J. Phys. Chem. 1992, 96,696-701
6%
For individual species, some examples of increased rate constants on profiles are given in Figure 6 for the WZ mechanism. The three species 0, H (Figure 6a), and OH are dominated by the reaction H + O2 M and have very similar sensitivities. As seen, an increase in the rate constant of a factor of 5 of that reaction produces lower H and 0 maxima improving the agreement with those from the experiments. Note that T a n g and Hampson in their review34recommend a rate constant of a factor of 2.5 higher than the value used by Warnatz at lo00 K. For 02, CO, and C 0 2 (Figure 6b), the reaction CO O H dominates, complemented especially by an influence of the reaction H O2 M. The Reaction Rate of CO OH. All the mechanisms give a slower conversion of CO to C 0 2compared with the experimental results. This is also supported by studies of flame 2. An increase in the rate constant for the reaction CO OH by about a factor of 1.25-1.5 at lo00 K,depending on the specific mechanism, gave an improved fit between the computed and the experimental profiles for C02. This means that a rate constant of about 2.5 X 10" cm3/(mol.s) is appropriate for that reaction at 1000 K. The corresponding rate constants for this reaction at lo00 K given by the DL, DN, MB, SD, WD, and WZ mechanisms are 1.75, 1.75, 1.75 2.02, 1.51, and 2.02 X 10" cm3/(mol.s). The determinations8J'*12 by the Louvain group for flames 1,2, and 3 gave values of 1.32, 1.34, and 1.75 X 10" cm3/(mol.s).
+
+ +
+ +
+
(34) Tsang, W.; Hampon, R. F. J. Phys. Chem. Ref.Dura 1986, 1087.
Conclusions All the mechanisms give a slower conversion of CO to C 0 2 compared to the experimental results. The computations, especially the ML mechanism but also the WZ and MB mechanisms, yield significantly higher values, up to a factor 2, for H atoms and 0 atoms than do the experimental results. Computations with the WZ and MB mechanisms produced species profiles almost identical to within 5% of each other with the modified WD mechanism results typically differing from these by 15%. For 02, CO, and C02, the ML mechanism produced profiles similar to the WD mechanism. The profiles from the DL and DN mechanisms were closer to the W Z and MB mechanisms for most of the species. A sensitivity analysis showed that the dominating reaction for the system, and especially for the major species profdes 02,CO, and C02, is CO OH. The reaction H + O2 M also has a significant influence, especially for the minor species.
+
+
Acknowledgment. This work was supported financially by the Swedish National Energy Administration, the Swedish National Board for Technological Development, the Sedish National Research Council, and the Volvo Foundation for Research and Education. We would also like to thank Owe Anderson for assistance with computer programming and Michael Tanoff for discussions about the manuscript. Reglptry NO. CO, 630-08-0; HZ,1333-74-0.
Kinetlcs and Mechanisms of the Reactions of CH2SH Radicals with 02,NO, and NO2 Christopher Anastasi,* Mark Broomfield, Department of Chemistry, University of York, Heslington. York. YO1 SDD, U.K.
Ole John Nielsen, and Palle Pagsberg Chemical Reactivity Group, Rise National Laboratory, DK-4000, Roskilde, Denmark (Received: January 8, 1991; In Final Form: July 30, 1991)
The pulse radiolysis/kinetic absorption technique has been used to study the reactions of CH2SH with 02,NO, and NOz. 1.9 X lo-'', and 3.5 X lo-'' cm3molecule-' s-I, respectively, were obtained at 1 atm and 298 Rate constants of 8.5 X K. Absorption spectra due to product species indicated that the reaction with Oz proceeds via addition. There is evidence to suggest that addition is also the pathway for the NO and NO2 reactions.
1. Introduction
Much recent interest has centered on the role of natural organic sulfur compounds in the chemistry of the atmosphere.' On a global scale, the anthropogenic and biogenic sources of atmospheric sulfur are comparable. However, the natural sources are primarily the reduced sulfur compounds dimethyl sulfide, dimethyl disulfide, and methyl mercaptan, rather than SO2, the predominant industrial emission. CH3S, and possibly CHzSH, are important intermediates in the oxidation processes of these naturally occurring sulfur compounds in the atmosphere. However, there is considerable doubt concerning the detailed oxidation routes of these specie^^-^ and also regarding their interaction with (1) Andreae, M.0.;et al. J . Geophys. Res. 1985, 90,12891. (2) Yin, F.; Grcsjean, D.; Seinfeld, J. H . J. Arm. Chem. 1990, 1 1 , 309. (3) Wallington, T. J.; Atkinson, R.; Winer, A. M.; Pitts, J. N. J . Phys. Chem. 1986. 90,5393. (4) Barnes, I.; et al. In Biogenic Sulfur in the Environment; Saltzmann, E., Cooper, W., Eds.; ACS Symposium Series no. 393; American Chemical Society: Washington, DC, 1989; p 464. (5) Daykin, E. P.; Wine, P. H. J . Geophys. Res. 1990, 95, 18547. (6) Balla, R. J.; Nelson, H. H.; McDonald, J. R. Chem. Phys. 1986, 109, 101.
