Kinetics study of the reaction of hydroperoxyl radical with sulfur


Nov 1, 1973 - Paul T. Roberts , Sheldon K. Friedlander. Environmental ... W. Hack , A. W. Preuss , F. Temps , H. Gg. Wagner , K. Hoyermann. Internatio...
0 downloads 0 Views 744KB Size


7614

A Kinetics Study of the Reaction of HO, with SO, and NO” W. A. Payne,lbt2L. J. Stief,2aand D. D. Davis* ,b,3 Contribution f r o m the Astrochemistry Branch, Laboratory f o r Extraterrestrial Physics, NASAIGoddard Space Flight Center, Greenbelt, Maryland 20771, and the Chemistry Department, University of Maryland, College Park, Maryland 20742. Received April 11, 1973 Abstract: Using the photochemical competitive isotope labeling technique, rate constants have been determined for reaction of the hydroperoxyl radical with the atmospheric gas SOZ. All measurements were made relative to the disproportionation reaction HOz €IO2-,H202 0 2 . At 300°K the rate constant for reaction of SOz with HO? was found to be (8.7 + 1.8) x 10-16 cm3molecule-’ sec-’. The quoted uncertainty reflects the uncertainty in the measurement of the rate constant ratio k2/(k7)”?. Because of existing uncertainties in the value of k;, however, the possible range of values in k z is 8.2 x to 1.5 X loeL5cm3molecule-’ sec-I. Also using the photochemical 1 8 0 2 competitive isotope labeling technique, it has been possible to estimate the value of the rate constant for reaction of HO? with NO. The rate constant for this system, k 3 ,was found to be 3 X 10-l3,ma molecule-’ sec-’ with the uncertainty in its value being a factor of *3. The possible importance of secondary reactions in the determination of k2 and k3is discussed.

+

+

e have previously used the photochemical ‘*OZ competitive isotope labeling technique4 to study the important reaction

W

HOz

+ CO +COz + OH

combined with the atmospheric processes 4 and 5 , is H

+ + +

+ +

OH CO +COz H 0 2 Nz +H01 Nz

(4) (5)

(1)

seen to be the chain conversion of SO2 to SOa and NO In that study, we found reaction 1 to be quite slow with to NOZ. The SO3 formed in reaction 2 would than be expected to react with H 2 0 (probably in a heterothe data yielding an upper limit for the rate constant for this process of k, = 1.5 X cm3 molecule-’ ~ e c - ’ . ~ geneous process) to form the familiar sulfuric acid aerosol. O n the other hand, the formation of NO, from These results clearly showed that reaction 1 cannot be reaction 3 would give rise to the photochemical producconsidered as an important process for the conversion tion of atomic oxygen which, in turn, could result in of CO to CO, in either the stratosphere or the tropothe production of O3and the further oxidation of a wide sphere of the earth. It also is now apparent that this assortment of hydrocarbons. reaction will be of negligible importance as a means of Since the mechanisms for conversion of NO to NO, maintaining the stability of the COz atmosphere on Mars.’ and SOz to SOaare now considered to be of major importance both in the natural and perturbed atIn this study, we have further explored the possible mosphere, we have undertaken a kinetic investigation important reactions of the hydroperoxyl radical with of reactions 2 and 3. The technique employed in this two additional atmospheric trace gases, SO, and NO. study has been the photochemical competitive The most likely reaction in both cases would consist of isotope labeling method. All measurements were the abstraction of an 0 atom from the HO, species with carried out at 298 “K. the production of the more reactive OH radical, i.e.

+ SOz +SOI + OH H 0 2 + NO +NOz + OH HOz

AH,,

=

-19 kcal/mol

(2)

Experimental Section

AH,,

=

-10 kcalimol

(3)

A schematic drawing of the photolysis system is shown in Figure 1. The gas handling system utilized in these experiments consisted of a conventional high-vacuum line equipped with high-vacuum Teflon-seated valves. The system was maintained H g free. LOW pressures (1 mTorr to 1 Torr) were measured with a 1-Torr Granville-Phillips capacitance manometer head: high pressures were measured with either a 200-Torr Granville-Phillips or a 0-760-Torr Wallace and Tiernan gauge. The reaction cell in all experiments was a 500-cm3 spherical Pyrex vessel equipped with a suprasil window which transmitted radiation at wavelengths >1650 A. The window was attached to the cell with Varian “Torr Seal.” The light source used throughout this study was a microwave powered mercury resonance lamp. The output of the lamp was examined on a McPherson 112 meter vacuum monocbromator and found to emit 98% of its radiation at 1849 and 2537 A with the intensity of the two lines being approximately equal. All other emission- lines from the lamp ( 2 % of total) were at wavelengths > 2537 A. Calculations based on known extinction coefficients7

