430
H. N.
Maier and F. W . Lampe
Heteronuclear Recombination of Chlorine and Iodine Atoms in the Flash Photolysis of Iodine Monochloride H. N. Maier and
F. W. Larnpe*
Department of Chemistry. The Pennsylvania State University, University Park, Pennsylvania 16802 (Received August 30 1972) Publ/cat/on costs assisted by The Petroleum Research Fund
The heteronuclear recombination of C1 and I atoms and the homonuclear recombination of I atoms have been studied by spectrophotometric observation of the IC1 reformation and 12 formation subsequent to the flash photolytic dissociation of IC1 in the presence of C02 and N2. A comparison of the specific reaction rates for heteronuclear and homonuclear recombination with CO2 as the third body strongly suggests that the radical complex mechanism proceeding uia 1 4 0 2 and CI-CO:! complexes is dominant.
Introduction Flash photolysis studies of bromine1-8 and iodine1-"15 have resulted in the establishment of absolute rate constants for the homonuclear recombination of Br and I atoms with a variety of substances acting as third bodies. In addition, a number of other kinetic studies have been reported16-21 in which the specific reaction rate for the homonuclear recombination of C1 atoms was evaluated. No such fundamental data appear to be available with respect to the heteronuclear recombinations of halogen atoms that are of major importance in the chemistry of a mixture of two C12)22923 or in the decomposition halogen gases ( e . g . , €31.2 of a heteronuclear halogen ( e . g . , ICl).24 The present paper describes studies of the flash photolysis of IC1 in which the specific reaction rate of the heteronuclear reconibination of C1 and I atoms was evaluated from spectrophotometric observation of the formation of I2 and reformation of IC1 subsequent to the flash.
+
Experimental Section The flash photolysis system was of conventional design. Two 10-pF, high-voltage, fast-discharge capacitors (Bonar, Long and Co., Ltd), connected in parallel, were discharged through two FPA-8-100 xenon flash lamps which were obtained from the Xenon Corp. Either a multiplepass photolysis cel125,2'jor a lO-cm, single-pass photolysis cell was mounted coaxially to the lamps, and both lamps and the cell were mounted in an elliptically shaped aluminum holder. A 200-W General Electric quartz-iodine lamp (6.6A/T4/CL), powered by a Tabtron power supply, was used to supply the analyzing light for the spectrophotometry. Collimating lenses, a Gaertner prism monochrometer, and a 1P28 RCA photomultiplier tube completed the optical train. The intensity of the analyzing light transmitted by the photolysis cell a t the selected wavelength (470, 5:!0, or 570 nm) was followed as a function of time after the flash by displaying and photographing the output of the phototnuitiplier tube on an oscilloscope. The concentrations of l2 and IC1 were followed spectrophotometrically a t 520 (or 570) and 470 nm, respectively. In the photolyses of ICI, filter solutions of Cu(NH3)42.+.27 were placed around each flash lamp in, order to reduce the scattered light (from the intense flash) at, the detection wavelengths of 470, 520, and 570 nm to acceptable levels. In experiments in which 12 was photodissociated in the The Journal of Physical Chemistry, Vol. 77, No. 4, 1973
