1616
The Quenching Reactions of the First Excited Singlet and Triplet States of Sulfur Dioxide with Oxygen and Carbon Dioxide' T. Navaneeth Rao, Susan S. Collier, and Jack G . Calvert Contribution from the Evans Chemical Laboratory, The Ohio Stale University, Columbus, Ohio 43210. Received October 9, 1968 Abstract: A determination has been made of the quantum yields of fluorescence (&) and phosphorescence ($J of sulfur dioxide excited at 2875 A and 25" in mixtures with added oxygen or carbon dioxide. From the variation
/ + ~[SO,], [O,], and [COz], and the published lifetime data for excited sulfur dioxide,296 estimates of r$r and q ~ ~ with of the rate constants (1.-mole-sec units) for the quenching reactions of both the first excited singlet ( 1 , 9 0 2 ) and triplet (SO2)sulfur dioxide molecules with S02, 0 2 , and C02 were derived: 1 . 5 0 2 SO2 -+ (2S02) (la), IS02 SOz S O 2 SO2 (2a), k l , kzn = (1.5 0.7) x 1010; l S 0 2 4- 0 2 (SO2-02) (lb), 'SO2 + 0 2 3.902 0 , (2b), k i b k2b = (0.47 rk 0.22) X 'so2 + C02 (s02-Co2) (IC), lS02 coz -+ 3s02 c02 (2c), kl, $- kzc = (1.1 i 0.5) x 1010; 3S02 -C SO2 (2S02) @a), ksa = (1.4 0.7) x 107; 3SO2 - 0 2 -+ (SO2-02) (8b), (1.1 f 0.6) X 1 0 7 I k g b 5 (5.9 31 0.6) X IO'; 3SO2 CO2 ( s 0 2 - 0 2 ) ( 8 ~ )ksc , = (0.87 I 0.50) x 107. The smallness of the rate constant for reaction 8b is unexpected and represents the lowest reported value for a triplet molecule quenching reaction by oxygen. -+
+
+
+
+
*
+
-+
*
-+
-+
P
revious studies of sulfur dioxide photochemistry suggest that some striking differences exist between the reactivities of the first excited singlet ('SO,) and triplet (3S02) states in sulfur dioxide. Fluorescence from the 'SO, state is quenched on virtually every collision with ground-state sulfur dioxide molecules,2- and, in the order of decreasing efficiency, C 0 2 , H,, 0,, and He quench 'SO, somewhat less rapidly than On the other hand, apparently the phosphorescence from the 3 S 0 2 state is quenched very inefficiently by ground-state sulfur dioxide. About one in lo3 or lo4 collisions are effective in deactivating this m o l e c ~ l e 5, ~, 6,and Caton and Duncan6 observed no preferential quenching of the 3 S 0 2 state in experiments with added oxygen. The apparent very low reactivity of the 3S02 species with oxygen appears to be unique among the known photochemical systems, and this observation should be verified and further defined in quantitative studies. Accurate rate data on the reactions of the 'SO2 and the 3 S 0 2 states with oxygen and other gases would be most valuable in the evaluation of the role of each state in the photochemistry of sulfur dioxide and in its mixtures with oxygen. These data would be of great practical value as well in the delineation of the paths of sulfur dioxide removal in the polluted atmosphere. In this work we describe the quantitative determinations of the rate constants for the quenching of both the 'SO2 and the 3S0, states by oxygen and carbon dioxide. These have been made through a study of the effect of oxygen and carbon dioxide on the quantum yields of fluorescence and phosphorescence of sulfur dioxide excited at 2875 8, and 25".
+
-+
+
The data allow some interesting conclusions concerning several aspects of the photochemistry of sulfur dioxide. Experimental Section All emission data were determined on an absolute spectrofluorimeter (Turner Model 210) using the procedures which have been described p r e v i o u ~ l y . ~The exciting light was at 2875 A with a 150-A band width. The second of the two methods outlined previously5 was employed in the determination of quantum yields of fluorescence (c$~) and phosphorescence (&,) of sulfur dioxide. Quantum yield data determined from S02-02 and SO,-CO, mixtures are summarized in Tables I and 11, respectively.
