J. Phys. Chem. 1983, 87, 1349-1352
1349
(C02Me)NOBz, 49838-52-0; t-BuNO=NOBu-t, 31107-20-7; Me5NOBz=BzNOMe5, 84802-30-2; Me4NOBz=BzNOMe4, 84802-31-3; C13NOBz=BzNOC13, 84802-32-4; (t-Bu),BzN(O)CHZ(CBHJNO2, 84802-33-5; (t-Bu),BzN(O)H, 40489-75-6; p nitrobenzyl bromide, 100-11-8;p-nitrobenzyl duryl nitrodide, 84802-29-9; methyl iodide, 74-88-4; N-methyl-N-tris(tert-buty1)phenyl nitroxide radical, 33721-34-5; N-methoxy-N-tris(tert-buty1)phenylanilino radical, 64191-72-6; benzyl bromide, 100-39-0.
We are grateful to Dr. M. Boyer for his generous gift of some nitroso compounds and to Professor F. Gerson for discussion and assistance in the building up of our ESR electrochemical cell. Registry No. t-BuNO, 917-95-3;p-(Me2N)NOBz,138-89-6; Me5NOBz, 65594-36-7; (t-Bu),NOBz, 24973-59-9; Me4NOBz, 38899-21-7; o-MeNOBz, 611-23-4;NOBz, 586-96-9; ClSNOBz, 1196-13-0;F5NOBz,1423-13-8; (MeO)5NOBz,84802-28-8;tris-
Study of NO, by Laser-Induced Fluorescence Takashl Ishlwata, Ichlro FuJlwara, Yuklo Naruge, Klnlchl Obl,
and Ikuro Tanaka*+
Department of Chemistry, Tokyo Inst/fute of Technology, Ohokayama, Meguro, Tokyo 152, Japan (Received: November 9, 1982; I n Final Form: January 21, 1983)
The nitrate radical, NO3,was directly detected by the laser-induced fluorescence method. The band structure of the fluorescence excitation spectrum was consistent with that of the absorption spectrum. The fluorescence spectrum excited at 662 nm corresponding to the 0-0 transition showed progressions with 1060- and 1480-cm-' intervals, which were assigned to the symmetric stretching (vl) and degenerate antisymmetric stretching (v3) modes of the ground state NO,, respectively. The fluorescence lifetime excited at the 0-0 band was estimated to be 2.8 ~s from the Stern-Volmer plots in the pressure region of 0.04-0.6 torr.
Introduction The nitrate free radical, NO3, has recently received increasing interest in connection with atmospheric chemistry due to the recognition of its role in chemical cycles involving oxides of nitrogen and ozone.1i2 The reaction NOz + O3 NO, + O2 (1)
-
is the principal source of NO3 in the atmosphere and its existence in ambient polluted air3 and the stratosphere4 has been observed by the optical absorption. The absorption spectrum of NO, consists of approximately 20 diffuse bands in the region of 665-400 nm.5f' Even though the spectrum a t longer wavelengths was photochemically inacti~e,~?' fluorescence has not been observed.8 The purpose of this paper is to report the detection of NO, by the laser-induced fluorescence technique, which is a satisfactory method for detecting free radicals in many cases. Very recently, McDonaldg also detected NO3 by the same technique in the pulsed photolysis system. Here, we derive the vibrational frequencies of NO3 in the ground state from the fluorescence spectrum and also discuss the fluorescence lifetime of NO3 in the excited state.
Experimental Section Excitation of NO3 was achieved by pumping with a pulsed tunable dye laser. Radiation from a dye laser entered and left the fluorescence cell (4 cm in diameter) through horizontal side arms having a set of antiscatter baffles. The emission was detected at right angles to the laser beam by a photomultiplier (Hamamatsu TV R-928). Glass filters were used in front of the photomultiplier to reduce the scattered light. The fluorescence signal was fed to a boxcar integrator (NF Circuit Design Block, BX531/BP-10) and was subsequently digitized and stored. The output was printed out or plotted on a chart recorder.