There is currently only an upper limit for the rate constant for the reaction of CH3S with 02,'which may be a significant atmospheric process. Although there have been several kinetic studies performed on oxidation systems involving CH3S, mostly using laser-induced fluorescence to detect the radical7**or IR spectroscopy to monitor the reaction p r o d u ~ t s , ~there J ~ are no reported measurements of CH2SH reaction kinetics. Studies of CH2SH would also yield useful 'insights into the kinetics and reaction pathways of the related species CH3SCH2,which is an important intermediate in the atmospheric oxidation of dimethyl disulfide. We have recently reported studies on the ultraviolet (UV) absorption spectrum and self- and cross-reaction kinetics of CH3S and CHzSH radicals using the pulse radiolysis technique." A composite absorption spectrum was obtained between 21 5 and (7) Tyndall, G. S.;Ravishankara, A. R. J. Phys. Chem. 1989, 93,2429. ( 8 ) Black, G.; Jusinsky, L. E. J . Chem. Soc., Faraday Tram. 11 1986,82, 2143. (9) Hatakeyama, S.; Akimoto, H. J . Phys. Chem. 1983, 87, 2387. (10) Jacox, M. E. Can. J . Chem. 1983,61, 1036. ( 1 1) Anastasi, C.; Broomfield, M.;Nielsen, 0. J.; Pagsberg, P. Chem. Phys. Lerr. 1991, 182, 643.
0022-365419212096-696$03.00/00 1992 American Chemical Society
I
I
,
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 697
Reactions of CH2SH Radicals with 02,NO, and NOz
OJO
1 OS5
A, = 0.081 A, = 0.033
1
0.10
0.05
t I
250
200
300
350 wavelength (nm)
....................................................................
Figure 1. CH,S/CH2SH composite absorption spectrum (see ref 1 I).
380 nm, which is reproduced in Figure 1. The region above 240 nm was assigned to CH2SH and the region below 240 nm was tentatively assigned to CH3Sradical absorption, except for a small peak at 220 nm which may be due to CHzSH absorption. Rate constants were obtained for reactions 1-3 (quoted in units of cm3 molecule-' SI). CH3S CHzSH CH3S
--
+ CH3S
+ CH,SH
+ CH,SH
products
kl = (4 f 2)
products
kz = (7 & 1) k3 = (10 f 3)
products
X
X X
lo-" (1) lo-"
(2)
lo-"
(3)
The observed absorption spectrum" is used in the present work to study the reaction kinetics of CHzSH radicals with Oz,NO, and NO,: CH2SH + 0, products (4) CH2SH CH2SH
+ NO
-
+ NOz
-
products
(5)
products
(6)
Under our reaction conditions, these reactions occur over time scales of between 5 and 30 y,while stable products are monitored at longer times (typically 40-100 ps). The identification of these products is discussed, and some consideration is given to the likely reaction pathways. 2. Experimental Section The pulse radiolysis/kinetic absorption apparatus has been described in detail previously.l2 Briefly, mixtures of CH3SH and SF6 are made up in a 1-L stainless steel reaction vessel, with 02, NO, or NO, as required. The vessel is irradiated with a 30-ns pulse of 2 MeV electrons from a field emission accelerator (Febetron 705B), and the decay of radicals is monitored by UV absorption. The analyzing light from a pulsed Xe lamp passes through the reaction vessel and is reflected by intemal aluminized mirrors to obtain path lengths of 40, 80, or 120 cm. Spectral discrimination is achieved with a 1-m grating monochromator (Hilger and Watts), and the light intensity is monitored with a photomultiplier (Hamamatsu). The signal is recorded on a transient digitizer (Biomation 8100) and transferred to a PDP-11 minicomputer for storage and analysis. Gas mixtures were prepared by admitting one component at a time into the reaction cell, and all mixtures were made up to 1 atm total pressure. The experiments were performed at 298 f 2 K. The radiolysis of SF, is a source of F atoms in which process 7 dominates, and reaction 8 has a negligible effect.13 The CH2SH
+ 2 MeV eSF6 + 2 MeV e-
SF6
-
+
+ 2F + eSF5 + F + e-
SF4
(7)
10
20
30
time @sed Figure 2. Experimental decay trace (with fit from full model superimposed) following radiolysis of a mixture containing 1 mbar of CH3SH, 2 mbar of 02,and 1 bar of SF, at 270 nm (298 K).
radical was generated by the reaction of fluorine atoms with methyl mercaptan (reactions 9 and 10).