Discussion of the Results The quenching rate constants for both 'SO, and 3S02 states of sulfur dioxide with SO,, 0 2 , and C 0 2 can be estimated from the emissiondatadetermined in this work. The quantum yields of fluorescence and phosphorescence from sulfur dioxide excited at 2875 A and 25" in SO2-0, and SO,-CO, mixtures can be described well in terms of an extension of the mechanism suggested previously for pure sulfur dioxide.
(1) Presented in part at the 156th National American Chemical Society Meeting, Atlantic City, N. .I. Sept , 1968. The authors acknowledge gratefully the support of this work through a research grant from the National Air Pollution Control Administration, U. S. Department of Health, Education, and Welfare, Public Health Service, Arlington, Va. (2) K. F. Greenough and A. B. F. Duncan, J . Am. Chem. Soc., 83, 5 5 5 (1961). (3) S. J. Strickler and D. B. Howell, J . Chem. Phys., 49, 1947 (1968). (4)H. D. Mettee, ibid., 49, 1784 (1968). ( 5 ) T. N. Rao, S. S. Collier, and J. G. Calvert, J . Am. Chem. Soc., 91, 1609 (1969). (6) R. B. Caton and A. B. F. Duncan, ibid., 90, 1945 (1968).
Joirrnal of the American Chemical Society 91:7 March 26, 1969
so2+ hv 'SO, + so2 'SO,
+ 0,
+
'SOz
(1)
-+
(2S02)
(la)
+
%02 SO,
+ co, 'SO,
+
(2a)
(1 b)
(SO,-0,)
.+ ?SO2
'SO,
+
+ O 2 ('I,'A, or 'I)
+
(S02-C0,)
-+
'So,
+ CO,
+
so2 + hvr
+
so,
-+
%02
(2b) (IC) (2c)
(3)
Table I. Quantum Yields of Fluorescence and the Ratio of br to the Quantum Yield of Phosphorescence (9,) for Sulfur Dioxide Excited at 2875 8, and 25” in Mixtures with Oxygen
Table 11. Quantum Yields of Fluorescence and the Ratio of $f to the Quantum Yield of Phosphorescence (G) for Sulfur Dioxide Excited at 2875 8, and 25” in Mixtures with Carbon Dioxide
Concentration, M x lo5
so2
+, x
0 2
1.52 1.52 1.52 1.52 1.65 1.65 1.65 1.65 1.65 I . 65 3.45 3.45 3.45 3.45 3.45 3.45 3.45 5.18 5.18 5.18 5.18 5.18 5.18 5.18 5.18 7.76 7.76 7.76 7.76 10.34 10.34 10.34 10.34
0.00 2.57 4.60 9.60 0.00 1.79 5.03 6.75 8.69 14.72 0.00 0.066 0.131 0.20 0.39 4.57 15.3
0.00 0.13 0.39 1.08 3.45 4.31 8.62 14.3 0.00 1.94 3.45 4.31
0.00 1.08 1.94 3.45
3s02 +
Concentration, M x lo5 103
8.55 5.24 5.00 3.26 7.94 6.10 6.21 4.52 4.69 2.26 3.92
... .*. 3.94 3.57 2.72 1.75 2.63 2.54 2.45 2.35 2.04 2.09 1.77 1.34
... ...
... ... 1.33 1.25
... 1.20
$rib,
so*
coz
...
1.65 1.65 1.65 1.65 1.65 3.45 3.45 3.45 3.45 3.45 3.45 3.45 3.45 4.31 4.31 4.31 4.31 4.31 5.17 5.17 5.17 5.17 5.17 7.76 7.76 7.76 7.76 9.48 9.48 9.48 9.48 9.48 9.48 9.48 IO.34 10.34 10.34 10.34 10.34
0.00 2.10 4.38 5.26 7.40
... ...
...
