5972
J . Phys. Chem. 1988, 92, 5972-5977
the case of a very weakly interacting metal-molecule system the interfacial barrier must give rise to a lifetime for the chargetransfer state that is long enough for a fast chemical step to take place. Evidence for charge transfer is the similarity of the frequency dependence of the photolysis and the SERS intensity profile shown in Figure 10. At a constant potential the photolysis rate reaches a maximum at nearly the same excitation frequency as the SERS intensity. From the potential dependence of the photolysis rate we cannot observe a resonance profile as seen in the SERS spectra since at around -0.5 V PNBA is electrochemically r e d u d . Thus, the present experiments cannot distinguish between models (i) and (ii). However, the increase in photolysis rate that occurs before the electrochemical reduction (-0.1 to -0.3 V) is evidence of a charge-transfer mechanism. The change of the potential in the negative direction will raise the Fermi level and result in the decrease of the charge-transfer threshold or to the barrier to tunneling. Thus a higher photolysis rate will be observed at more negative potentials. As a matter of fact, there are many other damping processes that will also be enhanced by the surface. The reason why onitrobenzoic acid or m-nitrobenzoic acid does not show photochemical reduction under the same conditions as in the case of PNBA might be attributed to the competition of the chemical step with other damping processes. Since adsorbed PNBA has its nitro group pointing out, it may be that PNBA is in a more favorable position to react with the solvent than o-nitrobenzoic acid or m-nitrobenzoic acid. On the other hand, reactivity differences between isomers might also be due to differences in
charge-transfer interactions. These differences would be expected to be especially pronounced for interactions through an extended 7r system as in the present surface complex. In conclusion, the change of the SERS spectra of PNBA on Ag island films and Ag electrode surfaces is attributed to a surface-induced photochemical reduction. Heating the sample does not cause the conversion of the spectra nor is the reaction rate vs laser power consistent with a reaction that is thermally induced. The potential dependence and frequency dependence of the photolysis rate also indicate that the change of the spectra is not thermally induced. It is found that both the rough metal surface and excitation with laser light is necessary for the surface-induced photochemical reaction. The reduction takes place at much more positive electrode potentials than a simple electrochemical reduction and with a much lower excitation energy (visible excitation) than a simple photochemical reduction which requires UV excitation. It is possible that the reaction product is either PABA or an azo compound or both. Although the molecular details of the mechanism require further study, it is clear that a charge-transfer mechanism may explain the potential and frequency dependence of the photolysis rate. Important clues to the mechanism may come if related photochemistry can be found and studied for other adsorbed molecules.
Acknowledgment. R.L.B. and J.R.L. are indebted to the PSC-BHE Research award program of the City University of New York (RF666367 and RF667261) and the National Science Foundation (CHE-87 11638) for financial assistance. Registry No. PNBA, 62-23-7; Ag, 7440-22-4.
Kinetics of the Reactions of C,H,S with NO2, NO, and 0, at 296 K Graham Black,* Leonard E. Jusinski, Chemical Physics Laboratory, SRI International, Menlo Park, California 94025
and Roger Patrick LSI Logic, Santa Clara, California 95050 (Received: February 1 , 1988; In Final Form: April 28, 1988)
The laser-induced fluorescence technique has been used to study the reactions of the ethylthio radical (C2H5S)with NO, NO,, and O2 at 296 K. The reaction of CzH5Swith NO involves a third body and has been shown to be in the transition region between the low- and high-pressure limits. Using an expression developed by Troe to fit the results, we have obtained values of the broadening factor F = 0.49foqE, k , = (5.2 f 0.5) X lo-" cm3 molecule-' s-l, ko(He,Ar) = (75.);: X cm6 molecule-2s-I, and ko(SF,) = (2.22,:) X cm6 molecule-z s-'. The reaction with NO2 has a rate coefficient of (9.2 & 0.9) X lo-" cm3 molecule-' s-I. No reaction could be found with 01,for which an upper limit on the rate coefficient cm3 molecule-' s-' was estimated. of 2 X
Introduction Alkylthio radicals (RS) are important intermediates in the reaction of reduced sulfur compounds (RSH, R S R , RSSR) with O H radicals in the atmosphere. They are also important intermediates in the photolysis and hydrogen atom reactions of these compounds. Although these compounds are minor constituents, they may play a part in the atmospheric sulfur cycle and contribute to the acid precipitation problem. It is, therefore, important to understand the atmospheric chemistry of these radicals. Until recently, the only reported absolute rate coefficients involved the HS radical.'-7 Recently, laser-induced fluorescence (1) Black, G. J . Chem. Phys. 1984, 80, 1103. (2) Black, G.; Patrick, R.; Jusinski, L. E.; Slanger, T. G. J . Chem. Phys. 1984, 80, 4065. (3) Bulatov, V. P.; Kozliner, M. J.; Sarkisov, 0. M. Khim. Fiz. 1984, 3, 1300.