presence of ICI, filter solutions of KzCr207 28 surrounded the flash lamps. This filter effectively prevents light from the flash lamps of X 95%, was purchased from Alfa Inorganics. It was purified by the method described by Cornig and K a r g e ~ degassed ,~~ on a vacuum line at -195", and distilled into a storage vessel equipped with a greaseless Vlrestef stopcock. The entire purification procedure was carried in a dim red light. The storage vessel was wrapped with aluminum foil and stored in the dark. Analytical Reagent Grade 1 2 was obtained from J. T. Baker Chemical (:o. Prior to each use it was degassed a t -195". Nitric oxide with a stated purity of >98.5% was obtained from Matheson Gas Products and was further purified using the method described by Kohout and Lampe.31 Research Grade ethylene, having a stated purity of >99.9%), was obtained from the Phillips Petroleum Co.; nitrogen and carbon dioxide were obtained in high-purity from Matheson Gas Products. All three of the latter gases were used as resewed. The magnitude of thermal effects resulting from a heating of the gas by the exothermicity of the reactions and a subsequent cooling of the gas a t the vessel walls was examined for all photolyses by the method of Willard, et al.1 It was concluded that negligible temperature rises of 1-2" occurred in the 1, photolyses. In the IC1 photolyses an average temperat ure rise of 25 i 5" took place, but the cool-
(1)
(2) (3) (4)
(5)
and it is in terms of this mechanism that we'shall consider our results. Figure 1 shows a typical plot of the concentrations of I2 and IC1 as a function of time after the photodissociation flash. The fact that the concentration of IC1 increases with time and does so in a manner quite similar to that of 12 indicates that (2) is playing only a very minor role under our conditions. This is, of course, to be expected in view of the much higher atom concentrations obtaining in the flash photolysis as compared with the low-intensity, continuous p h o t o l y s i ~ We . ~ ~ may, therefore, conclude that the rate equations describing the obsewations in Figure 1 may be written as d[I,l/dt =
h3(M)[MI
+ h,(lcl)[~c~lI
d[ICl]/dt = [I][Cl][ h,(M)[M] -d[I]/dt = Z(d[I,]/dt) -d[Cl]/dt = Z(d[Cl,]/dt
+
h,(ICI)[ICIJI
+ (d[lCl]/dt) + (d[lCl]/dt)
(6)
(7)
(8) (9)
where M = C02 or N2 Unfortunately we were unable to follow spectrophotometrically the formation of Cla in the presence of IC1, and the coupled differential equations do not lend themselves to tractable solutions. Hence, evaluation of k4, the heteronuclear specific reaction rate, must be done somewhat indirectly from the observdtions of the time dependence of [I4 and [ICl]. Values of k3(CU2) and k3(N2) are known from the literature14j32 and have been verified in our laboratory. However, k3(ICl) is not known and must be determined independently in an experiment in which ICI is present but in which the only reaction consuming I atoms i s (3). Accordingly, we flashed 12-ICl-CO2 mixtures, utilizing the K2C1-207 filter to prevent photodissociation of ICl, and followed the concentration of 12 as a function of time. Under these conditions, only (3) is occurring, and the instantaneous I atom concentration may be calculated from (30) J Cornig and R A Karges, lnorg Syn 1,155 (1939) (31) F C Kohout and F W Lampe, J Chem Phys 46,4075 (1967) (32) K E Russell and J Simons, Proc Roy Soc, S1.r A, 217, 271 (1953) The Journal ol Physical Chemistry, Vol. 77, No. 4, 1973
H. N. Maier and F. W. Lampe
432
:: 8.0
X
2 I
E!
-I
2.4-
X
s,
-a)0 EI
I
h
7.0
0
2.0-
E
m
E
1.6-
0
5.0
I
m
E 0
0
aJ m
1.2-
n
-l-4
Iy
N
0.80.4
-
L
0
LL-100
'0
200 300 400 500 600 time ( p s e c )
Figure 2. Second-order disappearance of I atoms by homonuclear recombination: P(CO2) = 306 Torr, P(iCI) = 8.8 Torr, P(l,) = 0.08 Torr. the measuremtmt of I2 concentration. The integrated rate equation for I atom consumption in this case is l/[II = l / [ I l o
+
+
21k((C02)[C02]
hdI2)1121
+
h,(lCI)[lCl]J1 ( I O )
A plot according to (10) of the data from a typical flash experiment is shown in Figure 2. If one corrects the slope of the line in Figure !? for the known contribution^'^ from COz and 12, one obtains h3(ICl). The average value found (300°K) from 1I such cixperiments is
I
I
I
I
I
I
I
100 2 0 0 300 400 500 600 time (psec)
Figure 3. Kinetic plots for IC1 and 11 and 12: 0, [ICI]; 0 , [ I * ] .