F + CH3SH
CH3S
+ CH3SH
CHZSH
F
+
+ HF
+ HF
(9) (10)
A high concentration of methyl mercaptan was desirable to ensure rapid and complete reaction of the fluorine atoms accordii to reactions 9 and 10. Methyl mercaptan absorbs strongly in part of our experimental region,14 which limited our concentrations to 1 mbar in experiments conducted at wavelengths below 270 nm. (1 mbar is equivalent to 2.5 X 10l6molecule cm-3 at 298 K.) It was also necessary to utilize the maximum p i b l e fluorine atom yield because of the low absorption cross-section of the radicals and hence the poor radical absorption signal-tenoise ratio. Balancing these demands to give the best signal yielded optimum experimental conditions of 1 mbar of CH3SH with up to 5 mbar of 02,NO, or NO2,in 1 bar of SF6. Absolute concentrations of fluorine atoms were determined by reaction with an excess of methane as previously des~ribed.'~The above conditions yielded an initial total radical density of 2.7 X 1015molecule ~ m - ~Ex. periments were carried out in the absence of CH3SH to ensure that the reactions of F atoms with 02,NO, or NO2 to form the corresponding adducts FOZ,FNO, and FN02 did not affect the observed spectra or kinetics under these experimental conditions. SF4 is a stable species with a very low absorption cross-section and does not affect our analysis. SF5and SF50zinfluence neither the observed spectra nor the chemistry on a 1Wps time scale.13J6
3. Materials High-purity, commercially available Ar and SF6 (AGA) and CH3SH, O,,NO, and NO2 (Matheson) gases were used directly from cylinders without further purification. 4. Results and Analysis 4.1. Reaction of CHzSHwith Oz. A set of experiments was performed in which mixtures of 1 mbar of CH3SH, 4 mbar of 02,and 1 bar of SF6 were radiolyzed, Figure 2 shows a typical
decay trace. The peak absorptions and decay kinetics were monitored at a range of wavelengths between 220 and 380 nm. This data was used firstly to obtain the ultraviolet absorption
(8)
(12) Pagsberg, P. B.; Eriksen, J.; Christansen, H. C. J. Phys. Chem. 1979, 83, 582. (13) Anastasi, C.; Muir, D. J.; Simpson, V. J.; Pagsberg, P. J. Phys. Chem. 1991, 95, 5791.
0
(14) Calvert, J. G.; Pitts, J. N . Photochemistry; Wiley: New York, 1966. (15) Pagsberg, P.; Sillesen, A.; Munk, J.; Anastasi, C. Chem. Phys. Lett. 1988, 146, 375. (16) Pagsberg, P.; Ratajczak, E.; Sillesen, A.; Jodkowski, J. T. Chem. Phys. Lerr. 1987, 141, 88.
Anastasi et al.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 1.2 1.0
0.8
0.6 0.4
0.00 200
0.2 250
350
300
wavelength (am) Figure 3. Residual absorption spectrum produced 100 after radiolysis of a mixture containing 1 mbar of CH3SH,4 mbar of 02,and 1 bar of SF6 (298 K) (absorption due to products of reactions 1, 2, and 3 subtracted).
