J . Phys. Chem. 1987, 91, 1793-1797
1793
Photolysis of Benzotriazole in Alcoholic Glass at 77 K Haruo Shizuka,lapdHiroshi Hiratsuka,lb Mamoru Jinguji,Ic and Hiroyuki Hiraoka*la IBM Research, Almaden Research Center, S a n Jose, California 951 20-6099 (Received: August 20, 1986)
The photolysis of benzotriazole (1) in alcoholic glass at 254 nm has been studied by UV absorption, emission, IR, and mass spectroscopies. Bond scission of the N-NH bond of 1 originates from the S1(.n,.n*) state to give the azo compound 2, having an absorption band at 423 nm and an IR absorption band at 2070 cm-'. This yellow azo intermediate is decomposed thermally or photochemically. Iminocyclohexadienylidene (3) with resonance structures of a carbene (3a) and a biradical (3b) may be produced as a colorless second intermediate. On the basis of reaction product analysis from the rigid-phase photolysis of 1 at 77 K compared to those in liquid solution and gas-phase photolyses, the reaction paths of 3 to yield aniline, o-anisidine, o-ethoxyaniline, and 1-cyanocyclopentadiene are discussed in terms of the spin states of 3.
Introduction Photochemical and thermal decompositions of triazoles have been studied e x t e n s i ~ e l y . ~ -The ~ loss of nitrogen from benzotriazole is assumed to generate the carbene intermediate 3 with its resonance biradical structure leading to the Wolff rearrangement, the insertion reaction, and the hydrogen atom abstraction. The photolysis of 1 is, therefore, of interest not only in photochemistry, but also in carbene chemistry.'&16 The triplet biradical 3, produced by the photolysis of 1 at 77 K, has been studied by ESR spectro~copy.'~However, no attention to the primary intermediate 2 produced photochemically by the N-NH bond scission of 1 has yet been given. In the course of a study on the photochemistry and photophysics of benzene derivatives,I8 we found that the primary process in the photolysis of 1 is N-NH bond cleavage from the Sl(x,a*)state to yield a yellow azo compound 2. Intermediate 2 was decomposed thermally or photochemically to 3 and a nitrogen molecule. We wish to discuss the reaction pathways of 3 in terms of the spin states of 3 on the basis of the results of the rigid, liquid, and gas-phase photolysis of 1. Similar reaction pathways involving triazolo[3,4-a]pyridine have been reported.lg Experimental Section The starting material, IH-benzotriazole (1, Gold-label grade), was obtained from Aldrich and was used without further purification. Methanol and ethanol were G R grade products from (1) (a) IBM Almaden Research Center. (b) Department of Chemistry, Tokyo Institute of Technology. (c) Present address: Yamanashi Medical College, Kitakoma-gun, Yamanashi. (d) Department of Chemistry, Gunma University, Gunma, Japan; Visiting Summer Faculty to IBM San Jose Research Laboratory. (2) Burgess, E. M.; Carithers, R.; McCullagh, L. J . Am. Chem. SOC.1968, 90, 1923. (3) Boyer, J. H.; Selvarajan, R. J. Heterocycf. Chem. 1969, 6 , 503. (4) Huber, A. J. J . Chem. SOC.C 1969, 1334. (5) Ohashi, M.; Tsujimoto, K.; Yonezawa, T. Chem. Commun. 1970, 1089. Jpn. 1972, (6) Tsujimoto, K.; Ohashi, M.; Yonezawa, T. Bull. Chem. SOC. 45, 515. (7) Crow, W. D.; Wentrup, C. Chem. Commun.1968, 1026. (8) WentruD. C.: Crow. W. D. Tetrahedron 1970. 26. 3965. (9) Gilchriit,' T. 'L.;Gymer, E. G.; Rees, C. W. i.Ciem. SOC.,Perkins Trans. I 1973, 1 , 555. (10) Trozzolo, A. M. Acc. Chem. Res. 1968, I , 329. (11) Kirmse, W. Carbene Chemistry, 2nd ed;Academic: New York, 1971. (12) Jones. Jr., M.; Moss, R. A., Eds. Carbenes, Vol. 1; Wiley-Interscience: New York, 1973. (13) Jones, Jr., M.; Moss, R. A., Eds. Carbenes, Vol. 2; Wiley-Interscience: New York, 1975. (14) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, CA, 1978; p 544. (1 5) Jones, W. M. In Rearrangements in Ground and Excited States, Vol. 1, de Mayo, P., Ed.; Academic: New York, 1980; p 95. (16) Wentrup, C. Ado. Heterocycl. Chem. 1981, 28, 231. (17) Murai, H.; Torres, M.; Strausz, 0. P. J . Am. Chem. SOC.1980, 102, 1421. (18) Shizuka, H.; Ueki, Y.; Iizuka, T.; Kanamaru, N. J . Phys. Chem. 1982, 86, 3327. (19) Chapman, 0 L.; Steridan, R. S.; LeRoux, J.-P. Rec. J . R . Neth. Chem. SOC.1979, 98, 334.