0022-3654/88/2092-5972$01.50/0
(LIF) spectra of CH3S were reported.8 This has led to the first measurements9J0 of the rate coefficients for the reactions of CH3S with NO, NO,, 02,and 0,. Very recently, LIF spectra of C2H5S were reported." Using the L I F technique, we report the first (4) Friedl, R. R.; Brune, W. H.; Anderson, J. G. J. Phys. Chem. 1985,89,
5505.
( 5 ) Schonle, G.; Rahman, M. M.; Schindler, R. N. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 66. ( 6 ) Stachnik, R. A.; Molina, M. J. J . Phys. Chem. 1987, 91, 4603. (7) Wang, N. S.; Lovejoy, E. R.; Howard, C. J. J . Phys. Chem. 1987,91, 5743. ( 8 ) Susuki, M.; Inoue, G.; Akimoto, H. J . Chem. Phys. 1984,81, 5405. (9) Balla, R. J.; Nelson, H. H.; McDonald, J. R. Chem. Phys. 1986, 109, 101.
(IO) Black, G.; Jusinski, L. E. J . Chem. SOC.,Faraday Trans. 2 1986,82, 2143. ( 1 1 ) Black, G.; Jusinski, L. E. Chem. Phys. Lett. 1987, 136, 241
0 1988 American Chemical Society
-Keactions . of
* ^
~
--2 -~ with .. .I5 3 NU^, N U , ana u2 3
.T-
A
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5973
direct measurements of the rate coefficients for the reactions of C2H5Swith NO2, NO, and O2 at 296 K.
Experimental Section The apparatus has been described previously.' Briefly, an excimer laser was used to generate ground-state C2H5Sradicals (diethyl disulfide, DEDS) or by the photolysis of C2H5SSC2H5 C2H5SH (ethyl mercaptan, EM) at 248 nm. Small (6-mm diameter) apertures, both inside and outside the photolysis cell, reduced scattered light and restricted the excimer beam to the center of the cell. Transmitted 248-nm energy was in the 1-10-mJ range for most of the experiments. The Quanta-Ray Nd:YAG dye laser, using Exciton LDS 821, provided =1-6 mJ ?f doubled light at 390-420 nm for exciting fluorescence on the A2Al-R2E transition of C2HsS in a beam of ~ 2 - m mdiameter. Almost all the measurements used excitation at 410.3 nm (tentatively assignedll as 3:). The dye laser beam was propagated through the cell along the center of the larger excimer beam but in the opposite direction. The lasers were operated at 10 Hz, and a variable delay could be introduced between them to follow the decay of the C2HsS radical. Experiments were performed under pseudo-first-order conditions. In the absence of any reactant, the C2H5Sradicals decayed via self-reaction (combination or disproportionation) and diffusion. When reactants were present in sufficient concentration, the decay of the C2H5Sradicals could be fitted by a single exponential over 2-6 lifetimes. The detection system consisted of a Heath EU-700, 0.35-m monochromator equipped with an RCA C3 1034A photomultiplier. For most of the experiments, the monochromator was operated in zero order and filters (Wratten 2E and a short-pass filter) were used to block laser-scattered light and transmit fluorescence in the 420-680-nm range. For measurements with the lowest concentrations of EM and DEDS, the monochromator was removed and the photomultiplier and filters used directly. The photomultiplier output was fed via a fast lOO-MHz, gain 100 amplifier (Pacific Photometrics Model 2A50) to a boxcar averager and then to a chart recorder. The DEDS (Pfaltz and Bauer, 97%) and E M (Aldrich, 97%) were degassed by several freeze-pumpthaw cycles and then used alone or as mixtures in argon. In view of the low vapor pressure of DEDS ( ~ 1 . 