0.0
2.0
1.0
3.0
additive concentration (mbar)
Figure 4. Plot of the first-order rate constant as a function of concen02, ( - * O - - ) NO, (-+-) N02. tration of additive (298 K): (-O-)
TABLE I: Results Obtained from Two Anaivsis Methods'
--
reaction CH2SH + O2 products (4) CH2SH + NO products (5) CH2SH + NO2 products (6)
rate constant (mixed-order analysis) (10-" cm3mokcule-' s-l)
rate constant (full trace analysis) (lo-" cm3molecule-l s-I) 0.85 f 0.10 1.5 f 0.2 3.8 f 1.0
0.79 f 0.40 1.6f 0.7 1.4f 0.6
'Note: The errors quoted for the mixed-order analysis are two standard deviations, derived from a linear least-squaresfit to the data. The errors quoted for the full trace analysis span the values extracted from the decay traces. spectrum of the oxygenated reaction products by monitoring the residual absorption after 100 fis. The concentrations of the products of reactions 1,2, and 3 were obtained by modeling the system, and their absorption cross-sections were deduced in our earlier work;" subtracting the contribution from these products to the residual absorption yields the absorption spectrum shown in Figure 3. Secondly, a rate constant for the reaction of CHzSH with O2 was obtained to corroborate a second series of measurements. In this subsequent series of experiments, the concentration of Ozwas varied in the range 0.5-3 mbar, and the effect on CH2SHkinetics was monitored at 270 nm. At this wavelength, the most favorable balance between a strong radical absorption and a weaker product absorption was obtained, which gives the best signal-to-noise ratio over the decay part of the trace. Analysis of the results was hindered by a high residual product absorption, as shown in Figure 2, effectively reducing the signal-to-noise ratio. The data analysis method was designed to take these absorbing products into account, as well as the presence of two radical species and a large second-order component in some of the decay traces caused by the high concentration of radicals which we were obliged to use. The experimental data from these experiments were analyzed in two ways. Firstly, the decay traces were fitted to a model with first- and second-order processes, containing the following reactions:
-
CHzSH + CH2SH CH2SH + O2
products
products
(2) (4)
In excess oxygen, CH2SH is removed via reaction 4 in a pseudo-first-order fashion. Thus, we can use a pseudo-first-order rate constant to describe this process, defined by k4/ = k4[02]. The residual absorption was subtracted from the measured absorption, and the pseudo-first-order rate constant was adjusted to give the best fit of the predicted absorption decay to the experimental decay trace. The rate constants obtained in this way were plotted against the concentration of oxygen, and a least-squares fit gave a value of (0.79 f 0.40)X 10-l'cm3 molecule-' s-l for k4,where the error quoted represents two standard deviations. The pseudo-first-order plot is shown in Figure 4 and the results summarized in Table I. The fitted lines do not pass through the origin because of the
TABLE II: Kinetic and Spectroscopic Data Used in Full Trace Analysis rate constant
----
(cm3 molecule-' s-I) ref 4 x 10-11 11 7 x 10-11 11 1 x 10-11 11 8.5 X lo-" this work 1.9 X lo-" this work 3.5 x 10-11 this work I x 10-11 11 2.1 x 10-11 11 2.9 X lo-" 6 6.1 X lo-" 2 3.0 X lo-" 2 absorption wavelength cross-section suecies (nm) (cm2 molecule-I) ref CHZSH 270 1.4X 11 CH2SH 290 8X 11 9 x 1049 products (from 1, 2, and 3) 270 11 7 x 10-19 products (from 1, 2, and 3) 290 11 products (from 4) 270 4.2 x 10-19 see text 1.1 x 10-18 products (from 5) 270 see text products (from 6) 270 9 x 10-19 see text products (from 6) 290 3.7 x 1049 see text 9.6 X CH$H 270 14 14 CH3SH 290 3.8 X 1.7 x 1049 CHjSNO 270 18
reaction 1. CH3S + CH3S products 2. CH2SH + CH2SH products 3. CH3S + CH2SH products 4.CH2SH + O2 products 5. CH2SH + NO products 6. CH,SH + NO2 products 9. F + CH3SH HF + CH3S 10. F + CH3SH HF + CH2SH 13. CH3S + NO CHjSNO 14.CHjS + NO2 CH3SO + NO 15. CHqSO + NO2 CHISO, + NO
--
--
-+
influence of reaction 3 removing CHzSH radicals. There is also an effect due to the concentration of CH2SH radicals a t the absorption peak being less than the fluorine atom yield because reaction of the radicals has already proceeded to an appreciable extent. This results in an overestimation of the importance of reaction 2, particularly a t higher O2concentrations, which tends to lower the estimate of k4 obtained using this method. This analysis does not take into account the details of absorption which is due to the products of reactions 1-4 and therefore has a complex time evolution. Also, other radical-radical reactions such as reactions 1 and 3 are not considered; it nevertheless serves as a first estimate of the derived rate constants. The second data analysis procedure was designed to include other reactions that may influence the observed decay and also the detailed contribution to the background absorption by the