7.14 7.65 9.35 10.37
...
... 7.14 7.28 7.35
... ... 10.3 10.9 7.14
... ...
7.57 8.28
...
8.78
10.01 7.14 8.07 8.35 8.64 7.14 7.35 7.50 8.00
co2-+ ( s o 2 - c o 2 )
(84
At this early stage of the research on sulfur dioxide photochemistry, it is not possible to separate the extent of chemical change and that of relaxation to ground-state molecules which occur in reactions la-c and 8a-c. The designation of the products of these reactions in parentheses implies that either one or both of these reactions paths occur and that there are no excited states of sulfur dioxide generated in these reactions. The Quenching Reactions of the First Excited Singlet State of Sulfur Dioxide with Oxygen and Carbon Dioxide. According to the mechanism outlined the quantum yield of fluorescence of sulfur dioxide in SO2-0, mixtures should be related to the concentrations of sulfur dioxide and oxygen by relation 9. We have tested the functional 1
--
4%
=
[O,]
klb
+ k2b
x 103
7.92 3.74 2.85 2.40 1.87 3.91
0.00 1.72 1.94 4.10 5.26 7.11 10.56 13.27 0.00 3.10 4.31 7.10 8.62 0.00 1.72 2.59 3.45 4.31 0.00 1.72 3.45 4.31 0.00 3.23 6.25 8.84 11.51 13.28 16.17 0.00 0.86 1.72 2.59 3.45
... ...
1.93 1.73 1.40 1.15 0.988 3.15 2.09 1.81 1.47 1.29
...
...
... ... ... ... ... ... ... 1.45 1.07 0.961 0.879 0.781 0.698 0.623
...
...
... ... ...
$fl$Ll 7.14 5.33 5.36
... 4.25 7.14 7.41 7.30 6.22
,,. 5.91
... ...
...
...
... ...
...
7.14 7.06 6.99 6.02 6.28 7.14 7.89 6.70 7.30
...
... ... ... ... ... ...
7.14 7.31 6.84 6.75 6.88
within the experimental error of the data this is the observed result in Figure 1 ; the slopes ( M - ’ x of the least-squares lines are’ 2.0 f 0.8, 1.9 0.5,2.1 0.2, 2.4 I 0 . 3 , and 2.2 f 1.5 at [SO,] ( M x 10’) = 1.65, 1.52, 3.45, 5.19, and 10.34, respectively. for sulfur dioxide in Expression 10 should describe experiments with added carbon dioxide. The functional 1
--
(bf
= [CO,]
k,,
+ k,,
+
k3
+
k3
[SO,](k,,
+ k 2 8 )+ k3 + k , + k , k3
(9)
form of (9) using the data of Table I for a series of runs at different sulfur dioxide and oxygen concentrations. In a given series of runs made at a fixed [SO,], we would expect a linear relation between libf and [O,]. In theory the intercepts of a series of such plots at different fixed [SO,] values are expected to be a function of [SO,], but the slopes of the series of lines should all be equal. Indeed
form of (10) is tested in Figure 2 using the data of Table 11. Again within the experimental error the slopes are equal for the different series at fixed [SO,] as theoretically expected; the slopes (M-’ x of the least-squares lines are 5.2 f 0.7, 5.6 k 0.4, 5.2 f 0.2, and 5.4 k 0.6, at [SO,] ( M x 10’) = 1.65, 3.45, 4.31, and 9.48, respectively. (7) In every case in this work the error limits shown represent the95% confidence limits (twice the standard deviation) as determined by the method of least squares.
Rao, Collier, Calvert 1 Quenching Reactions of Excited States of SO2 with 0
2
and CO2
1618
4
8 [CO&
12 16 MxlG
Figure 1. Plots of the reciprocal of the quantum yield of fluorescence from sulfur dioxide, excited at 2875 A and 25" in mixtures with oxygen, vs. the oxygen concentration for several values of [SO,], M x IO5: 0, 1.52; A, 1.65; E, 3.45; 0 , 5.18; M, 10.3.