An examination of the literature has revealed that for only one of these reactions, (3), has a n experimental measurement been made, and this measurement was of an indirect nature.6 The possible importance of reactions 2 and 3, when (1) (a) The results from this research were first reported at the Symposium o n “Sources, Sinks, and Concentrations of CO and CHI in the Earth’s Environment,” a joint meeting of the AGU and AMS, St. Petersburg, Florida, Aug 1972. (b) The research reported on here is part of a thesis to be submitted to the Graduate School of the University of Maryland in partial fulfillment of the requirements for the Masters degree in Chemistry. (2) (a) NASAiGoddard Space Flight Center; (b) University of Maryland. (3) This author would like to acknowledge the Climatic Impact Assessmcnt Program, Office of the Secretary, Department of Transportation, and the National Science Foundation RANN Program (Grant No. GI-363338X) for support of this research. (4) D. D. Davis, W. A. Payne, and L. J. Stief, Science, 179, 280 (1973). ( 5 ) M. B. McElroy andT. M. Donahue, Science, 177,986(1972). (6) H. S. Johnston and Zafonte, work cited in National Bureau of Standards Report No. 10447.

Journal o f t h e American Chemical Society 1 95:23

1 November

(7) (a) I<. Watanabe, M. Zelikoff, and E. C . Y. Inn, Air Force Cambridge Research Center Technical Report No. 53-23 (1953); (b) N. Washida, Y. Mori, and I. Tanaka, J. Chem. Phys., 54,1119 (1971); (C) D. Golomb, K. W. Watanabe, and F. F. Marmo, {bid.,36,958 (1962); (d) B. A. Thompson, P. Harteck, and R . R. Reeves, Jr., J. Geol. Res., 68,6431 (1963).

14, 1973

for the gases HzO, CO, O?, Nz, and N O showed that none of these gases or any of the possible products from this system (e.g.,COp and HzOJ absorbed 2537-A radiation significantly at the pressures used. Although SO2 does absorb significantly at 2537 A ( k = 5 cm-I atm-1),7d control experiments discussed in the next section showed that this does not lead to formation of the products C1602 or C16,1802.In theseoexperiments, therefore, no attempt was made to block the 2537-A line. Actinometry at 1849 A was carried out using two methods. In the first approach. measurements were made on the amount of COz produced when 20 Torr of HzOwas irradiated in the presence of 1 to 3 Torr of CO. This method assumes the quaatum yield for dissociation of HPO into H and OH is unity at 1849 A and that there are no loss mechanisms for OH other than reaction with CO to produce Cot. The second method involved the photolysis of N20 at 1849 A where the quantum yield for NP production is 1.44.8 The results of both methods agreed to within 10% and a value of 3.5 X 1015photons/cm*/sec was obtained for the intensity of the lamp. The 1 8 0 z (9973 used in this study was obtained from Miles Laboratories, Inc., in 100-cm: break-seal bulbs. A mass spectrum analysis showed that the major impurities in the gas were C1802 and C168180P.These impurities were removed by passing the OP through a liquid NP trap filled with glass beads. Subsequent analysis showed this to be an efficient method of removing the COP impurity. Carbon monoxide and Nz were obtained from the Matheson Co., Inc. The nitrogen (99.999z purity) was used without further purification; traces of iron carbonyl were removed from the C O with a liquid NPtrap filled with glass beads. The SO? was obtained from Baker Chemical Co. After degassing at - 196”. mass spectrum analysis showed the impurity level to be <0.05%. The gas NO was Matheson Co. chemical grade; it was purified by first degassing at -196” followed by distillation from a trap at -183“. A mass spectrum analysis of the purified gas indicated that the NOz level was <0.1%. In a typical experimental run, the procedure was to prepare the reaction mixture (Ht0, CO, Oz. Nz, and SOP), allow 10-15 min mixing time, and then commence irradiation of the mixture with the resonance lamp for a period of 1 to 3 min. After the desired irradiation time, the condensable products and reactants were trapped a t -196” and noncondensable products and reactants pumped away. The products and reactants remaining were then allowed to warm to room temperature and subsequently the cold finger on the photolysis vessel was reduced to -130” using an n-pentane slush. At this temperature C02, which was the major product of interest in this study, has a vapor pressure of 2 Torr, SOP has a vapor pressure of 5 mTorr, and all of the remaining products and reactants have vapor pressures of < 1 mTorr. Thus, the COP produced in the reaction was measured without significant interference from other gases. The COP product was then transferred to a removable collecting vessel and attached directly to the inlet of the CEC 21-620 mass spectrometer for isotopic analysis. In all experiments reported on in this study, the source of H atoms was the photolysis of 20 Torr of HzO: Based on the extinction coefficient7aof 1.5 cm-l atm-l at 1849 A , it was calculated that 30% of the incident radiation at this wavelength would have been absorbed.