A nitrogen laser (4 mJ/pulse, 5-10 Hz) pumped dye laser (Molectron DL 14) was used for following the fluorescence decay and measuring the fluorescence excitation spectrum. The duration of the laser pulse was about 8 ns and the bandwidth was about 0.01 nm. The dyes Rh-610, Rh-640, and 0x-725/Rh-610 (Exciton Chemical Co.) covered the wavelength region between 595 and 677 nm. The output power was monitored by a photodiode. In order to measure the fluorescence spectrum, we used a flash lamp pumped dye laser with a duration of 0.6 ps and a bandwidth of 0.1 nm as the excitation source. The fluorescence was dispersed by a monochromator (Nikon G-500, 1200 grooves/", blazed at 500 nm). The NO, radical was formed from the reaction of nitrogen dioxide with ozone at room temperature by using a flow reactor in order to avoid the accumulation of products in photochemical reactions. Ozone generated by a commercial ozonizer was collected on silica gel a t 195 K and was purified by a bulb-to-bulb distillation prior to each experiment. The purity of ozone stored in a large bulb was determined to be more than 95% by optical absorption at 254 nm. Nitrogen dioxide was prepared by oxidizing nitric oxide with an excess amount of oxygen overnight and purified by freeze-pump-thaw cycles until a white solid was obtained. These gases were handled in a conventional greaseless vacuum system and the flow rates were controlled by needle valves. The pressure was measured by an MKS capacitance manometer. (1) P. J. Crutzen, Q.J. R. Meteorol. SOC.,96,320 (1970). (2)H.S.Johnston, Science, 173,517 (1971). (3)U.Platt, D.Pemer, A. M. Winer, G . W. Harris, and J. N. Pitts, Jr., Geophys. Res. Lett., 7 , 89 (1980). (4) J. F. Noxon, R. B. Norton, and W. R. Henderson, Geophys. Res. Lett., 5, 675 (1978). ( 5 ) R. A. Graham and H. S. Johnston, J.Phys. Chem., 82,254(1978). (6)D.A. Ramsay, R o c . Colloq. Spectrosc. Int., loth,1962,583(1963). (7)F.Magnotta and H.S. Johnston, Geophys. Res. Lett., 7,769(1980). (8)F.Magnotta and W. J. Marinelli, unpublished results cited in ref
11.
'Also Adjunct Professor a t t h e Institute for Molecular Science. 0022-365418312087-1349$01.50/0
(9)J. R. McDonald, private communication.
0 1983 American Chemical Society
1350
r'I 5,
The Journal of Physical Chemistry, Vol. 87, No. 8, 1983 " " " '
"
"
Excilatlon spectrum o! NO3 ,*----
'I
Ishiwata et al.
(a)
..-
Fluorescence
spectra of NO,
'
660
670
W A V E L E N G T H inmj
Flgure 1. Fluorescence excitation spectrum of NOB. A nitrogen laser pumped dye laser was employed as a pumplng source, using dyes Rh-610 for 595-620 nm, Rh-640 for 620-650 nm, and Rh-610/0x-725 for 650-675 nm. The dashed lines show the relative laser power.
Results Figure 1shows the fluorescence excitation spectrum of NO, obtained by monitoring the emission through a Toshiba R-72 cutoff filter (50% transmission a t 720 nm). NO3 was produced by admixing NO2 (1 mtorr) with an excess amount of O3 (0.5 torr), where the NO, fluorescence signal was linearly proportional to the initially added NO2 up t o 2 mtorr. Six broad bands could be observed with a laser bandwidth of 0.01 nm, and a profile of the spectrum was fairly consistent with the absorption spectrum taken earlier.6J0J1 The longest wavelength peak lying a t 661.8 nm is firmly assigned to the 0-0transition and the strong peak a t 623.5 nm is the first member of the progression in the symmetric stretching vibration (950 cm-') identified by Ramsay.6 It has been shown that the bands a t 627.7 and 604.6 nm are involved in the progressions with 820and 1453-cm-' intervals,1° respectively, although the assignments are ambiguous. Weak bands are also seen a t 637.3 and 614.1 nm. Figure 2 shows the emission spectra observed following the single vibronic-level excitation of NO3. When the dye laser was tuned a t 661.8 nm corresponding to the 0-0 transition, the emission spectrum exhibited a distinct vibrational structure as shown in Figure 2a. The strong peak a t 711.5 nm was about 1060 cm-l below the excitation wavenumber and the following member of this progression appeared a t 770 nm. The second intense peak at 733.5 nm was about 1480 cm-' below the laser wavenumber and the peak a t 794.5 nm would be a combination band of these two vibrations (1480 + 1060 cm-'1. Two other weak peaks a t 678.7 and 697.0 nm have a spacing of 380 and 760 cm-l off the excitation wavenumber, respectively. The excitation spectrum in Figure 1corresponds to the absorption spectrum of NO, and the emission spectrum in Figure 2a is far different from the fluorescence spectrum of NO2 excited a t the same wavelength, so that this emission should be attributed to the transition originating from the electronically excited state of NO,. On the basis of published thermochemical data,12 the energetic wavelength threshold for the following dissociation products can be calculated: NO, hu NO(X211) 02(X32c,) X < 8 pm (2)
+
-
~~
~~~~
-
+
NO(X211)+ 02(a1A,)
+
NO(X211) O2(b'Bg+) N02(22A,)+ O(,P) ~
~
< 1.1pm X < 700 nm X < 580 nm X
(3) (4) (5)
~
(10) D.N. Mitchell, R. P. Wayne, P. J. Allen, R. P. Harrison, and R. J. Twin, J. Chem. SOC.,Faraday Trans. 2,76, 786 (1980). (11)W. J. Marinelli,D. M. Swanson, and H. S. Johnston, J. Chem. Phvs.. 76. 2864 (19821. 112)D:R. St& and H. Prophet, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 37 (1971).