10
-
2
6
8 1 M x 105
[soj,
0
Figure 3. Plots of the intercept to slope ratios from the leastsquares lines of Figure 2 for SO,-0, mixtures and Figure 3 for SOz-CO, mixtures; the slopes and intercepts of these plots are used to derive estimates of rate constants for 'SO, quenching by SOz, OZrand C 0 2 .
slope ratios (taken from Figure 1 for 0, and Figure 2 for CO,) vs. [SO,] should be a linear function. The test of this dependency is shown in Figure 3. Within the experimental error the theoretically expected linear relation between the variables is seen to hold well for both the oxygen and the carbon dioxide data. The slope and intercept for the least-squares lines in Figure 3 yield the following rate constant estimates.
N
1
I
5
IO
I
15 cod, M x 105
1
I
20
25
(siope SOz-02)Fig = k l a li1b +
+
Figure 2. Plots of the reciprocal of the quantum yield of fluorescence from sulfur dioxide, excited at 2875 A and 25" in mixtures with carbon dioxide, vs. the carbon dioxide concentration for several values of [SO,], M x l o s : 0, 1.65; 0, 3.45; A, 4.31; 0, 9.48.
From the intercept to slope ratios of the plots in Figures 1 and 2 it is possible to derive the estimates of the desired quenching rate constant for the 'SO, species reacting with oxygen and carbon dioxide. In theory the intercept to slope ratios for the SO,-O, mixtures are given by relation 11 and those for the SO,-CO, mixtures by the analogous relation 12. The plot of the appropriate intercept to
intercept [=]Fig
4
2
=
klc
+ kzc
+
(slope S0,-C02)Fig
3
=
k,, + klc
k3
k2a =
+ k2c
(0.85 (intercept S02-C02)Fi, =
= 3.18
k2n k2b
f 0.33
1.36 rt: 0.01
1.8) x
l./mole
+ k4 + k5 -k1C
+ k2c
(0.23 rt: 0.07) x lO-'l./mole Taking the inverse of the lifetime estimate of Greenough and Duncan, for k, + k4 + k5 = (2.38 i 0.13) x lo4 sec-', we find k,, + k,, = (0.90 k 1.9) x 10" I./(mole sec) (O,-SO, data); k l , + k,, = (1.4 & 0.4) x 10" (c02-s0, data); klb + k,, = (0.28 i 0.60) x 10"; klC + kZc= (1.03 f 0.30) x 10" l./(mole sec). Within the experimental error the two independent estimates of k , , + k,, derived here agree with one another and with the independent estimate of this rate constant sum derived recently from fluorescence studies of pure SO, : k , + k,, = (2.2 & 1.0) x 10" l./(mole sec).' Probably the best estimates of the quenching constants which we can derive from the present data are from an average of the estimates
Journal of the American Chemical Society 1 91:7 March 26, 1969
1619
+
of k , , k,, and the fairly accurate rate constant ratios derived from the slopes of Figure 3. With this procedure we estimate
+ k,, = (1.5 f 0.7) x 10" l./(mole sec) k,b + kzb = (0.47 f 0.22) X 10" l./(mOle SeC) k , , + kZc= (1.1 i 0.5) x IO" I./(mole sec) k,,
The Quenching of the First Excited Triplet of Sulfur Dioxide with Oxygen and Carbon Dioxide. A consideration of the variation of the ratio of the quantum yield of fluorescence to that of phosphorescence of sulfur dioxide with added gas allows the determination of the rate constants for 3 S 0 , quenching by the added gas. From the mechanism outlined, the variation of Qf/Q, for sulfur dioxide in mixtures with oxygen should follow relation 13. In our previous study we have derived the (+f/$p)([SoZlk2,
k6
+ k7)
(ilopr)F i g 4
+ (k6 + k7)/kEb
= [So21k8a/kSb
(I6)
(13)
estimates k,, = (0.18 J. 0.08) x IO" I./(mole sec), and k , = (1.5 5 0.8) x IO3 sec-'. From these values it can be seen that the following inequality holds for our conditions: ([SO,]k,, + [ 0 2 ] k 2 , >> ) k , . Simplification of 13 can be made neglecting k,.'