Results and Discussion As was described in an earlier publication from our laboratories4 the photochemical 180z isotope labeling technique involves a measurement of the relative rates of reaction of HOz with a known reactant (in this case either SOs or NO) us. HOz itself. In this system, therefore, the extent of each reaction is given by the rate of formation of C16s1802us. CI6O2 (see reaction schemes 6, 4, 5 , 2, 4’, 7, and 8). The results from the SOz experiments are shown in Table I. Not included in Table I are the results from several control experiments. These included : experiments in which H 2 0 was left out of the photolysis mixture and the SO, pressure varied from 0.2 to 0.5 Torr; experiments in which the Hg resonance lamp was left off and the mixture allowed to stand for a period of 20 min; (8) M. Zelikoff and L. M. Aschenbrand, J . Chem. Phys., 22, 1680 (1954).

-

7615

High Vacuum System 0-250 torr Pressure Ow91

0-1 torr Pressure Gouge

.... .

IS

.1

Spec Collecting Vtssel

” .-O-ring . loin1

Used in the 1849 A Photolysis of Mixtures O f H2Ot CO, oLa’’r, and N2 ( A r ) .

*ppWpS

Figure 1. Photolysis apparatus used in the ‘ 8 0 2 competitive isotope labeling technique involving kinetic studies on the reaction of

Hop. Table I. Reaction of HOz with SOza

sot,mTorr 82.5 82.5 82.5 109.0 175.0 175.0 250.0 325.0 400.0

CO, Torr 1.o 1.o

1.o 1.o 10.0 10.0 10.0

10.0 10.0

Nz, Torr 18.0 18.0 18.0 18.0 9.0 9.0 9.0 9.0 9.0

Carbon dioxide yield, molecules ~ 1 1 sec-l 1 ~ ~ x 10-12 Cl6O2 C1G~180~ 12.1 13.1

12.8 13.1 11.5 15.7 11.8 10.2 11.8

3.7 5.3 2.2 5.8 7.0 9.9 12.8 15.4 20.2

a Pressure of reactants: H t 0 = 20 Torr: 1 8 0 2 = 1 Torr. Photolysis time: 1-3 min. Intensity at 1849 A : 3.5 X l O I 5 photons/ cm*/sec.

experiments with CO left out of the photolysis mixture; and finally, experiments in which a Corning 7-54 filter was used between th,e Hg lamp and the reaction cell to eliminate the 1849-A Ijne while transmitting approximately 4 0 z at 2537 A . In all these control experiments less than 1 mTorr of C o n was detected. This is to be compared with a yield of 40 mTorr of C o n , the smallest amount of CO, formed in the experiments reported in Table I. The most noticeable features of the data presented in Table I are the steady increase in &!16,1a02 with increasing amounts of SOn and the reasonably constant Rc1602. Over the range of 82.5 to 400 mTorr of SOn,the ratio of Rcis,Iso,/Rci~,changes from approximately 0.3 to 1.7. This qualitative observation would indicate that HOz is being formed under our experimental conditions and that its reaction with SOn does occur at a measureable rate. The relatively constant RclEo, indicates that, under the conditions of the experiments in Table I, addition of increasing amounts of SOn did not lead t o loss of O H and that all available OH radicals were quantitatively converted to COz. Experiments at 0.3 Torr of SOz (HzO = 20 Torr, 1802 = 1 Torr) further showed that while RCjEo2 increased with an increase in

Payne, Stief, Davis

A Kinetic Study of the Reaction of HO? with SOYand NO

7616 'I

I

I

I

I

I

Table 11. Variation of Light Intensity and l8O2Pressurea

I

x 10-15, photons/ cmZ/sec

kp X 10'6, cm3 molecule-' sec-

I"

1S02,

~~

Range of SOa pressure, mTorr

Torr ~

~~

~

1 .o 2.0 1 0

3.5 3.5 14.0

-~

8.7 f 1.8b 8 1 2 12 Zk 4

82.5-400 140-400 25M00

~

Pressure of reactants: HzO = 20 Torr, CO = 1 Torr, N z = 18 Torr. Photolysis time: 1 to 4 min. See Table I and Figure 2. a

I

I

I

I

I

I

2

4

6

8

IO

12

CSOzl

MOLECULES/CM3 X

Figure 2. A plot of the ratio, R C I R . I ~ ~ ~ ~ ( RasCa~function E ~ ~ ) ' / of ~, the SOz concentration where Rcii.lso, and RCl60, are the rates of formation of the photolysis products C 1 6 ~ * 8and 0 p C160p in units of molecules cm-3 sec-1.