0 1 '
3
"
1
650
"
"
1
'
700 (nn,l
'
1
'
1
750
"
"
1 '
800
WAVELENGTH
Flgure 2. Fluorescence spectra (unmected for the detection system) of NO8excited at (a) 661.8 and (b) 604.4 nm. The dashed line in Figure 2a shows the spectral sensltivlty of the detection system.
Energetically, reactions 2-4 may occur in the wavelength region of the NO, absorption shown in Figure 1. It is then conceivable that the chemiluminescence of NO2due to the reaction
NO
-
+ 0,
NO2* + O2
(6)
would contribute to the emission observed at the excitation of NO3 other than the fluorescence of NO3. In this case, the spectral feature of the observed emission should resemble that of the NO 0,chemiluminescence. The short wavelength limit of this broad emission is around 600 nm and ita intensity rapidly increases with an increase in wavelength. l3 Figure 2b presents the emission spectrum excited a t 604.4 nm. The spectrum shows a structureless emission which underlies the same two peaks a t 711 and 733 nm as those in Figure 2a with the strong one a t 662 nm being coincident with the 0-0 transition. It should be noted that the spectral profiles observed with excitation a t 627.7 and 623.5 nm were very similar to that observed a t 604.4 nm except that the broad underlying emission became weaker. The observation of the 0-0 transition in Figure 2b gives additional evidence for the identification of the emitting species as NO3and indicates some vibrational relaxation in the upper state. The broad emission overlapped with the NO, emission may be due to the chemiluminescence of NOz in the NO + 0,reaction, as discussed above. The fluorescence decay of NO3was measured with excitation a t the 0-0 band (662 nm), where only the fluorescence of NO, was observed. Cutoff filters, Toshiba R-64 and Corning 7-59, were used to reduce the scattered light. Figure 3 shows some typical logarithmic plots of fluorescence signal against time. The decay fitted single exponential and was followed a t least for two lifetimes. The decay constant was computed by means of a linear least-square fit to the data, starting 0.2 p s after the laser pulse. The total depopulation rate constant (It) of an electronically excited NO, radical is described as follows:
+
12 = kf
+ ko,[O31
(7)
where kt is the inverse of the collision-free fluorescence (13)J. C. Greaves and D. Garvin, J. Chem. Phys., 30, 348 (1959).
The Journal of Physical Chemistry, Vol. 87, No. 8, 1983
Study of NO, by Laser-Induced Fluorescence I
I
TABLE I: Assignments of Vibrational Frequencies for NO, ( c m - ’ )
I
Fluorescence decay of NOj
Pressure 4
r
(Torr) a) 0 1 4 0 b) 0 . 2 5 5
\.
1351
(PSeC) 1 .91 1 ,52
symmetry no. a1:, a2
e’ e’
vl
v2 vj u4
mode symstretch op deform degstretch deg deform
NO, 1060 1480
380
NO,-Q 1050 831 1390 7 20
From ref 22.
3
-C
2
,
1
‘,\ * .
,
A.
3
2
1
Time ( p sec )
Flgure 3. Logarithmic plot of fluorescence decay of NO, excited at 661.8 nm.