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 699
Reactions of CH2SH Radicals with 02,NO, and NO2 various reaction products. A complete model of the reaction scheme was set up as follows, using rate data summarized in Table 11: F CH3SH -+ HF + CH3S (9)
+
F + CH3SH
-+
HF
+ CH2SH
(10)
CH3S + CH3S -,products
(1)
CH2SH + CH2SH -,products
(2)
--
CH3S + CH2SH CH2SH
+ O2
products
products
(3)
(4)
The reaction of CH3Swith O2is too slow to be significant under our reaction condition^.^ Rate constants for reactions 1, 2,3, 9, and 10 were taken from our earlier work.'' Absorption crosssections for the various absorbing species are shown in Table I1 and were obtained by using the literature value for methyl mercaptanI4 and once again taking values for CH2SH and the products of reactions 1,2,and 3 from our earlier work." In order to model the time evolution of the absorption, an estimate is required for the absorption cross-sectionof the products of reaction 4. The final concentration of these products was estimated by running the above model, using the rate constant for reaction 4 deduced from the simple mixed-order analysis above. Their contribution to the final absorption was calculated by subtracting the contributions to the experimental final absorption due to the products of reactions 1-3 and taking into account the consumption of methyl mercaptan. The remaining absorption was assumed to be due to the products of the reaction between CH2SH and 02.The absorption cross-section was estimated using the BeerLambert law in the usual way and found to be consistent across the range of O2concentrations. The absorption cross-section of the products of reaction 4 was incorporated into the model, which was then used to yield a prediction of absorption as a function of time after the electron pulse. This predicted profile was fitted to the whole experimental decay trace by variation of k4. The results obtained lie in the range (0.85 f 0.10) X IO-'' cm3 molecule-' s-I, which is close to the value obtained from the simple analysis. The error in this case represents the range of values obtained, the results are summarized in Table I, and a fitted profile superimposed on the correspondingexperimental trace is shown in Figure 2. Various checks were carried out on this analysis. Firstly, the checks described in our previous work to ensure that the only signifcant reactions of fluorine atoms were with methyl mercaptan to yield CH3S and CH2SH in the ratio 1:3 were carried out. Secondly, the rate constants for reactions 1,2,and 3 were varied over their range of experimental uncertainty with no significant effect on the estimate of k4. Thirdly, the relative contribution to the residual absorption of reaction 4 was varied by a factor of 2. This variation extended the range of values obtained for k4 to (0.6-1.0) X lo-'' cm3 molecule-' s-I, while the quality of fit to the experimental data was much reduced. Fourthly, the fitting procedure was carried out on traces from the spectral scan in the presence of 4 mbar of oxygen, in the region 280-340 nm. The mean value of k4 obtained from these traces was (0.8 f 0.2)X IO-'' cm3 molecule-' s-I, in very good agreement with the results obtained at 270 nm. Although the signal-to-noiseratio of these traces was poorer, the rate constants obtained served as a check to the above procedure. A likely product from reaction 4 in view of the known chemistry of CH20H with O2 is H02, which has a broad and featureless absorption spectrum in the range 210-260 nm.17 The signal was monitored in the region 230-240 nm where there is a "window" in the C H g H radical absorption spectrum, but no slowly decaying product absorption which could be assigned to H 0 2 was observed. 4.2. Reaction of CH2SH with NO. The reaction kinetics of CH2SH radicals with NO were studied by monitoring the decay (17) Pagsberg, P.; Munk, J.; Anastasi, C.; Simpson, V. J. J . Phys. Chem. 1989, 93, 5162.
of CH2SH in an excess of NO. Preliminary experiments were carried out at wavelengths between 240 and 290 nm to locate the optimum wavelength for monitoring CH2SHdecay. The residual absorption (Af)due to the products of reactions 1, 2, 3, and 5 dropped from about 0.2at 240 nm to 0.1 at 290 nm, and as before, this background absorption reduces the effective signal-to-noise ratio of our data. Mixtures containing 5 mbar of CH3SH with NO concentrations in the range 0.5-2 mbar and 1 bar of SF6were then irradiated, and the decay in absorption was monitored at 270 nm. The two analysis procedures employed for the oxygen system were used to obtain values for k5,the rate constant for reaction
5: CH2SH
+ NO -,products
(5)
Using reactions 2 and 5 in the simple analysis method yields the plot of pseudo-first-order rate constant against NO concentration shown in Figure 4,from which a value of k5 = (1.6f 0.7)X l e ' ' cm3 molecule-' s-' was obtained. In applying the second analysis method, it was necessary to consider reaction 13 in addition to reactions 1,2,3,5,9,and 10.