[O2Ik8b
intercept
Similarly the function 17 is expected to describe the intercept to slope ratio for the C02-S02 data in Figure 5.
+ [O2lk2b + k S )
= ( k 3 / k 6 ) ( [ S O ~ I k ~ a[OZlk8b +
the data at several concentrations of sulfur dioxide (Tables I and 11) and are plotted for the 0 2 - S 0 2 data in Figure 4 and for the SO,-CO, mixtures in Figure 5. Indeed, the theoretically expected equality of the slopes is observed within the somewhat larger experimental error in these series:' for the 0 2 - S 0 2 runs the least-squares slopes are 4.0 f 0.5,4.2 f 0.4,4.1 f 0.3, 5.3 k 0.6,and 5.1 f 0.7 for [ S O , ] ( M x lo') = 1.65,3.45,5.18,7.76, and 10.34, respectively; or the C0,-SO, runs the least-square slopes are 2.6 5- 0.8, 3.5 f 0.8, 3.2 f 1.6, 4.6 & 3.3, and 3.7 I 1.5 at [SO,] ( M x IO') = 1.65, 3.45, 5.17, 7.76, and 10.34, respectively. The intercept to slope ratios taken from the data of Figure 4 should in theory be a linear function of [SO,].
+ k6 + k 7 )
(I4) Similarly the function 15 can be shown to apply for.the C0,-SO, mixtures for the conditions employed here.
intercept
+ (k6 + b ) / k S c
= (F )
[S021kSa/kL3c
Fig 5
(17)
The intercept to slope ratios from Figures 4 and 5 are plotted as a function of [SO,] in Figure 6. Within the experimental error the data for 0 , - S 0 2 and CO,-SO, mixtures follow the theoretically expected linear relationships. From the slopes and the intercepts derived by a least-squares treatment of these data, we estimate the following rate constant ratios. (Slope soz-02),ig6 = k,,/k,b
=
1.27 f 0.26
(slope S0,-C0,),ig6 = k,,/k,, = 1.62 k 0.48
+
(intercept S O , - O ~ ) F = ~(k6 ~ ~ k,)/k,b=
+ k6 + l i 7 )
(15) From the present study all of the quantities to the left of the equality sign in (14) and (15), other than k 2 , / k z ,and k z a / k z chave , been determined. The needed rate constant ratios cannot be evaluated directly. However, it is probably a reasonable assumption that the fraction of the quenching collisions which result in spin inversion in 'SO, is independent of the nature of the colliding gas; that is, k,a/(kla kza) = k 2 b / ( k l b + k 2 b ) = k J ( k i C kzc). For this condition k2,/kZb= ( k l , + k z , ) / ( k l b+ k,,) = 3.18 andk,,/k,,= (kl,+ k J ( k l C + k 2 c )= 1.36. Forthecase of 'SO,-0, collisions there must be some question as to whether there is an apparent enhancement of the spin inversion of the 'so2 with k2,/k2, < ( k l a k 2 , ) / ( k l b+ kzb). At this point we will accept the first hypothesis, and a test of the alternate hypothesis will be made later. For runs at constant [SO,], one expects a linear relationship between the function on the left of eq 14 and the [ O , ] ,or eq 15 and the [CO,]. As with fluorescence quenching studies we expect in theory that different series of runs, each at a fixed [SO,], would have identical slopes, (k,bk,/k,k,,) for 0 2 - s 0 2 runs, and (k,,k,/k,jk,,) for C0,-SO, runs, but the intercepts would be a function of the [SO,]. Functions 14 and 15 have been derived from [CoZlk8c
+
(8) The neglect of kS introduces a maximum error of 4 % in the runs at [SO,]= 1.65 x Mand [O,] = [CO,] = 0. I n all other runs the error is much less.