kx!d6-
(set“)-
=,A,
of a Jahn-Teller instability. Nonempirical calculations by Walker and Horsley16 assuming D 3 h symmetry showed a 2A2/ground state and low-lying 2E”(0.36 eV) and 2E’ (1.77 eV) excited states. More recently, an ab initio MO LCAO UHF calculation was made by Lund and T h u o m a P to examine the stability of the Y-shaped geometry. They confirmed that the ground state of NO possessed D3h symmetry with a bond distance of 1.24 The geometry of NO3 has been studied in various environments by ESR. Some w o r k ~ ’ ~were J ~ consistent with 2A21symmetry in the ground state having a threefold symmetry axis, while others20!21 suggested a Y -shaped C2, structure when NO3 was present in the crystal of urea nitrate. This discrepancy is probably due to the effect of the crystal field upon the orbital energies. Wood and LOZOS’~ proposed that hydrogen bonding to urea protons lowers the energy of an orbital on oxygen, inverting the order of the a2/ and e” levels and distorting the ESR spectral parameters. In Da symmetry supported by the recent work as noted above, NO3 should possess two stretching modes (a< + e’) and two bending modes (a2’ e’). It is worthwhile to discuss the assignments of the vibrational frequencies in the ground state obtained from the fluorescence spectrum in comparison with the analogous nitrate ion (NO,-). The nitrate ion is known to belong to a symmetric planar group. The assignments of the vibrational frequencies are obtained from the literature,22as shown in Table I. The highest occupied molecular orbital of NO3 as well as NO, has a2/ symmetry which is nonbonding with respect to the N-0 bonds. It is then reasonable to expect that the vibrational frequencies of the stretching modes are similar for these species and the wavenumbers for v1 and v3 can be assigned to be 1060 and 1480 cm-’, respectively. It is interesting to note that the excitation spectrum in Figure 1 seems to show the corresponding two vibrational frequencies, 950 cm-’ for v t and 1435 cm-’ for v3, in the upper state. Since the out-of-plane bending mode ( v 2 ) is symmetrically forbidden, the degenerate deformation mode (v4) would be assigned to 380 cm-’. This was confirmed by the observation of a hot band a t 679 nm in the absorption ~ p e c t r u m .The ~ band at 760 cm-l, then, seems to be an overtone of the v4 mode. The diffuseness observed in absorption bands of NO3 by Ramsay6 under high resolution raises the question whether NO3 in the excited state undergoes predissociation or some other process such as a radiationless transition. Information on the collision-free fluorescence lifetime is very valuable to discuss this point, since the absolute ab-
661.8 nm
1.0 .
I
+
1 0
0.1
0.2
0.3
0.4
0.5
0.6
Pressure ( T o r r )
Flgure 4. Stern-Volmer plot of fluorescence llfetimes of NO3 excited at 661.8 nm.
lifetime (7f). The second term represents the removal of the electronically excited NO3 by O3which is the dominant collision partner, since the initial concentration of NO2was kept less than 1 mtorr. Figure 4 shows the variation of It with total pressure in the region of 0.04-0.60 torr. From these data, the extrapolated (zero-pressure) lifetime of Tf was estimated to be (2.8 f 0.3) X lo4 s and the quenching constant was determined to be Ito, = (3.9 f 0.3) X lo-’’ cm3 molecule-’ s-’.
Discussion The geometry and electronic structure of NO3 have been studied theoretically and experimentally; Walsh14 predicted that the NO3 radical had Dah symmetry and that the allowed transition in the visible region was (e’)3(e’’)4(a2’)2:2E’
-
(e’)4(e’’)4(a2’)1: 2A2’
However, the extended Huckel and INDO molecular orbital calculations by 01sen and Burnelle15 indicated a Y-shaped geometry (C,) with the 2B2ground state. They explained the distortion from trigonal geometry in terms (14) A. D. Walsh, J. Chem. SOC., 2306 (1953). (15) J. F. Olsen and L. Burnelle, J. Am. Chem. SOC.,92, 3659 (1970).