1X 10" molecules cm-3 at 296 K), it was used as a 1:1260 mixture in argon. Most of the experiments used E M (vapor pressure = 1.5 X lOI9 molecules cm-3 at 296 K) as a 1:lO mixture in argon. The other gases used were supplied by Matheson Gas Products with purity 99.5% mininum and were used without further purification. The NO2 was a 1% mixture in helium. The NO was used either pure (CP Grade; 99% minimum) or as 5 or !O% mixtures in argon. The gases passed through flowmeters and were mixed prior to entering the photolysis cell. The cell was equipped with MKS pressure gauges and was evacuated with a small rotary pump which gave .=2 s for the residence time of the gas in the cell. Results and Discussion Although both DEDS and EM produce C2HSSradicals at 248 nm C2H5SSC2H5+ hv (248 nm) 2C2H5S (1) C2H5SH
+ hv (248 nm)
-
C2H5S+ H
(2)
other photodissociation channels can occur. In studies of E M at 254 nm, two studies12J3have found evidence for C-S bond rupture with a quantum yield of ==O.l. C2H5SH hv (254 nm) C2H5 S H (3)
+
-
+
The H , C2H5, and S H produced in (2) and (3) can give rise to secondary production of C2HsS radicals by the reactions (12) Dzantiev, B. G.;Shiskov, A. V.; Unukovich, M. S. Khim. Vys. Energ. 1969, 3, 111 .
(13) Bridges, L.; Hemphill, G . L.; White, J. M. J. Phys. Chem. 1972, 76, 2668.
H
-
+ C2H5SH
C2H5
SH
C2H5S+ H2
+ CzHsSH
+ C2H5SH
CzHsS
AH = -12 kcal mol-'
+ C2H6
C2HsS + H2S
(4)
AH = -6 kcal mol-] (5) AH = 0 kcal mol-] (6)
Of these reactions, reaction 4 is likely to be the most important because of the large yield ( ~ 0 . 9 of ) H atoms. In the previous work, this reaction was used to explain the large quantum yield for H2 production. There does not appear to be a determination of the absolute rate coefficient for reaction 4. However, previous relative rate coefficient studiesI4 can be used to determine k 4 / k 7 = 0.85 f 0.14 where k7 refers to the analogous reaction of H atoms with CH3SH. H + CH3SH CH3S + H2 (7) --+
When this ratio is combined with the recently determinedI5 value of k7 = 2.0 X cm3 rnoleculed s-' at 296 K, k4 = 1.7 X cm3 molecule-' s-I. The similar rates should not be surprising since D(CH3S-H) and D(C2H5S-H) are approximately equal.16 In the absence of added reactants, a long-lived component to the decay of the C2H5Sradicals in E M was observed and thought to originate from reaction 4. If the analogy with (CH3S), at 254 nm17 holds, then photodissociation of DEDS at 248 nm should proceed by reaction 1 and by a clean source of C2HsSradicals. However, at 248 nm (1 15 kcal mol-'), there is more than sufficient energy for complete dissociation to C2H5 and S2 DEDS
+ hv (248 nm)
-
2C2H5 + S2
(8)
which requiresI6 98.5 f 3 kcal mol-'. Although secondary reactions involving the C2H5 radicals can then occur, this chemistry is less likely to regenerate C2HSSradicals. Both E M and DEDS were used as sources of C2HSSradicals in these studies, and the E M concentration was varied by ~ 2 0 , to be sure that the results obtained were the same and presumably, therefore, not affected by the secondary chemistry that can occur in these systems. The photodissociation process (1) is exothermic by 41 kcal mol-' and process (2) by 23 kcal mol-'. It has been shown'* that most of the excess energy in (2) appears as translational energy of the H atom and little is left in internal excitation