CO pressure from I to 10 Torr, there was no change in RC160? in the presence of 19 Torr of CO. This further indicates complete scavenging of OH at the pressure of CO employed. To treat the data presented in Table 1 quantitatively, a similar approach was used as reported earlier for the HO, CO system, In this case, the following reaction scheme was employed.

+

k6 + IIY + H + 'BOH '60H + C'60 C"Oz + H ka H + + M 9 H'SOz + M (M = Nz, HzO) H'SOZ + +S'6,16"80~+ "OH 'SOH + Cl60 +C'6s1sOz+ H H'80p + +Ht'SOz + ks + HzO +HzSO,

HzO

(6)

k4

----f

'802

(4) (5)

i;?

S'602

(2)

li0

(4')

kl

HI802

'802

S16,'6,'803

(7)

(8)

The photochemical decomposition of H 2 0 z has been neglected in the above reaction scheme since its contribution to the formation of C16,1s02 is very small. Also the reaction HO2 CO + COz OH has been omitted from the above scheme due t o its extremely small rate constant < cm3 molecule-1 sec-I. As discussed above, the labeled CO, produced in reaction 4' is a direct measure of the extent of reaction 2. From a steadystate treatment of reactions 6, 4, 5 , 2, 4', and 7, the following relationship may be derived

+

+

where R c ~ and ~ , R ~ ~ c~are ?~ the~ rates ~ of~ formation of Journal of the American Chemical Society

1 95:23

labeled and unlabeled CO, in units of molecules cm-3 sec-I. From the mathematical expression above, a plot of the ratio R(C 16,1s02)/(R(C1602))'/2 against [SO,] should yield a straight line with the slope equal t o the rate constant ratio k2/(k7)'/'.Figure 2 shows the results of such a plot. For those pressures where there were two or three separate experiments, the point shown represents the average value of R(C16*1802)/(R(C160~))i/2. The line drawn through the points is based on a weighted least-squares treatment of the data. From the slope and using the preferred valueg of k7 = 3.3 X lo-'* cm3 molecule-' sec-', we obtain the result kz = (8.7 f. 1.3) x 10-I6 cm3 molecule sec-'. The quoted error limit was obtained by assigning an experimentally determined uncertainty (*l5x) to each value of R(C16t1SO)/ (R(C1602))'/' X in Figure 2 (based on uncertainties in measurements of the heights of the mass 46 to 44 peaks and the time of photolysis) and by drawing lines of maximum and minimum slope through the indicated error bars. Although a slight negative intercept is observed in Figure 2, within the experimental uncertainty of these measurements this intercept could also be given a small positive value. The latter case is what would be expected since a very small amount of C16j1*02 would have been formed via the photolysis of the product Hz "Oz. In addition t o the experiments reported in Table I and Figure 2, a study was made of the effect of increasing the pressure of and the effect of increasing the light intensity. These experiments were each done at three different SO, pressures and the data treated quantitatively as described above to derive values for k,. Good linear plots of R(C16~180~)/(R(C1G0~))1/2 us. [SO,] were obtained. The results of these experiments are summarized in Table 11. It may be seen that within the experimental error, increasing the '*Ozpressure from 1 to 2 Torr or increasing the light intensity from 3.5 X 10'j to 1.4 X 1016 photons cm-, sec-I has no effect on the value derived for the rate constant, k,. In addition to the reaction scheme 6, 4, 5, 2, 4', 7, and 8, several other processes must also be considered for their possible influence on the observed ratios of R c ~ ~ ~ ~ ~ ~Many ? / R ofc ~these ~ o ~reactions . have already been discussed in an earlier publication* and, thus, will not be treated here in any detail, i.e.

+ h v (1849 A)

'SO2 "0

+ "0

CIS0

+ M --+ C'6

+ H'sOz

H + H"0p H ' 8 0 3

+

2180 l802

+M

+"OH + ' 8 0 2 +'SOH + "OH

+"OH

+ "Oz

( 9)

(10)

(1 1) (12) (13)

(9) (a) A. C. Lloyd, National Bureau of Standards Report NO. 10447, 1971; (b) D. L. Baulch, D. D. Drysdale, and A. C. Lloyd, "Evaluated Kinetic Data for High Temperature Reactions," Vol. 1, Butterworths, London, 1972.

