photons was employed providing sufficient energy to generate HONO O('D.2) in nitrogen as well as in argon matrices. Again, little or no HONO was observed in solid argon. Since the primary photoprocesses are likely to be identical in both matrices, it must be assumed that HONO O('D2) are also produced in argon but recombine efficiently to regenerate the precursor HON02, leading to an apparent inhibition of this particular channel. In nitrogen matrices O('D2) can be scavenged by the matrix material to form N20 via reaction 7, preventing further recombination. The formation of Cz,-HN02 was not observed at E. 1 248 nm, and we may therefore assume that it involves a highly energetic process. Comparison of the kinetic profiles of C2"HN02 and HOONO, shown in Figure 6, suggests that C2"-HNO2 is likely to be formed through an additional primary channel, rather than reaction of excited fragments, as it is formed more readily than HOONO in the initial stages of photolysis. An additional primary pathway that is thought to occur only at shorter wavelengths (A < 200 nm) is the formation of 3HONO:7
+
+
HONO,
+ hv -3HON0 + O(3P)
( 1b')
The fate of the excited ,HONO molecule can be quenching, decomposition, or isomerization. The former two processes would not produce any products other than those already observed. However, the latter process may provide a plausible channel for the formation of the thermodynamically less stable isomer of HONO, C2,-HN02. An isomerization will proceed more readily via the trans form of HONO, as it allows the H
I
0 4 0
5
15
10
20
25
30
TIME [MINI
Figure 6. Normalized kinetic profiles of HOONO (1701 cm-I) and Czy-HN0z (1598 cm-I) in solid argon.
atom to move more easily toward the nonbonding electron pair located on the N atom.
This process must compete with the recombination reaction
+
3HONO O(3P) to reform the precursor. Since O(3P) cannot react with N2 to form N20 due to a symmetry correlation, it may equally proceed in argon as well as in nitrogen matrices. A recombination of C2"-HNO2 O(3P)to produce nitric acid is less likely to proceed since, unlike HONO, all electrons on the central N atom are involved in bonding. Such steric criteria could account for the fact that in argon matrices the formation of C2"-HNO2 is favored to the more stable linear cis- or transHONO isomers. Although this mechanism for the production of C2,-HN02 is quite feasible, another possible mechanism exists and should be considered: ,HONO may be generated with enough energy to dissociate, forming H NO2. This would lead to photolysis channel IC, which becomes thermodynamically allowable at 192
+
+
nm: HONO,
+ hv - H + NO, + O(3P)
(IC)
C2,-HN02 could then be formed by recombination of H and N02. The preferential formation of C2,-HN02 over HONO via H NO2 has also been suggested in a theoretical study by W a l ~ h .However, ~~ matrix studies have conclusively shown that only cis- and trans-HONO are produced in this process.26 Primary photoprocesses of matrix guests are thought to be independent of the host material. However, since molecular oxygen absorbs light below 240 nm, the use of oxygencontaining matrices is expected to reduce the intensity of the incident 184.9 nm radiation emitted by the low-pressure mercury lamp drastically. Upon UV absorption atomic oxygen and ozone may be formed as follows:
+
+ hv - O(3P) + O(3P) 0, + 0(3P) + M 0,+ M 0,
-
A
< 242 nm (8)
(9)
The photodissociation of HONOz will therefore proceed mainly
J. Phys. Chem., Vol. 99, No. 27, 1995 10829
Photochemistry of Nitric Acid
+
at 253.7 nm, with OH NO2 as the most likely primary products that may subsequently react with O(3P)atoms or ozone. In the atmosphere peroxynitric acid is almost exclusively formed via reaction 10, the association of the HO2 radical with NO2. However, in oxygen-containing matrices two additional processes, the photooxidation of HOONO by O(3P) atoms (1 1) or ozone (12), may also be envisaged:
+ NO, -H02N02 HOONO + 0 - H02N02 H02
HOONO i- O3
-
H02N02 -I- 0,
upon nitric acid photolysis at A < 200 nm. The importance of nitrous acid formation at shorter wavelengths is further highlighted by the detection of C2,-HN02, presumably generated via a related HON02 photolysis channel, the formation of 3HON0 O(3P). These results show that matrix isolation provides a useful tool to complement time-resolved techniques employed in gas phase photolysis studies.
+
(10)
Acknowledgment. The authors thank Dr. Roger Grinter and Dawn Thomas for useful discussions concerning the assignment and formation of C2,-HN02.
(11)
References and Notes
(12)
The HO2NO2 yield was found to increase at the expense of the HOONO yield as higher oxygen concentrations were used, culminating in the sole formation of HOzN02 in pure oxygen matrices. As observed for nitrogen matrices, such a reduction of the HOONO yield can be attributed to a less efficient OH NO2 cage recombination process due to hydrogen bonding of OH with the matrix material. Higher oxygen concentrations also enhance the likelihood of the OH fragment reacting with O(3P) atoms or 0 3 to form HO2. Subsequent cage recombination with NO2 would lead to the formation of H02N02. HONO was also formed, which can be attributed to the scavenging of O(3P)by 0 2 , thereby preventing recombination. C2,-HN02 was observed at low oxygen concentrations but not in pure 0 2 matrices. It is not clear whether this effect arises from quenching of 3HON0 by oxygen or simply the reduced intensity of incident 184.9 nm radiation.
+
Conclusion The primary photochemistry of nitric acid in the lowtemperature solid state corresponds well to published gas phase results. However, the secondary chemistry is largely dominated by cage recombination processes and the stabilization of reactive molecules by the cryogenic environment. Upon irradiation at 184.9 and 253.7 nm OH and NO2 are produced, which can subsequently recombine in the solid to form peroxynitrous acid (HOONO), the thermodynamically less stable isomer of nitric acid. Secondary photolysis is likely to yield HO2 and NO, essentially repartitioning the observed HOflO, ratio in the gas phase. In oxidizing solids peroxynitric acid (HO2NO2) is preferentially formed, presumably via HO2 NO2. The detection of cis- and trans-HONO and N20 in solid nitrogen indicates the occurrence of the primary pathway HONO O(lD), which is believed to play an increasingly important role
+
+
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