0022-3654/87/2091-1793$01.50/0
Wako. Isopentane (GR grade) was further purified by passing it through a silica gel column. The experimental procedures and apparatus for absorption, emission, lifetime, and emission quantum yield measurements were the same as those reported elsewhere.20 EPA (diethyl ether: isopentane:ethanol = 5:5:2 by volume) and MeOH-EtOH (methano1:ethanol = 1O:l by volume) mixtures were used as solvents for spectrophotometry and photochemical experiments at 77 K, respectively. Methanol was used as a solvent for the photolysis of 1 at room temperature. The concentration of 1 was M. All samples were degassed by freeze1 X 10-4-5 X pump-thaw cycles. The low temperature experiments for spectrometry and the photolysis in EPA glass at 77 K were carried out with a cryostat (Oxford DN704). A quartz tube in a quartz Dewar bottle was used for the photolysis in MeOH-EtOH glass at 77 K. For the gas-phase photolysis at 373 K, a quartz cell (600 mL) containing 400 mg of 1 was used. It was heated to 373 K after degassing at 77 K. The IR spectra of 1 prior to and following irradiation at 254 nm (3 mW/cm2) for 6 h were recorded with a Perkin-Elmer 521 grating IR spectrometer. The IR sample was prepared by melting benzotriazole powder between the NaCl plates in a sandwich-like structure. Low-pressure mercury lamps with a Vycor glass filter or a 254-nm interference filter and a medium-pressure mercury lamp with a Corning glass 5-61 filter were used for the 254- and 420-nm radiation sources, respectively. After irradiation at 254 nm for 1 h the photolyzed MeOH-EtOH glass of 1 (0.05 M) at 77 K was warmed to room temperature, and then the solution was irradiated again at 254 nm after cooling it at 77 K. This procedure was repeated three times. After the photolyzed solution were warmed, the solvent was evaporated completely and the residue was analyzed by mass spectroscopy (Hitachi-Perkin-Elmer Model RMS-4). For identification of photoproducts, the condensed photolyzed solution was injected into a 4-ft Apiezon gas chromatograph column to isolate each photoproduct; the products were identified by infrared and mass spectra, and also by their retention times in the column. For the 254-nm gas-phase photolysis at 373 K, the gaseous photoproducts were introduced into the mass spectrometer directly. Thin layer silica gel chromatogram sheets were also used for separation and identification of the photoproducts.
Results and Discussion Absorption and Emission Spectra of Benzotriazole. Figure 1 shows the absorption spectrum of benzotriazole (1, ca. 1 X lo4 M) in EPA glass at 77 K. Benzotriazole has two absorption maxima at 254 and 276 nm in the UV range. Judging from the mirror image relationship between the absorption and fluorescence spectra, one finds that the latter band corresponds to the fluorescent state; the 0-0 transition energy was determined to be 4.19 (20) Shizuka, H.; Tobita, S. J . Am. Chem. SOC.1982, 104, 6919.
0 1987 American Chemical Society
7 I
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Shizuka et al.
r’
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\ I
\ - .
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1
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200
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, L
400
600
500
Wavelength/nm
i
0.2 0.1 0.0
--
-
(4) 200 K -
0.3 0, 0.2 -
-
Figure 1. Absorption and emission spectra of 1 in EPA glass at 77 K.