Nouember 14, 1973

7617

Suffice it to say that in all cases in which SO2 was absent from the reaction mixture, processes 9-13 could account for at most 6 z of the C16,1802 formed, and this number is more likely t o be < 2 z . With SO, added to the system, a large number of new reactions become possible, e.g.

+ SO2 + M +HS03 + M S16G + h v (1849) -+- S l 6 0 + I6O + Sl602 + M s'600s+ M OH

(14) (15)

I6O

(16)

--f

160

H

+ BO2 + M -+- 1 6 , 1 S , 1 8 0 3

+ 1 6 , 1 8 , 1 8 0 3 +180H +

(17)

16,1802

(18)

+ SO2 + M +HSOa + M + CO + M ---tHCO + M SO2 + /ZY (2537) +SOz*' +/ZV + SO, H

(19)

H

(20)

soz* +so2* so2*3+ iao2+s i 6 , 1 6 , 1 8 , 1 8 0 ~ H

(21) (22)

3

1

(23)

+ S16,16,18,1801+S16816,1803+ 180H

+ C'60 *C'6,1802 + SI602 + H -+- 'SOH + S1602

S16.l6.18O3

S16,16,1803

(24) (25) (26)

Qualitative evidence for the possible importance of reaction 14 was the observation that, at low CO pressures (1 Torr), addition of higher pressures of SO, (>0.2 Torr) led to a decrease in R C I G ~These ?. data are summarized in Table 111. From a steady-state treatTable HI. Reaction of O H with Sop R q O 2 molecule . (3177-3 sec-1

x

SOz, mTorr 0 250 325 400

10-13

5.2 3.7 3.0 2.6

a Pressure of reactants: H 2 0 = 20 Torr, CO = 1 Torr, l80z = 1 Toy, N2 = 18 Torr. Photolysis time: 1 min. Intensity at 1849A: 1.4 X 10lsphotons c m P sec-1.

ment of reactions 6, 4, and 14, the following relationship may be derived

where ROCO? is the rate of formation of C1602in the absence of SO2, and RcO,is the rate in the presence of SOe. Thus, a plot of (RO - R)/Rous. S 0 2 / C 0 should give a straight line with zero intercept and a slope equal to kI4(M)/k4. The data in Table I11 were used t o prepare such a plot. From the slope of 1.4 and a value of k4 = 1.3 X IO-l3 cm3 molecule-' sec-',l0 we obtain the result k14(M) = 1.9 X cm3 molecule-' sec-I. For our system, this implies k,, = 1.5 X cm6 molecule-2 sec-' for M = N2 H,O (18 and 20 Torr, respectively). The only value for kli that we are aware of in the literature is that calculated by Wheeler l 1 from measurements by McAndrew and Wheeler" on the effect of SO2 on recombination rates in propane-air flames. Wheeler estimates kI4+,, = 1.1 X cmG molecule-2 sec-l at 2080°K. Fair and Thrush12 have

+

(10) F. Stuhl and H. Niki, J. Chem. Phys., 52,3671 (1972). . 66,229 (1962). (11) R. Wheeler,J. P h j . ~Chem., (12) R. W.FairandB. A.Thrush, Trans.FuraduySoc., 65, 1550(1969).