T. E. H. Walker and J. A. Horsley, Mol. Phys., 21, 939 (1971). A. Lund and K. Thuomas, Chem. Phys. Lett., 44, 569 (1976). D. E. Wood and G. P. Lozos, J . Chem. Phys., 64, 546 (1976). A. Reuveni and Z. Luz., J. Magn. Reson., 23, 271 (1976). G. W. Chantry, A. Horsfield, J. R. Morton, and D. H. Whiffen, Mol. Phys., 5, 589 (1962). (21) T. K. Gundu Rao, K. V. Lingam, and B. N. Bhattacharya, J. Magn. Reson., 16, 369 (1974). (22) G. Herzberg, “Molecular Spectra and Molecular Structure. 11. Infrared and Raman Spectra of Polyatomic Molecules”,Van Norstrand, New York, 1945, p 178. (16) (17) (18) (19) (20)
J. Phys. Chem. lB83, 87,1352-1357
1352
sorption cross section has been published. We used the following formula derived by F O r ~ t e rto~ calculate ~ a radiative lifetime: 1 / =~ 2.88
X
1 0 ~ 9 n 2 ( g l / g , , ) ~ ~ -( 2v) -~ oc (-v ) / D )dD
(8)
where iio is the wavenumber of the 0-0 transition and n is the refractive index of the surrounding media. go and g, represent the degeneracy of lower and upper states, respectively. There appeared to be some disagreement in four quantitative s t ~ d i e s ~ Jof~ the J ' ~absolute ~~ absorption cross section of NO, regarding the magnitude and shape. Marinelli, Swanson, and Johnston" showed that the integrated absorption cross section of the four studies are coincident with each other within 20% and recommended that ref 5 and 10 give the best estimate of the integrated absorption. If we assume that the excited state is degenerate (2E'),the radiative lifetime derived from the cross sections of Graham and Johnston (400-704 nm)5 is about 1.4 ps, which is shorter than our extrapolated fluorescence lifetime (2.8 ps) by a factor of 2. This fact clearly indicates that the nonradiative process including the predissociation to NO + O2 does not dominate the fate of NO3 excited at 662 nm. Magnotta and Johnston' directly measured the wavelength dependence of photoproducts (NO and 0) by resonance fluorescence coupled with the pulsed photolysis of NO3. They reported that the quantum yield for NO2 + 0 was ca. 0.6 and that for NO + O2 was ca. 0.3 at 600 nm. These yields decreased with increasing wavelength and (23) Th. Forster, "Fluoreszenz Organischer Verbindunger", Vanderhoeck and Ruprecht, GBttingen, 1951. (24) H. S. Johnston and R. A. Graham, Can. J. Chem., 52,1415 (1974).
only slight photolysis was observed at 623 nm. Further, the strong 0-0 band at 662 nm was photochemically inactive with respect to dissociation. Our result that the nonradiative process does not proceed in NO, excited at 662 nm is consistent with their observation. The contribution of dissociation processes becomes dominant at wavelengths shorter than 662 nm. However, it is difficult to measure quantitatively this process since the apparent lifetimes are affected by the appearance of NO + 0, chemiluminescence as well as the occurrence of a vibrational relaxation. We plan further investigations to obtain spectroscopic and photochemical information about NOg. For example, a high-resolution spectroscopic study of the infrared absorption at 1480 cm-' is in progress from which detailed information about the structure of the ground state NO3 will be obtained.25 Also a molecular beam experiment coupled with laser-induced fluorescence is attempting to examine the radiationless processes in the excited state under the collision-free condition. Moreover, a simplified spectrum of NO, in a cold jet would be extremely useful for the analysis of the excited state. Acknowledgment. This research was performed under a joint program with the Institute for Molecular Science. We thank Dr. J. R. McDonald for a helpful discussion and showing us his results prior to publication. We are also grateful to Drs. H. Akimoto and N. Washida for the loan of an ozonizer. Registry No. NO3, 12033-49-7. (25) T. Ishiwata, K. Kawaguchi, E. Hirota, and I. Tanaka. D, symmetry of NO3 in the ground state was indicated by infrared diode laser spectroscopy of the NO3 v3 band.
Perturbations around Steady States in a Continuous Stirred Tank Reactor K. Bar-Ell" Department of Chemistry, Tel-Aviv University, Tel A viv 69978, Israel
and W. Geiseler Institute of Technical Chemistry, Technical University of Berlin, Berlin, West Germany (Received: June 6, 1982)
Perturbations around steady states of a system of cerium or manganese bromide and bromate ions in a sulfuric acid solution, in a continuous stirred tank reactor (CSTR), are investigated. Two kinds of perturbations are discussed, namely, small-speciesand large-constraint ones. The feasibility of measuring a particular rate constant by following the system behavior after a small perturbation is analyzed. Some general theorems regarding this behavior are obtained. Large perturbations, namely, those that cause a transition of the system from one steady state to another, are analyzed and an approximate relationship between the perturbations' intensity and length is developed. The results are in good agreement with the experimental data.
Introduction open systemin a continuous~ystirred tank reactor (CSTR) can exist in more than one steady state, depending on the conditions in which it is operated. A typical such systemwhich shows this steady-state mulor manganous ion oxidation by tiplicity is the bromate ions in sulfuric acid solution. Such a system was shown by Geiseler and Follner' to exist under certain ex-
perimental conditions in two stable steady states, while under other conditions only one steady state is possible. In this paper we want to examine the dynamical behavior of a system initially in a steady state under the influence of perturbations, and to study the feasibility of such experiments in order to get a better understanding (1) W. Geiseler and H. Follner, Biophys. Chem., 6, 107 (1977).
0022-3654/83/2087-1352$01.50/00 1983 American Chemical Society