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t ::$-
o.2 n ”. i,
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(6) 300K -
0.3 -
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300
400
500
600
700
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1
800
Wavelengthlnm Figure 2. The absorption spectra of (a) the yellow intermediate 2 produced by the photolysis of 1 in EPA glass at 77 K at 254 nm for 30 min, (b) a blue product which was produced by warming the photolyzed glass to room temperature very quickly, and (c) the photolyzed solution which was warmed to room temperature after irradiation at 420 nm at 77 K. eV. The fluorescence quantum yield and lifetime were 0.1 (ttO.05) and 5 (f0.5) ns, respectively. The fluorescent state was assigned as the SI(*,**) state as will be discussed later. The excitation spectra, monitored at their maxima (315 nm for fluorescence and 430 nm for phosphorescence), were similar to the absorption spectrum. The energy level for the phosphorescent state was estimated to be 3.02 eV. The phosphorescence quantum yield and lifetime were 0.024 (0.003) and 4.45 (0.03) s, respectively, indicating that the phosphorescent state is TI(*,**). Primary Photochemical Reaction of Benzotriazole in EPA Glass at 77 K . When 1 in EPA glass (1.1 X 10-3M) at 77 K was irradiated at 254 nm in the presence of an interference filter, the rigid matrix changed from colorless to yellow as shown in Figure 2a. The new absorption band in the range 350-550 nm (A, = 423 nm) is due to the yellow intermediate produced by the photolysis of 1. The spectrum clearly exhibits vibrational structure. The yellow intermediate can be ascribed to the diazo compound 2 on the basis of IR spectral evidence at 77 K and the assignment of the UV spectrum, as will be described later. No ESR signal due to 2 was detected, indicating that 2 should be a nonradical species.
Wavelength/nm Figure 3. The spectral change of the yellow intermediate 2 in EPA glass at 77 K during a slow warming process to 300 K. Each temperature was kept within &2 K for 30 min.
of 1 are similar to those of acetanilide. We, therefore, assume that the photochemical bond scission of 1 upon direct excitation occurs from the SI(*,**) state. This bond scission is also classified as a-cleavage of azo compounds.14 The electronic level of the l(n,s*) state seems to be blue-shifted due to the electron-donating NH group adjacent to the diazo group, especially in polar media. When the irradiated EPA glass was warmed to room temperature very quickly, the yellow intermediate 2 in Figure 2a changed to a blue product as shown in Figure 2b. The blue product may be an azo compound produced by the reaction of the yellow azo compound 2 with the ground state of the starting material 1. It is known that the photochemical formation of azo compounds from 1 -phenylpyrazolones results from the reaction of a photochemically produced diazo compound with the ground We tried to isolate the blue product, state of phenylpyrazol~nes.~~ having ,A, = 660 nm, but this compound was thermally unstable. When the yellow glass at 77 K was warmed slowly, only a trace amount of the blue product was obtained as shown in Figure 3. In this case, the yellow diazo compound 2 was thermally decomposed to iminocyclohexadienylidene(3) and a nitrogen molecule in the warming process as shown in eq 2. The yellow intermediate
3a
1
2
This finding shows that the loss of a nitrogen molecule from 1 at 254 nm does not take place directly, but it occurs stepwise. For direct excitation, the NH-N bond dissociation of 1 is similar to that (@-bondscission) of acetanilide at 254 nm, which has been shown to originate from the SI(*,r*)state; the electronic features ~~
(21) Ellis, R. L.; Kuehnlenz, G.; Jaffe, H. H. Theor. Chim Acta 1972, 26, 131.
800
3b
2 was unstable at temperatures higher than 110 K. The intermediate 3 at 77 K has a triplet biradical structure, as shown by ESR spe~troscopy.~’ 3 is considered to be an intermediate in the Wolff rearrangementsJ6 and in the reaction of 1 with solvents.2-4,6 The yellow intermediate 2 in EPA glass at 77 K was further irradiated at X = 420 nm. The yellow color was bleached upon (22) Escande, A.; Galigne, J . L.; Lapasset, J. Acta Crystallogr.,Sect. B . 1974, B30, 1490.
(23) Shizuka, H.; Tanaka, I. Bull. Chem. SOC.Jpn. 1968, 41, 2345. (24) Shizuka, H . Bull. Chem. SOC.Jpn. 1969, 42, 52, 909. (25) Tsutsumi, K.; Takagishi, I.; Shizuka, H.; Matsui, K. J . Chem. Soc., Chem. Commun. 1976. 685.