M
(1.38 f 1.72) x (intercept S02-C02)Fi,6= (k6
+ k,)/k,,= (2.21
i 3.0)
x 10-5 M
Using the inverse of the lifetime of 3 S 0 2 from Caton and Duncan,6 k6 + k , = (1.43 f 0.16) x 10'sec- ',the above rate ratios can be used to calculate two independent estimates of k,,.' O
k,,
=
(1.3 f 1.7) x lo7 l./(mole sec) (SO,-0, data)
=
(1.1 f 1.5) x lo7 l./(mole sec) (SO,-CO,
data)
These values are equal to one another within the error limits, and they compare well to recent estimates of k,, derived from very different experiments; k,, z 2.5 x IO7 from the Strickler and Howell37 study of the dependence (9) The method used to determine the values of 4D involves the difference between the total intensity of emission, fluorescence plus phosphorescence, at several wavelengths, and the fluorescence intensities at these wavelengths. Thus there is considerably more error present in the values of &/bP than those of reported in this work. (10) The 3 S 0 2 lifetime estimate of Caton and Duncan6 must be accepted with some reservation since their results showed no detectable dependence on the pressure of sulfur dioxide up to 300 p. However, the rate constant data of Strickler and Howell,3 Rao, Collier, and C a l ~ e r t , ~ and the present data suggest that a significant decrease in T (from 55 to 280%) should have been observed over this pressure range. It seems probable that their estimateof T for Pso, = 0 is nearlycorrect, but the large scatter in the data may have masked the expected pressure effect on the lifetime.
Rao, Collier, Calvert 1 Quenching Reactions of Excited States of SO2 with
0 2
and COZ
1620
I
[Oz], MX105 Figure 4. Plot of a function of Or/$, (relation 14 of the text) vs. oxygen concentration; data from experiments with SO, excitation at 2875 8, and 25" in SO2 mixtures with oxygen and at fixed [SO,], M x l o 5 : 0,1.65; A, 3.45; a, 5.18; A, 7.76; 0, 10.3.
I
I
1
2
4
6
EOJ,
I
8
MX106
Figure 5 . Plot of a function of bf/$D(relation 15 of the text) vs. carbon dioxide concentration; data from experiments with SO, excitation at 2875 8, and 25" in SOz mixtures with carbon dioxide 1.65; A , 3.45; a, 5.17; U, and at fixed [SO,], M x 10': 0, 7.76; A, 10.34.
of & on [SO, ] at low [SO,], and k,, = (0.49 k 1.4) x IO7 I./(mole sec) from the Rao, Collier, and Calvert' study OF the quantum yields of 3S0,-sensitized biacetyl phosphorescence. The rate constant ratios derived here and the best estimate of k,, (from the average of all the available estimates of this constant) yield the following rate constant estimates. k,, = (1.4 -t 0.7) x lo7 l./(mole sec) n
k 0.6) x io7 ]./(mole sec) k,, = (0.87 k 0.50) x lo7 I./(mole sec) k8b = (1.1
+
x
10-
0,(32) +
3s0,
a
i+
+
+ OJA,
(W Reaction 2b' is energetically possible for the vibrationally nonequilibrated, excited singlet molecules ('S0,V) formed at 2875 A." However, we did not observe a special high efficiency of quenching of 'SO2 species by oxygen. It quenches 'SO2 at only four-tenths of the rate of CO,, at about the same relative efficiency as one would expect if both CO, and 0, quenching of 'SO, involved vibrational relaxation processes only. Although k , , dominates over k2a,5it is possible that k,, is much greater than k l b ,so that the rate constant sums observed experimentally, k , , + k,, and k , , + k,,, may tend to mask the true effect of oxygen in ( 1 b) and (2b) in these experiments. It is important to test the effect of the possible inequality, k 2 , / k , , < (kla + k 2 a ) / ( k l b+ k,,), on the 'SO,-O, quenchingrate constant estimates derived in this work. The maximum value which k,, can have is k,, = k,, + k,, = 0.47 x 10" I./(mole sec), estimated in this study. The value k , , = 0.12 x 10,' l./(mole sec) can be estimated from the average value, k,, + k,, = 1.5 x 10" l./(mole sec), derived above, and k Z a / ( k l + a k,,) = 0.080, found in our 1s020
-
y
In view of the surprisinhy small magnitude of k,,, one must check carefully the one assumption made in its estimation from the quenching data; namely, is the approximation, k,,/kzb = ( k l , k z , ) / ( k l b k,,), justified? It is theoretically possible that oxygen enhances the spin inversion of 'SO2 through the reaction
+
15
P
I5
5-
1x1
(11) The singlet-triplet energy separation in SO2, Es: - ETo1 -N 13 kcal/mole; A E = 22.6 kcal/mole for 0 2 ( 3 X ) + OZ('A) and 37.7 kcal/mole for 02(3Z)+ 02(lZ).