of the C2H5Sradical. In contrast, in ( l ) , after subtracting translational energy, the rest of the excess energy must be divided between the two C2HsS radicals that are produced. The excess energy in (1) manifests itself as a longer time for the C2H5Sradicals to relax into the ground vibrational level and the LIF signal from the ground state to appear. In fact, this limited the use of DEDS at low argon pressures when the internal relaxation took > 10 I.LS and was noticeably slower than the appearance time of the LIF signal when EM was used. C2HsS + NO2. For these measurements, the EM/DEDS concentration was (0.8-16) X loi4molecules ~ m - and ~ , the NO2 concentration was varied over the range (0-11) X lOI5 molecules ~ m - ~Most . measurements were made under conditions where the initial C2HSSradical concentration was 10l2molecules or less (calculated from the EM/DEDS concentrations, the measured absorption cross sections," and the laser flux at 248 nm). Initial measurements indicated that the NO2 fluorescence excited by the dye laser at 410.3 nm was long-lived (at low buffer-gas pressures) and, because it occurs in the same region as the C2HSSfluorescence, interfered with the measurements of the C2H5Sradical decay. This problem was solved by working at a high buffer-gas concentration ([Ar] = 1.3 X 10'' molecules ~ m - ~which ), shortened the NOz fluorescence lifetime to 2 2 ns (14) Steer, R. P.; Knight, A. R. Can. J . Chem. 1969, 47, 1335. (15) Wine, P. H.; Nicovich, J. M.; Hynes, A. J.; Wells, J. R. J . Phys. Chem. 1986, 90, 4033. (16) Benson, S. W. Chem. Reu. 1978, 78, 23. (17) Balla, R. J.; Heicklen, J. Can. J . Chem. 1984, 62, 162. (18) White, J. M.; Johnson, R. L.; Bacon, D. J . Chem. Phys. 1970, 52, 5212.
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The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
Black et al. 10
[NO2]- 4 . 2 ~ 1 moleCUle 0 ~ ~ Cm3 d) [NOp]. 1 . 0 6 ~ 1 molecule 0 ~ ~ cm3
C)
8
-- 6 --s v)
In
,
4
2
4
0
8
12
16
20
0
TIME (10-es)
Figure 1. C2HsSradical decay vs time after CzHSSHphotodissociation at 248 nm in the presence of various NOz additions. [C2H5SH]= 8 X 10') molecules in [Ar] = 1.3 X I O l 9 molecules ~ r n - ~ . (the quenching rate coefficient measured for argon wasp.4 X lo-'' cm3 molecule-I s-') but does not quench the C2H5SA2Al emission." While removing the interference from NO2 fluorescence, this solution did prevent measurements of the rate coefficient at low buffer-gas pressures. Such measurements would be required to detect a three-body reaction, since at the high argon pressure used to suppress the intereference from NO2 fluorescence, a three-body reaction would likely be close to its high-pressure limit and, therefore, insensitive to small variations in the argon pressure. Some measurements of the C2HSSradical decay with increasing NO2 addition are shown in Figure 1, and the decay rates from these and other measurements are shown in Figure 2. The slope of the line in Figure 2 gives (9.2 A 0.9) X lo-" cm3 molecule-' s-l for the removal of C2HSSby NO2. This can be compared to similarly large values for NO2 reaction with H S (values cover the range (0.24-1.2) X lo-'' cm3 molecule-' s - ' ) ] , ~ - ~and with CH3S (1.09 X 10-l' cm3 molecule-' S-I).~ Because of the large rate coefficient observed, and because the extent of NO2 photodissociation at 248 nm is