The Journal of Physical Chemistry, Vol. 91, No. 7, 1987
Photolysis of Benzotriazole
1795
TABLE I: Mass Spectroscopy Data of the Products in the Photolyses of 1 at 254 nm in the Rigid, Liquid, and Gas Phases‘ MS re1 int temp, time, 6, mle 4, mle 5, m/e 5’, mle phase K h 91 93 123 136 MeOH-EtOH 77 3 32 100 10 6.6 (1O:l) glass MeOH, liquid 300 2 6 100 31 gasb 373 3 100 5 I
“For details, see the text. *The relative intensity of the nitrogen molecule (mle 28) was 810.
I
I
1
I
chemical conversion of 1 (less than 5%). The IR results strongly support the reaction path of eq 1-3. Photochemical Reactions of Benzotriazole in Rigid, Liquid, and Gas Phases. The photolysis of 1 at room temperature in methanol gives aniline (4) and o-anisidine (5) in low yields, as reported by Boyer and S e l ~ a r a j a n . ~
I
4
2.4
2.2 2.0 1.9 1.8 Wavenumber/103cm-’ Figure 4. The IR spectral change of 1 with and without irradiation at 254 nm: (a) before irradiation, (b) after irradiation for 6 h at 254 nm, and (c) after bleaching the yellow color by irradiation with the 420-nm light for 10 min. irradiation for 3 min as shown in Figure 2c. It is well-known that photoelimination of N z is a general reaction for azo comThe photoelimination of 2 can be expressed as
5
Similar reaction of 1 in MeOH is reported to afford 5 as the major product (70% yield) along with 4 (30% yield).6 The reason for the discrepancy in the reported yields is probably due to polymer formation which was not taken into account in the latter; our experiments showed significant polymerization. The gas-phase pyrolysis of 1 has been studied by Wentrup7s8in detail; the main .reaction is Wolff rearrangement to yield 6. (5) 6
This colorless intermediate 3 is the same as that in eq 2, which is produced by the thermal decomposition of 2. The quantum yield for photoelimination from 2 seems to be very large ( 1). These findings show that the photochemical decomposition of 1 does not occur directly from 1 to 3. The yellow intermediate 2 is decomposed photochemically or thermally. I R Measurements of Benzotriazole Prior to and Following 254-nm Irradiation at 77 K . W e tried successfully to measure the IR absorption spectrum of the yellow intermediate 2 produced by the photolysis of 1. The neat sample of 1 was irradiated at 254 nm at 77 K for 6 h in a vacuum glass cell fitted with two NaCl windows. The sample changed from colorless to yellow with time. Typical IR absorption spectra are shown in Figure 4. The new IR band at 2070 cm-I appeared upon irradiation of 1 at 254 nm at 77 K (Figure 4b). There was no absorption band at 2070 cm-’ prior to the irradiation (Figure 4a). The yellow sample was then irradiated at X = 420 nm for 10 min at 77 K, resulting in the loss of color. The IR absorption band at 2070 cm-’ completely disappeared upon irradiation at 420 nm, as shown in Figure 4c. This IR band is assigned to the stretching vibration of the diazo group of 2. Its frequency is very close to those (2040-2083 cm-I) of the vinyl azo compounds produced by the photochemical reactions of 3 H - p y r a ~ o l e s . ~N=N ~ stretching frequencies of the azo compounds are reported to be in the range 2080-2125 an-’.*’The reason there is no clear difference in the IR spectra shown in parts a and c of Figure 4 may be due to the small amount of photo-
In order to confirm the reaction pathways of 1, the photolyses of 1 in various phases at 254 nm were carried out. At first we examined the photolysis of 1 (0.05 M) in MeOH-EtOH (1O:l) glass at 77 K. The rigid matrix at 77 K was irradiated at 254 nm for 1 h and became yellow, and the matrix was melted by slowly warming to room temperature. The photolyzed sample was then cooled by liquid nitrogen and was irradiated again at 254 nm at 77 K for 1 h. This procedure was repeated three times. The solution was evaporated by the usual method. The viscous red-brown products obtained were injected into a gas chromatograph with 4-ft Apiezon column to isolate each product; the products confirmed by their IR, mass, and/or retention times were aniline, (4, m / e 93 (loo)), o-anisidine ( 5 , m / e 123 (lo)), oethoxyaniline (5’, m / e 137 (6.6)), and 1-cyanocyclopentadiene ( 6 , m / e 91 (32)). The viscous products were also fractionated in thin layer chromatograms by using chloroform as a developing solvent. Because gum formed inside the gas chromatograph column, accurate quantitative analyses were difficult but following qualitative discussions based on parent peak intensities in lo-’ A/unit, as shown by the values in parentheses next to the m / e values, may be justified; the data are summarized in Table I. The results show that 4 and 6 are the major products, judging from the high intensities of the anilino-radical cation ( m l e 92 (29)) and (C5H4)+( m / e 64 (22)) peaks, the latter produced by elimination of hydrogen cyanide from 6. The parent peak of the starting material 1 was very small ( m l e 119 (l)), and no corrections due to the formation of 6 by mass fragmentation of 1’’ were necessary. The same products were formed regardless of whether or not the 420-nm irradiation was carried out. The
(26) Palmer, G. E.; Bolton, J. R.; Arnold, D. R. J . Am. Chem. Soc. 1974, 96, 3708. (27) Ege, G.; Gilbert, K.;Hahn, B. Tetrahedron Lett. 1979, 1571.