determined that k19 < 1.5 X cm6 molecule-2 sec-' at 298°K and that reaction 14 is the major contributor to the value for k14+,,deduced by Wheeler. Thus, values for kla at 2080°K obtained from flame studies and at 298" from the present work are both of the order of cm6 molecule-2 sec-'. More recently Davis and Schiff l have made preliminary measurements of OH SO, He -+ HSO, which gave k14= 2 X (ifactor 2) cmGmolecule-2 sec-I. It is expected that H,O would be at least 10 times more efficient as a third body than He. Both processes 16 and 17 involve the initial formatiqn of l6Ofrom the photodecomposition of Sl6OZat 1849 A, reaction 15. The rate of production of l60will b? significant since the SO, absorption coefficient at 1849 A is 100 cm-I atm-l.7~ However, using available rate constants for reaction 16 and 17,14,15 it is seen that the dominant reaction is 16 (e.g., kl6/kI7E 50). From the above arguments is also follows that reaction 18 can be discarded as unimportant. Because significant amounts of S 16,16,1903 could be formed via reaction 2 and 16, it is of some importance that reactions 25 and 26 be shown to be negligible compared with reaction 8. In this case, even though rate constants are not available for these three processes, arguments can be given which show reaction 8 to be the dominant reaction path. Evidence supporting this conclusion is found in the observation that aerosol was produced in the photolysis vessel for photolysis times of approximately 3 min (note: no quantitative data were taken at times longer than 3 min). This presumably would follow from reaction 8 and the subsequent condensation reaction between H,S04 and H 2 0 molecules. A final observation, which would tend t o rule out reaction 26 as significant, is the results from those experiments carried out at increased light intensity (Table 11). The argument here is that since reaction 26 involves two reactants, both directly dependent on the light intensity, the rate of reaction 26 should depend on the square power of lo. However, in experiments in which lowas increased by a factor of 4, no change (within the experimental uncertainty) was observed in the value of the rate constant k,. The sequence of reactions 21-24 represents still another mechanism which could have produced H'*0 and hence C1G,180z in this study. The important intermediate in this scheme is the yet unidentified species S 16,16,ls~lsOa. The SO4 species has previously been suggested as a possible important intermediate in the photooxidation of SOz in the atmosphere.16 In our system, if this reaction had been important, it would again be expected that the measured rate constant, kp, would have shown a significant dependence on the light intensity. As indicated in the previous discussion of reaction 26, no such dependence was observed even with a fourfold change in Io. The final set of secondary reactions which needs t o be considered in the SOZ-HO, system are processes 19 and 20. If important, they would have tended to make the observed rate constant too low. In this case, a com-

+

+

(13) D.D.DavisandR.Schiff, unpublisheddata. (14) D. D. Davis, R. Schiff, and S. Fischer (0

+

+

Son M), paper in preparation. (15) R. E. Huie, J. 7.Herron, and D. D . Davis, J . Phys. Chem., 76. 2653 (1972). (16) P. Urone and W. H. Schroeder, Enciron. Sci. Techno/., 3, 436 (1969).

Payne, Stief, Davis

1 A Kinetic Study ofthe Reaction of H 0 2 with SO2 and NO

7618

parison of the rate constants for reaction 5 with those of reactions 19 or 20 ( e . g . , k: = 5.4 X lo-,, cm6 molecule- sec- ',l7 k,, = 1.5 X 10- ,cm6 molecule- sec- ',1 2 and kso = 0.8 X cm6 molecule-2 sec-',I8 clearly shows that even with the maximum pressures of 0.5 Torr of SO, and 10 Torr of CO present, the rate of reaction 5 at 1 Torr of 0, would be some 60 times greater than that for reaction 19 or 20. In addition, the fact that the observed rate constant k , did not change when the pressure of l8OZwas increased from 1 to 2 Torr (Table 11) shows that reactions 19 and 20 were not important compared with reaction 5. As indicated in the introduction, n o previous measurements of the rate constant for reaction 2 have been reported in the literature. However, since a direct comparison with other values cannot be made, it is perhaps important that upper and lower limits t o kz be discussed in terms of the reference reaction 7. An examination of the l i t e r a t ~ r efor , ~ example, shows that although the preferred value for ki is 3.3 X cm3 molecule-' sec-', values for this rate constant range from a low of 2.9 X lo-', to a high of 9.5 X cm3 molecule-' sec-l. If one takes this range of values t o represent the upper and lower limits for the rate constant of reaction 7, then it follows that the reported rate constant for reaction 2 could have maximum and minimum values of 1.5 X 10-'5 and 8.2 X 10-lF c m 3 molecule- ' sec- ', respectively. The results from experiments involving the reactant NO, reaction 3, were somewhat more difficult to interpret than those from the H02-S02 system. These results, therefore, should be considered as only semiquantitative in value. In the investigation of reaction 3, initial experiments were carried out in which 50 to 100 p of N O was added to gas mixtures consisting of 20 Torr of HzO, 1 Torr of CO, 1 Torr of and 20 Torr of Nz. However, in contrast to the results from the S02-H02 system, the NO-HOZ system was found t o be very sensitive to addition of even small amounts of NO. Thus, in one experiment 50 mTorr of N O was added t o the above gas mixture with the result that the rate of production of CO, increased by about a factor of 5 and the ratio of labeled to unlabeled CO? was -3: 1. Also, the absolute yield of labeled CO, was nearly eight times greater than the amount of added NO, indicating that not only was N O reacting very rapidly with HO, to form the product NO,, but that the NO, was being converted back to NO. The most likely reaction t o explain the latter observation would be process 27, Le. H

+ N1'.180Z --+

N"0

+

" 6 0

+ "OH + '*OH

(27)

In subsequent experiments to minimize the possible reaction of H with NOz, the pressure of N O was reduced to 1 mTorr and the light intensity was reduced by a factor of 20 (photolysis time = 1 min). In this case the maximum amount of NO, that could have been present was 1 mTorr. Thus, the rate of H atom removal Diu reaction 5 would have been -5 times faster than by reaction 27 (k2i = 4.8 X lo-" cm3 molecule-' ( I 7) W. Wong and D. D . Davis, Int. J . Chem. Kine?.,in press. (18) J. J. Ahumada, J. V. Michael, and D. T. Asborne, J . Chem. Phjas., 57,3736(1972).