CH3S + NO
+
CH3SNO
(13)
The literature value was used for the rate constant of reaction 132 and for the absorption cross-section of CH3SN0.'* The absorption cross-section of the products from reaction 5 was estimated in the same way as for those from reaction 4,and again these were found to be consistent across the range of NO concentrations. The results were in the range k5 = (1.5 f 0.2) X lo-'' cm3 molecule-' s-', again similar to the value given by the simple analysis. The checks described above for the oxygen system were also carried out on these results. No effect on the estimate of k5 was observed on varying k l , k2, and k3 or on varying the relative contribution to the final absorption from reaction 5. 4.3. Reaction of CH2SHwith NO2. A similar series of experiments was performed to investigate the reaction kinetics of CH2SH radicals with NOz. Preliminary experiments showed residual absorptions of 0.23 at 270 nm decreasing to 0.06 at 310 nm. The concentration of NO2was varied in the range 0.4-2.5 mbar, and the decay in absorption was monitored at both 270 and 290 nm. The simple analysis procedure was used as described in section 4.1,using reactions 2 and 6:
CH2SH + CH2SH -,products CH2SH
+ NO2 -,products
(2) (6)
and a value of k6 = (1.4f 0.6)X lo-'' cm3 molecule-' s-' was obtained using this method. In using the full data analysis method, it was necessary to include reactions 14 and 15 in the kinetic model, in addition to reactions 1, 2, 3, 6,9,and 10:
CH3S + NO2 CH3SO + NO2
-
CH3SO + NO
(14)
CH3S02 + NO
(15) Literature estimates were used for these reactions: and CH3S0 and CH3S02were assumed to have negligible absorption crosssections at 270 and 290 nm. The self-reaction of NO2 was not a significant loss process for this r e a ~ t a n t . ' ~The absorption cross-sections of the products from the reaction of CH$H with NOz were estimated as described for the oxygen system and were similarly consistent across the range of NO2 concentrations. However, a value of k6 = (3.8 f 1.0) X lo-" cm3 molecule-' s-' was obtained, which is much higher than that obtained in the simple analysis method. The checks described for the oxygen system were carried out on this result; no effect on the estimate of k6 was observed on varying k l , k2, and k3 or on introducing +
(18) Niki. H.; Maker. P. D.;Savage, - C. M.; Breitenbach, L.P. J . Phys. Chem.. 1983, 87, 7. (19) Borrell, P.; Cobs, C. J.; Luther, K. J . Phys. Chem. 1988, 92, 4377.
Anastasi et al.
700 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 TABLE IIk Comparison of tbe Rate Constants for the Reaction of CH#H rad CHZOH with 0 2 , NO, atid NO2 reaction k (cm' molecule-I s-I) ref CH2SH + O2 products (4) 8.5 X this work 8.6 X 17 CHIOH + O2 products (1 0) CH2SH NO products (5) 1.9 X lo-" this work products (19) 2.5 X lo-" 17 CH2OH NO 3.8 X lo-" this work CH2SH NO2 products (6) 2.3 X lo-" 17 CH2OH NO1 products (20)
+ + + +
----
TABLE IV: Estimates of Enthalpies of Formation and Reaction#
reactant and product
CH2SH 168
5. Discussion
CH2SH 168
CHzSH + 0 CHzSH + 0
2
-+
2 --c
HSCH2OO
CH2S + H02
(4b)
In our experiments, we investigated these reaction pathways by looking for a slowly decaying absorption in the region 230-240 nm which could be assigned to H 0 2 . No such absorption was observed, which indicates that either H 0 2 is not present, or it is reacting very rapidly to give concentrations too low for us to monitor. Under the experimental conditions used in our study, the expected half-life of H 0 2 due to its self-reaction is greater than 100 ~ s , ~ suggesting O that the reaction of CH2SH with O2 proceeds via the addition route (4a). This would result in a peroxy radical, similar to those obtained in oxygen addition to alkyl radicals.21 Also, although the S-H bond is weaker than the 0-H bond, there is no driving force for the abstraction reaction from CH2SH (reaction 4b) because of the thermodynamic instability of CH2S (AHf = 140 kJ mol-', estimated from heats of formation for CH20, CO,, COS, and C S F ) . This is the most significant difference between the chemistry of CH2SH and CH20H,with the consequence that addition to CHSH is favored. Our estimates of the exothermicitiesof these reactions are shown in Table IV. Furthermore, the only reported absorption features for CH2S are in the region 185-21 5 nm,23indicating that the residual absorption spectrum observed in the presence of oxygen (Figure 3) cannot be assigned to this species. This spectrum may be due to the peroxy radical resulting from the addition reaction (4a). If this is the case, then we can use the fact that no decay was observed over 100 ps to estimate an upper limit on the self-reaction rate cm3 molecule-I s-I, comconstant for this radical of 5 X parable with other peroxy radical self-reactions. The broad absorption between 240 and 300 nm is characteristic of peroxy radicals, although some weakening of the 0-0 bond by the thiyl (20) Kurylo, M. J.; Oullette, P. A,; Laufer, A. H.J . Phys. Chem. 1986, 90, 437. (21) Finlayson-Pitts, B. J.; Pitts, J. N. Atmospheric Chemistry; Wiley: New York. 1986. (22) Benson, S. Thermochemical Kinetics; Wiley: New York, 1968. (23) Drury, C. R.; Mode, D. C. J . Mol. Spectrosc. 1982, 92, 469.