(28) Ohashi, M.; Tsujimoto, K.; Yoshino, A.; Yonezawaw, T. Org. Mass. Spectrom. 1970, 4 , 203.
2
hu2 (420 nm)
-N1
-3
(3)
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Shizuka et al.
1796 The Journal of Physical Chemistry, Vol. 91, No. 7 , 1987
photolysis of 1 in MeOH-EtOH (1O:l) glass at 77 K can be expressed as
'
hv2(420nm) or
hv,(254nm)
MeOH-EtOH at 77 K"
-Nz
A - 3 - 4 + 5 t
cxoCzH5 +
6
(6)
NH2
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5'
e 0.2
It should be noted that in the rigid-phase photolysis of 1 at 77 K the Wolff rearrangement product 6 was obtained in a larger amount relative to that in the liquid-phase photolysis. The photolysis of 1 in MeOH at 254 nm at room temperature for 2 h gave red-brown oily products together with 4 ( m / e 93 (loo)), 5 ( m / e 123 (31)), and 6 ( m / e 91 ( 6 ) ) . The result agrees with the literature3q6except for the minor product 6. Thus, the Wolff rearrangement is involved in the photolysis of 1 in MeOH to a small extent even at room temperature. The gas-phase photolysis of 1 at 254 nm at 373 K for 3 h gave nitrogen ( m / e 28 (810)) and red-brown polymers deposited on the wall of a reaction cell along with small amounts of the Wolff rearrangement product 6 ( m / e 91 (100)) and aniline (4, m / e = 93 (5)). The photoproducts 6 and 4 (minor) are exactly the same as those in the pyrolysis of 1 as reported by Wentrup and Chow.* The absence of molecular hydrogen in the gas-phase photolysis indicates that N-H bond scission of 1 is negligibly small. The experimental results are accounted for by the following scheme: hvtl254nm)
1 -*
*
hvz(420 nm)
orA,-NZD
-. ,
200
I
300
400
500
814
Wavelengthlnm
Figure 5. The absorption spectra of 1 in EPA glass at 77 K: (a) before and (b) after irradiation at 254 nm for 10 min, and (c) the differential spectrum ((b) - (a)) which corresponds to that of the yellow intermediate 2 with the electronic transition moments.
and 33would result in population of the singlet state of 3 to some extent leading to the insertion reaction with ROH. (7)
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a
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-
--
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The bond cleavage process 1 2 originates from the lowest excited state Sl(a,a*)state as described above. Nitrogen elimination from the yellow diazo compound 2 takes place thermally or photochemically to give the singlet intermediate '3. The insertion reaction ki of the singlet carbene '3a on the 0-H bond of an alcohol R O H may result in the formation of substituted product 5 and 5' and this process should be competitive with the intersystem crossing process kiscfrom the singlet state '3 to the triplet ground state 33 of iminocyclohexadienylidene (3). The 33 species may react with hydrogen-donating solvents to yield the reduced product 4 (ka). Hydrogen atom abstraction reactions of carbenes (which may be a triplet) from hydrocarbon solvents are well-known.I0 The hydrogen abstraction involving 33a is similar to that of triplet triazinyl nitrer~e.*~ It is also assumed that the biradical 33b may react with hydrogen-donating solvents to give the reduced product.I6 However, it is indistinguishable at the present time which state is the reactive state for hydrogen atom abstraction, since they have the resonance structure 33a 33b. It is noteworthy that the ratio of 4/(5 5') in the rigid-phase photolysis at 77 K increased significantly compared to that in MeOH at room temperature and in contrast the amount of the insertion product of 5 or 5' via '3a is markedly reduced. The singlet-triplet energy separation in 33seems to be several kcal/mol and the intermediate 3 at 77 K should be a triplet. 1 was photolyzed in the glass at 77 K and then warmed to room temperature,
+
-
(29) Goka, T.; Shizuka, H.; Matsui, K. J . Org. Chem. 1978, 43, 1361.