Journal of the American Chemical Society 1 95:23

1 November

sec-'; ks = 4.3 X cm6 molecule-' sec-l for M = H,O), 19,20 Since the kinetic expression for the ratio of R C 1 6 , l 8 O 2 / [RC1602]1'2 with N O as the reactant would be of the same form as the equation for the reactant SOz, it follows that when

then k2(SO2)= k,(NO). Under the conditions outlined above (1 mTorr of N O and photolysis time = 1 min), it was found that

and the ratio of labeled t o unlabeled COz was 1.4. An examination of the SOz data shows that at an SO2 concentration of 11.7 X 10': molecules cm-, a similar Using the value for R C NM O ~ / [ R C I ~was O ~ ] ~measured. /? expression k2(SOZ) = ks(NO), a value of k 3 is thus calculated of -3 X cm3 molecule-' sec-I, the uncertainty being at least a factor of i3. A comparison of the rate constant reported in this work with the only other number published in the litercm3 molecule-' sec-') shows a disature6 ( k , 1: crepancy of nearly two orders of magnitude. Other values for k 3 , which are at this writing unpublished, are those reported by Niki2' and Simonaitis and Heicklen.?, NikiZ1has an indirect measurement which suggests a value of -4 X 10-13 for k , ; whereas, Simonaitis and Heicklen in a photochemical study have given a value of k3 2 1.5 X lo-', cm3 molecule-' sec-'. It would appear, therefore, that all recent measurements of kaare in reasonably good agreement.

Summary and Conclusions Using the photochemical 'SOz competitive isotope labeling technique, rate constants have been determined for reaction of HOz with SOz and NO. At 300°K the rate constants for these two processes were found t o be 8.7 x and 3 X lo-', cm3 molecule-' sec-', respectively. The reasonably fast rate constant found for the HOz-NO system would clearly indicate this process as being of major importance in the net conversion of N O to NOz in the lower atmosphere and the dominant reaction of the HOz species. On the other hand, the rate constant found for reaction 2 would initially indicate that this process is probably only of minor importance in the conversion of SOz t o SOa in the unperturbed as well as urban atmosphere. However, until more realistic estimates can be made of the HO, concentration, under widely differing meteorological conditions. the actual importance of this process must remain somewhat uncertain. In the lower stratosphere the rate of reaction of SO2with HOz would (19) R . F. Hampson, Ed., "Survey of Photochemical and Gas Kinetics Rate Data for Twenty-Eight Reactions of Interest in Atmospheric Chemistry," typescript, National Bureau of Standards, 1973, J . Phys. Chem. Ref.Data, submitted for publication. (20) Climatic Impact Assessment Program, Monograph NO. 1 , "The Natural Stratosphere," typescript, April 1973, prepared by the Scientific Panel on the Natural Stratosphere, Ft. Lauderdale, Florida. (21) H. Niki, private communication. (22) R . Simonaitis and J. Heicklen, report presented at the CIAP Workshop, National Bureau of Standards, Oct 1972.

14, 1973

7619

+

now appear t o be more significant than the SOz O(3P) M reaction in leading t o the conversion of SOs to SO3and sulfuric acid aerosol. The point should

+

+

also be made that based on these results the OH SO2 M reaction would appear t o be of major importance in both the troposphere and stratosphere.