0
+ NO 91
+ NO
CH2SH 168
91
CH2SH 168
H S C H 2 0 0 (4a) 48
CH2S 140
0
CHzSH 168
CH2SH 168
+ O2
+ 02
CH2SH 168
significant absorption due to CH3S0and CH3S02. It can be Seen from Table I1 that the removal rate of CH3S radicals by NO2 is twice the rate of removal by NO, and consequently we would expect secondary reactions to be more important in this case. It is clear that the simple model of the decay is insufficient in this case, and a full description of the system is required to obtain a reliable estimate of k6.
These are the first kinetic measurements on the reactions of CH2SH radicals with 02,NO, and NO2. The results are summarized in Table 111, along with those obtained in a preceding study of the reactions of hydroxymethyl radi~a1s.I~The rate constants we have derived are similar to those measured for the equivalent oxygen-containing species; however, our evidence suggests that the reaction pathways are different. There are two possible reaction routes for the reaction of CH2SH with 0,; addition of oxygen to the carbon center or abstraction of hydrogen.
-
Mor (298 K) (kJ mol-')
-
CH2S 140
-
33
33
+ NO2 33
21
H S C H 2 N 0 (5a) 87
+ NO2
+ NO2
+ HOz (4b) + H N O (5b) 100
H S C H 2 N 0 2 (6a) -56
+ NO (6b)
HSCH20 34 CH2S 140
91
+ HN02 ( 6 ~ ) -78
AH0z9a (kJ mol-')
-.I20 -7
-I 72 -18
-257 -76 -139
ONote: Enthalpies estimated from ref 21, using a calculated radical stabilization energy for CH2SH from ref 26. Figures in bold are measured values, and figures in italics are estimated values. group is indicated. Pagsberg et al.17 observed evidence for both abstraction and addition processes in the equivalent reaction of the oxygen-containinganalogue, hydroxymethyl, with 02:
CH2OH + 0 CH2OH + 0
2
2
HOCH2OO
( 16a)
CH2O + HO2
(16b)
+
-+
In their study, absorption around 230-240 nm due to HOz was observed, indicating that abstraction via reaction 16b is the major pathway. However, a residual product absorption at higher wavelengths was also observed, suggesting the formation of a peroxy-type adduct via a minor reaction pathway (16a). The residual absorption following reaction with O2was monitored between 290 and 360 nm over a time scale of 400 ps. Below 330 nm, the residual absorption did not decay over this period, whereas above this wavelength a first-order decay was observed. Decay half-lives varied between 36 ps with 20 mbar of O2and 63 ps with 4 mbar of OF This suggests the onset of an absorption above 330 nm due to a reaction product which may be decaying by reaction with 02.The HS radical is one possible product of the oxidation process, as outlined below, but as it absorbs between 323 and 329 nm24this absorption cannot be assigned to HS. The reaction product is as yet unidentified. The reactions of CH2SHwith NO and NO2 may also proceed via addition and/or abstraction:
CH2SH + NO
+
HSCHZNO
(58)
CHzSH + NO --c CH2S + HNO
(5b)
CH2SH + NO2 --c HSCH2N02
(64
CHzSH + NO2
+
[HSCH,ONO]
CH2SH + NO2
+
+
HSCH2O + NO (6b)
CHZS + HONO
(64
Regarding the possible abstraction products, the only reported UV absorption bands of HNO are below 210 nm,25outside our experimental region, and H N 0 2 does not exhibit the same absorption cross-section at 270 nm as our observed residual abs0rption.2~The estimated enthalpies of reaction are shown in Table IV, indicating that reaction via addition to the carbon center (reactions Sa and 6a) is the most favorable route. Adjacent thiyl and nitrogen-containing groups may increase the intensity of the (24) Okabe, H . Photochemistry of small molecules; Wiley: New York, 1978. (25) Callcar, A. B.; Wood, P. M. Tram. Faraday SOC.1971, 67, 3399. (26) Pasto, D. J.; Krasnansky, R.; Zercher, C. J . Org. Chem. 1987, 52, 3062.