It is postulated that for the two spin states in thermal equilibrium with intersystem crossing k , and its reverse intersystem crossing k'i, is relatively fast.I2 It is of interest that the insertion reaction product 5' is comparable to that of 5 in eq 6 in spite of the small ratio of EtOH (Me0H:EtOH = 1O:l by volume). This result indicates that the insertion reaction of the singlet carbene '3s is electrophilic; the charge density on the proper oxygen atom of EtOH is larger than that of MeOH because the electron-donating power of the ethyl group is greater than that of the methyl group. As for the Wolff rearrangement in the gas-phase pyrolysis of 1, the rearrangement product 6 is assumed to be produced via the carbene 3a although the spin state is unknown.I6 In the gas-phase photolysis of 1, the main product was 6 along with a small amount of 4 as described above. We examined the mercury-sensitized reaction of 1 in the gas phase at 254 nm at 373 K. The products were almost similar to those obtained in the gas-phase direct photolysis at 254 nm as shown in Table I. The acetone-sensitized reaction of N-benzoyltriazole gives the Wolff rearrangement product in solution.6 These facts may indicate that Wolff rearrangement occurs from the triplet carbene 33a. Considering the fact that the rearrangement product 6 increases considerably on going from solution-phase to rigid-state photolysis at 77 K the triplet state 33aseems to be the reactive state. In this case another intersystem crossing between 33a and 6 should be involved. However, the above results do not signify that there is Wolff rearrangement from the singlet carbene 13a, since the singlettriplet equilibrium of 3 may exist at room temperature as shown in eq 7 and therefore 3 may rearrange on a singlet surface. An ab initio study of simple ketene shows that the Wolff rearrangement occurs on a singlet reaction path.30 A further investigation of the electronic spin state of 3 as regards the Wolff rearrangement is needed. Absorption Bands of the Parent Molecule 1 and the Yellow Intermediate 2 . Figure 5 shows the absorption spectra in EPA glass of 1 at 77 K (a) before and (b) after irradiation at 254 nm for 20 min. The difference in absorbance between these spectra ((b) - (a)) is shown in Figure 5c. (30) Tanaka, K.; Yoshimine, M. J . Am. Chem. SOC.1980, 102, 7655.
J. Phys. Chem. 1987, 91, 1797-1802 TABLE 11: Transition Energies E and the Oscillator Strengths f for 1 and 2 by CNDO/S MO CI Calculations comcd absomtion band EIeV f character 4.19 0.0137 U H* 4.32 0.0994 H H* I1 (276-nm band) 4.79 0.0023 U H* 111 4.87 IV (254-nm band) 0.3513 H H* V 5.53 0.2471 H H* 0.3294 H H* VI 6.03 VI11 6.22 0.0001 U H*
--+
2
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0.0000
a-a*
0.4319 0.0036 0.0325 0.0829 0.2548
H
0.0000
x
VI11
1.52 3.01 3.67 4.36 4.61 4.99 5.27 5.43
0.0010
(U
IX
5.95
0.4672
H
1 I1 (423-nm band) 111
IV V VI VI1
U
'IT
H H
H* H* H*
H* H*
1797
a-a* transition (IV). M O calculations indicate that the lowest excited state is cr-a* (I). However, the cr-a* transition (I) was shifted to the blue in polar media, whereas the a-r* one (11) was shifted to the red; hence the latter becomes the lowest excited state. This assignment is consistent with the experimental data on the fluorescence of 1 in EPA glass at 77 K. These calculated results are summarized in Table 11. The calculated results of the yellow intermediate 2 are also shown in Figure 5c in comparison with the differential spectrum ((b) - (a)). By analogy, the 423-nm absorption band of 2 was assigned to the first T-T* transition (11), and the intense band below 250 nm was assigned to the fourth (VI) and/or fifth (IX) transition. There is a hidden a-u* transition at 814 nm in 2. The calculated bond population indices of the N-NH bond of 1 has the smallest value in its lowest excited singlet states S1(a,a*), indicating a primary photochemical bond scission in agreement with the above observation.