X
105 M
Figure 6. Plots of the intercept to slope ratios from the leastsquares lines of Figure 4 for SO2-Oz mixtures and Figure 5 for SOz-C02 mixtures; the slopes and intercepts of these plots are used to derive estimates of rate constants for 3 S 0 2 quenching by SO2, 02,and CO,.
previous study.' Now taking the ratio k,,lk,, = 0.26 instead of 3.18 used previously, the data of Table I can be used to calculate function 14, and plots similar to those in Figures 4 and 6 can be made as before. This treatment gives a slope, k8,/k,, = 0.24 5 0.05, and an intercept, ( k 6 k7)/k,, = - (0.39 k 0.29) x M . Taking k,, = 1.4 x lo7 l./(mole sec), the average value of the previous estimates, and neglecting the impossible negative number of the less accurate intercept, we find k,, < (5.9 k 0.6) x IO7 l./(mole sec). Thus we can conclude that the true value of k,, lies in the range (1.1 k 0.6) x lo7 < k8b < (5.9 5 0.6) x io7 I./(mole sec). Itseemsiiighly probable to the authors that the lower limit of k , , i S the best estimate of the quantity; in the same calculation in which this estimate was made, CO,-SO, and 02-SO2
Journal of the American Chemical Society 1 91:7 1 March 26, 1969
+
1621 data gave near equal estimates of k8,, and these estimates were also equal within the experimental error to the other estimates of this constant from entirely independent and different kinds of experiment^.^'^ However, the same plot whose slope was used to derive the high estimate of k8, gives the impossible result of a negative value for the intercept, presumably equal to (k6 + k,)/ksb. Comparison of the Rates of 'SO, and 3S02Quenching Reactions. The results of this study confirm the unusual reactivity difference between the singlet and triplet excited states of sulfur dioxide. It is interesting to compare the ratio of quenching rates observed in this study for SO,, CO,, and 0, molecules. rate of 'SO, with SO, = 3.18 f 0.33 rate of 'SO, with 0,
rate constant for the quenching reaction with oxygen is veryunexpected: (1.1 -t 0.6) x io' < k8b < (5.9 f 0.6) x lo7 l./(mole sec). However, we believe that this result is correct; it is also consistent with the qualitative observations of Caton and Duncan6 who reported that the presence of a small amount of oxygen in sulfur dioxide gave no observable effect on the lifetime of 3S0,, and there was no preferential quenching of 3S0, by oxygen over that by sulfur dioxide. One might have expected that energy transfer from 3S02to 0, would occur efficiently by reaction 8b', forming singlet oxygen, as well as by (8b), forming vibrationally excited ground-state oxygen molecules (3c).