701
J. Phys. Chem. 1992,96,701-710 electronic absorption spectra of the products, and this would account for the &crease in magnitudeof the product absorption cross-sections from reactions 5 and 6. These results suggest that, in an atmospheric context, CH2SH will be removed by reaction with 02.This is in contrast to the low reactivity of CH3Swith respect to oxygen where the possibility of significant reaction with N O and NO2 must be considered. Similar arguments apply in the case of hydroxymethyl and methoxy radicals; hydroxymethyl radicals are removed exclusively by reaction with oxygen, whereas reaction of CH30 with NO and NO2 may be significant, particularly in areas of high NO, concentrations. However, the detailed chemistry of CH2SH and CH20H appear to be very different. Hydroxymethyl radicals will yield H 0 2 and formaldehyde via reaction 15b, whereas CH2SH will yield a variety of oxidized products, following initial addition of oxygen:
CHzSH
+0 2
-
+ NO HSCH20 + 02
HSCHzOz
HSCH20
+
+
HSCH202
HSCH2O
HSCHO
HS
+ NO2
+ HOz
+ CHI0
(4a) (17) (18) (19)
A likely intermediate is the HS radical which will be oxidized to SO2;however, other oxygenated sulfur species may be formed.
Acknowledgment. We would like to thank Mrs. Jette Munk of Rista National Laboratories for her valuable assistance in conducting the experiments. We gratefully acknowledge the support of the SERC and National Power PLC for the award of a CASE studentship for M.B. Registry NO. CHISH, 17032-46-1; 02,7782-44-7; NO, 10102-43-9; N02, 10102-44-0.
Photophysical Properties of Styryl Derivatives of Aminobenzoxazinones S. Fery-Forgues: M. T. Le Bris,? J.-C. Mialocq,' J. Pouget,+W. Rettig,o and B. Valeur*vt Laboratoire de Chimie GEngrale (CNRS URA 1103). Conservatoire National des Arts et Mgtiers, 292 rue Saint-Martin, F-75003 Paris, France, CEAICE-Saclay/SCM/CNRS URA 331, F-91191 Gif-sur Yvette, f i r Physikalische und Theoretische Chemie, Strasse des I7 Juni 112, France, and Iwan-N.-Stranski-Institut 0-1000 Berlin 12, Germany (Received: February 25, 1991; In Final Form: August 6, 1991)
The spectra, fluorescence quantum yields, and lifetimes of styryl derivatves of (dimethy1amino)benzoxazinone in various solvents are reported. The results show that the main chromophoreis the aminobenzoxazinone moiety and the para-substituted styryl moiety is a substituent as a whole. Therefore, these compounds do not behave as stilbene derivatives in spite of analogy in chemical structure. The effect of the substituent in the para position of the styryl moiety has been carefully examined with the help of quantum mechanical calculations by PPP and CNDO/S methods. The possibilities of internal rotation leading in particular to twisted intramolecular charge transfer (TICT) states are discussed.
SCHEME I
Introduction
Photophysics of fluorescent dyes is a field of constant interest because a better understanding of the excited-state properties helps in the design of new molecules offering the best performances for a given application: laser dyes, probes for polymeric, micellar and biological systems, molecules for nonlinear optics, for molecular devices, etc. Generally, each class of dyes has its own characteristics, e.g., ranges of absorption and emission wavelengths. Coumarins, rhodamines, and oxazines, are well-known families. Aminobenzoxazinone (1) derivatives, a class of dyes mainly developed by some of us,14 exhibit very different properties ac-
R.
--cHo
DFSBO
R-
-H
802-H
1
cording to the nature of the substituent R2, but the common feature of these dyes is the presence of electron donor (amino group) and electron acceptor (carbonyl group and heterocyclic nitrogen of oxazinone) moieties which leads to an intramolecular charge transfer upon excitation that results in a large increase of the dipole moment and hence a strong solvatochromiceffect. Rullitre et al.'** investigated the photophysics of some of these compounds on the picosecond scale to study solvation dynamics at a molecular level. The laser properties of these compounds were also ~ t u d i e d . ~ . ~ whom correspondence should be addressed. '*To Conservatoire National des Arts et MQiers. rCEA/CE-Saclay. Iwan-NStranski-Institut.
0022-365419212096-701$03.00/0
U
Among aminobenzoxazinones, the styryl derivatives (Scheme I) are of particular interest. Special attention was first paid to ( 1 ) (a) Le Bris, M. T.J . Hererocycl. Chem. 1984, 21, 551. (b) Le Bris, M.T.Ibid. 1985, 22, 1275. (c) Le Bris, M. T.Ibid. 1989, 26, 429. (2) Le Bris, M. T.;Mugnicr, J.; Bourson, J.; Valeur, E. Chem. Phys. Le??. 1984,106, 124.
0 1992 American Chemical Society