u* H*)
H*
From this differential spectrum it can be seen that two new absorption bands appeared upon irradiation at 254 nm; one broad absorption having vibrational structure at 423 nm, as stated above, and an intense absorption band below 250 nm. M O calculated results with a CNDO S/CI method 21 using X-ray analysis of the benzotriazole crystal structure22are shown in Figure 5a in comparison with the absorption spectrum. The electronic transition moments are also shown in the figure. The observed absorption spectrum is in good agreement with the calculated results. The absorption band at 276 nm was assigned to the first a-a* transition (11). The other band at 254 nm was assigned to the second
Summary Bond cleavage of the N-NH bond in the SI state results in an azo compound having an absorption band at 423 nm and an IR absorption band at 2070 cm-I. This yellow intermediate decomposes thermally or photochemically to give rise to iminocyclohexadienylidene with resonance structures of a carbene and a biradical. Mechanisms leading to the photoproducts have been discussed. Acknowledgment. We thank Dr. M. Yoshimine of IBM Research for discussion on M O calculation. Registry No. 1, 95-14-7; 2, 106627-10-5; 3a, 59046-30-9; 3b, 54260-83-2;aniline, 62-53-3;o-anisidine,90-04-0;o-ethoxyaniline,9470-2; 1 -cyanocyclopentadiene,20830-58-4.
Fluctuation Spectra of NiSO,, MgSO,, and ZnSO, at an Anion-Exchange Membrane Interface Michael E. Green* and Roseline Rodneyt Department of Chemistry, The City College of the City University of New York, New York, New York 10031 (Received: July 15, 1986; In Final Form: November 17. 1986)
The voltage fluctuation spectra of 0.0175 and 0.023 M ZnS04, MgSO,, and NiS0, have been studied at an anion-exchange membrane, in the frequency range 500 Hz to 10 kHz. The constant currents used were 0.9-1.5 times the critical current density (CCD) at the membrane, with the data analyzed at 1.2-, 1.3-, and 1.5CCD. The essential features of the spectra include a straight-line portion below 2.3 kHz, with slope on a log power-log frequency plot increasing with current. In the higher frequency range, where noise above background was weak, there is a spectrum resembling diffusion noise in those cases where sufficient noise exists for quantitative data to be obtained. ZnS04 spectra are weaker than NiS04 spectra; this may be understood as a consequence of the differences in rates of ionic association reactions. The results are interpreted, overall, in terms of a neutral depleted layer outside the double layer at the membrane surface. Pulses due to an instability in this layer are responsible for the observed noise. Constraints on the model limit the possible dimensions and electric fields in the layer. A typical set of consistent values would be E = 1 X V m-l, thickness 200 nm, concentration 6 X 10" M, and pulse duration 2 X s. No alternate mechanism appears likely to be able to account for the observed fluctuation spectra.
Introduction The transport noise of ion-exchange (electrodialysis) membranes has been studied with various ionic solution^.^-^ We now report the spectra of ion at an anion-exchange membrane, with three different cations, Zn2+,Mg2+,and Ni2+. Differences in the ionic association rate of the cations with S042-were expected to produce differences in the power spectra of the fluctuations.
All previous work with ion-exchange membranes has shown that the source of the electrical noise is the membrane-solution interface. Only counterions (to the membrane's fixed charge) can enter the membrane; the number of ions required to carry the imposed current becomes greater than the number which can be supplied by diffusion and by drift in the presence of the field. At sufficiently high currents the interface region becomes largely
*To whom correspondence should be addressed. 'Current address: Temple University Medical School, Philadelphia, PA 19122.
(1) Stern, S. H.; Green, M. E. J . Phys.Chem.1973,77,1567. (2) Fang, Y.; Li, Q.; Green, M. E. J. Colloid Interface Sci. 1982,88, 214. (3) Fang, Y . ;Li, Q.; Green, M. E. J. Colloid Interface Sci.1982,86, 185.
0022-3654/87/2091-1797$01.50/0
0 1987 American Chemical Society