+
%02 02(3Z) -+ SOa
+ O#A,
IZ)
(8b')
Reactions analogous to (8b'), involving triplets of polynuclear aromatics, dye molecules, etc., are very fast (near the collision number in the gas phase or diffusion con rate of 'SO, with SO, = 1.36 0.01 trolled in the liquid phase) and are believed to be imrate of 'SO, with CO, portant steps in the photooxidations of these comp o u n d ~ . Since ~ ~ oxygen quenches 3S0, at about the rate of 'SO, with 0, same rate as does carbon dioxide where electronic energy = 0.43 & 0.04 rate of 'SO, with CO, transfer is impossible, reaction 8b' does not occur efficiently, if at all, for our conditions. Kawaoka, Khan, and rate of 3S02 with SO, K e a r n ~have l ~ ~considered the theory of the quenching of = 1.27 & 0.26 rate of 3S0, with 0, organic triplet states by oxygen and found that the most important contribution to the oxygen quenching of tripletrate of 3S0, with SO, state molecules is by transfer of electronic energy to oxygen = 1.62 0.48 rate of 3S02 with CO, rather than by enhanced intersystem crossing. They further showed that the rapidity of the electronic energy rate of 3S0, with 0, transfer was in large part due to the participation of = 1.28 0.54 rate of 3S0, with CO, charge-transfer states in the quenching process.'3b It may be significant that oxygen has little or no effect on the enhancement of the 3S0, c SO, absorption, and chargeThe comparisons show that there is a surprising similarity transfer states, often invoked to explain 0, enhancement in the quenching rates of 'SO, with SO,, O,, and CO,. of the singlet-triplet band, l4 seem to be of little importance It has been shown that the rate constants for these reacin the SO,-0, system. l5 We may propose the hypothesis tions are all near the collision number. The above results that the relative slowness of energy transfer to oxygen confirm the order of the rates of quenching reported by from 3S02 in reaction 8b' is related to the relative unGreenough and Duncan' several years ago: k,, > k,, importance of charge-transfer states in the 02-3S02 > k'b. One can rationalize these results through the collision complex. picture proposed by Douglas" and Strickler and H ~ w e l l . ~ In our continuing research on the photochemistry of They have considered the 'SO, state as a composite of sulfur dioxide, we are attempting to correlate the photoexcited singlet, triplet, and ground vibronic states. physical primary processes, delineated in this and our Presumably collisional relaxation is fast since the 'SO, previous work,' with the photochemical changes which molecule behaves like a highly vibrationally excited have been observed in sulfur dioxide containing systems.16 ground-state molecule. One expects from this hypothUnder study are the rate constants for sulfur trioxide esis that the efficiency of various gases for 'SO, quenching formation from the reactions of 'SO, and 3S02 species or vibrational relaxation would be related to the number with sulfur dioxide and oxygen, and those for sulfinic acid of degrees of internal freedom of the colliding molecule. and other products from the reactions of the 'SO, and the The observed near equality of the quenching rates for SO, 3 S 0 2species with the paraffinic and olefinic hydrocarbons. and CO,,which possess near equivalent number of internal degrees of freedom, and the somewhat lower rate for 0, (13) For references to the extensive original literature, see (a) C. S . with the smaller number of internal degrees of freedom Foote, S . Wexler, W. Ando, and R. Higgins, J . Am. Chem. Soc., 90, are trends consistent with this hypothesis. 975 (1968); (b) K. Kawaoka, A . U. Khan, and D. R. Kearns, J . Chem. Phys., 46, 1842 (1967). Perhaps the most surprising result from this work is the (14) H. Tsubomura and R. S. Mulliken, J . Am. Chem. Soc., 82, 5966 demonstrated unselective quenching of the 3S0, species. (1960). Oxygen is a less efficient quencher of %O, than sulfur (15) S. S . Collier, unpublished observations on direct singlet-triplet absorption in sulfur dioxide-oxygen mixtures. dioxide, and it is only slightly more effective than carbon (16) For a recent comprehensive review of the present literature dioxide. However, the very low absolute value of the concerned with the photochemical studies of sulfur dioxide in a variety (12) A. E. Douglas, J . Chem. Phys., 45, 1007 (1966).
Rao, Collier, Calvert
of mixtures, see P. Uroneand W. H. Schroeder, J . Environ. Sci. Technol., in press.
1 Quenching Reactions of Excited States of SO2 with 0 2 and COZ