+

Temperature Jump Relaxation Kinetics of the Chelation of Nickel( 11) and Cobalt( 11) with Some Aromatic Ligands’ Joseph C. Williams and Sergio Petrucci*

Contribution from the Polytechnic Institute of Brooklyn, Brooklyn, New York 11201. Received February 21, 1973 Abstract: The temperature jump relaxation kinetics of NiZ+ with anthranilate ion at 25” and salicylate and 5-sulfosalicylate ions at temperatures between 15 and 35’ and Coz+ with salicylate and 5-sulfosalicylate ions at 20” have been studied and the results reported here. In all cases the forward rate constant k f appears to be significantly smaller than that required by the “normal” Eigen chelation mechanism in terms of the product (l/S)Kok,, (with 1jS a statistical factor, generally equal to 0.2, KOthe Fuoss outer-sphere preequilibration constant, and k,, the solvent exchange rate constant determined from nmr measurements). For nickel(I1) sulfosalicylate, kr is the same, within experimental error, as for nickel(I1) salicylate. However, because of the extra charge of the sulfonate group and consequently larger KO,one would expect a much larger rate constant k f for this ion with respect to the salicylate ion. That this is not the case is suggested by the activation parameters AHf* and AS[* being comparable for both ligands. The preceding is reflected in measurements of complexation rates of Co2+with salicylate and sulfosalicylate ions, the rate constants being within a factor of 2 of each other. A discussion of the statistical reason for this similarity in the rate constants for the salicylate and sulfosalicylate ions is given in terms of charge delocalization with respect to the point of attack at the reaction site. For nickel(I1) salicylate comparison between AHex* and ASex with the quantities (AHf - AHo) and (ASr - ASO)(with AH0 and AS0 the outer-sphere activation parameters) suggests that the source of the deviation from the normal chelation mechanism is entropic.

*

*

*

T

he kinetics of chelation of ionic complexation reactions has been the subject of many discussions and investigations in the past.2 One may discuss the equilibrium between a divalent metal cation MSe2+(with S a solvent molecule) and a monovalent bidentate ligand (L-L)- in terms of the Eigen multistep mechanism

e

MSa*+,L-L- F? (MSa-L-L)+ k-2

e (M&(Z)+

kr

the chelate complex. By applying the rate equations t o scheme 1 and imposing a steady state condition on the intermediate, d(MSj-LL)+/dt = 0, with the additional requirement of the preequilibration of the first step, one obtains the relations3p4in terms of the overall (1) This work is part of thc thesis of Joseph C. Williams in partial fulfillment of the requirements for the degree of Ph.D., at the Polytechnic Institute of Brooklyn. Support by the IBM Corporation in the form of a graduate fellowship to J. C. W. is acknowledged. (2) (a) F. Bas010 and R. G. Pearson “Mechanism of Inorganic Reactions,’’ 2nd ed, Wiley, New York, N. Y.,1967, p 224. (b) R. G . Pearson and 0. P. Anderson, Inorg. Chem., 9, 39 (1970), and literature references quoted therein. (3) G. G. Hammes and J. I. Steinfeld, J. Amer. Chem. Soc., 84,

4639 (1962). (4) M. Eigen and L. DeMaeyer in “Techniques of Organic Chemistry,” Vol. VIII, Part 11, 2nd ed, S . L. Fries, E. S. Lewis, and A. Weissberger, Ed., Interscience, New York, N. Y., 1963, p 895.

Kok&3/(k3

+ k-z) +

(2)

(3) For an interchange dissociative process (Id), the second step in the above scheme will have a rate constant k2 equal to k,, (the one for solvent exchange) within a statistical factor5s6l/S /v 0.2.5r6 K Omay be calculated through the Fuoss relation’

k3

,L\

=

kr = k-&-3/(k3

k-2)

K O = (47rNa3/3000)ebe-bZu~( Ifza)

F-S

where MSe2+,L-L- is an outer-sphere ion pair, (MSjL-L)+ the metal ligand monodentate complex, and /

rate constants k f and k r .

(4)

where the symbols b = /zlzs/e2/aDkTand yi2 = e-oza~(l+xa) are7 the Bjerrum parameter and the activity coefficient according t o Debye and Hiickel. k,, is the experimental pseudo-first-order constant for solvent exchange as determined by nmr 1 7 0 line broadening.* It is apparent that in the majority of the cases, if (kalk-2) >> 1

kf = Kok2

=

(l/S)Kok,,

k r = k--2(k--3/k 3)

(5)

and the closing of the chelate ring is faster than the breaking of the first metal-ligand bond (“normal chelation”). Evidence of the converse condition (ks/k-?)<< 1 has ( 5 ) C. H. Langford and T. R. Stengle, Annu. Reo. Phys. Chem., 19, 193 (1968). (6) C. H. Langford and T. R. Stengle, Coord. Chem. Reu., 2, 349 (1967). (7) R. M. Fuoss, J. Amer. Chem. Soc., 80,5059 (1958). (8) T. J. Swift and R. E. Connick, J . Chem. Phys., 37, 307 (1962); 41, 2553 (1964).

Williams, Petrucci

1 Chelation of Ni2+ and Coz+ with Some Aromatic Ligands