12
formation according to eq
obeyed in the mathematically more complex situation that precludes analytic integration of (6)--(9). Differentiating (11) and (12), and referring to (6) and ( 7 ) , we obtain for the initial rates, where the subscript zero indicates initial conditions, the following (d[I,]/dt), = l/a
=
[Ilo21h,(M)[M]
+
h,3(ICI)[IC:111 (13)
(d[ICl]/dt), = l/g = [I]JCI],( h , ( M ) ( M ]
+
h.i(ICI)[ICI]J (14)
which, as expwted, is comparable to the value of 3.80 X l O - - 3 0 crr16 molwule sec 1 reported14 for h3(12). Referring again to Figure 1, and the formation of IC1 and 12 subsequent to the flash photodissociation of ICl, one may show that the time dependence of the concentrations may be written empirically as
when a, b, g, :ind Ii are constants. The validity of (11) and (12) and the evaluatian of the constants may be seen from Figure 3 in which t,'A[Iz] and t/A[ICl] are plotted as a function of t . The empirical constants evaluated from the slopes and intercepts of the straight lines are a = 0.57 x 1 0 - 2 0 cm3 sec molecule - 1 , b = 1.18 x 10--16 cm3 molecule- 1, g = 0.28 x 10 - 2 0 cm3 sec molecule--', and h = 4.48 x l W 1 7 cm3 inolecule~-l.It should be pointed out that (11) is the form of the integrated rate equation for 12 formation in t,he mathematically simple case in which I atoms react only by (3); and, since all atom combination reactions are a t least comparable in rate, it is not surprising to find the same functional form, (11) and (12), being The Journalot Physical C,'lemistry, Vol. 77, No. 4 , 1973
Dividing (14) by (13) and assuming that 1110 = j C l ] ~yields the result a/g =
1 h,(ICI) +
~,(M)([Ml/[ICIl)J/( hdICI)
+
h,(M)([Ml/[IC11)1 (15) Since a / g is determined experimentally (cf. Figure 3) and the denominator is known, we obtain easily values of h4(ICl) + k4(M)([M]/[ICl]) as a function of the concentration ratio [M]/[ICl]. It is then simply a matter of plotting the numerical values of the numerator of (15) us. [MI/ [IC11 to obtain hd(IC1) and h4(M). Such plots for M = NZ and M = CO2 are shown in Figure 4. The rate constants evaluated are shown in Table I where they may be compared with homonuclear recombination rate constants for I and C1 atoms. With respect to the efficiency of the product molecule of the recombination reaction acting as a third body or chaperon, IC1 falls intermediate between 12 and Cl2 as might be expected. It is a very efficient third body for the homonuclear recombination of iodine atoms, although not as efficient as 12, again as one might expect. The most surprising result shown in Table I is the much greater effi-
Heteronuclear Recombination of CI and I Atoms
433
TABLE I: Recombinatian Rate Constants for I and CI Atoms ___I
i @ ' k ( M ) , cm6 molecule
I+I+M
1 280 17 f l C
+12+M
I+CI+M +ICI+M CI + CI + M +GI? M
+
Reference 32 293°K
3 72" 43 IF This work 325°K
I
I u)
E
V
.-t
8
0
EICII Figure 4. Dependence of heteronuclear recombination rate constant on (M]/[ICI]:0, M = N2; 0 ,M = CO2.
ciency of N2 and C02 in promoting the heteronuclear recombination as opposed to the homonuclear recombination of I atoms. This result may be rationalized in terms of the radical-molecule complex theory12.14,33J4 in which it has been showG4 that the third-order rate constant may be written as
where 3 E is the exoergicity of the reaction
M
-+
RM.
(17)
In general, chlorine atom bonds are stronger than iodine atom bonds and this would be reflected in a greater exoergicity for (17) when R = C1 than when R = I. Then, given essentially equal encounter frequencies for the reactions 1 I- lClM
and
5 5 4 ~ 0 5 ~
Reference 21 298°K
In
+
248 f 5OC 119 f 14c
12f01d
Reference 14 295'K
V 0)
R.
sec
380b
0 82b
*
2
-
-
IC1
+
M
(18)
1 + IM I2 + M (19) one would expect, in view of (16), a greater third-order rate constant for the heteronuclear recombination, (4), than for the homonuclear recombination, ( 3 ) . From stud-
ies of the temperature dependence of k3(CO2), Porter34 has estimated AE for the formation of the 1 4 0 2 complex to be 2.35 kcal/mol. Assuming the validity of (16) one finds that the reasonable value of' AE = 3.85 kcal/mol for the Cl-CO2 exoergicity will explain the difference in the values of k3(CO2) and kd(C02). A similar result would also apply to ks(N2) and k4(N2) although L E values for I-N2 are not known. On the other hand, it would be much less likely that CO2 and N2 would be 11 and 13 times more efficient, respectively, in deactivating ICl* than they would be in deactivating I2* which would be required in terms of the energy transfer theory.35 Excitation i n t h e Primary Products. With the filter employed in the IC1 photolyses there is insufficient energy in the absorbed light quanta to produce I(2P1,2) atoms. It is possible that C1(2P1,2) is formed in the primary photodissociation, but, in view of the rapid collisional spin conversion to C1(2P312),36it is most probable that the excited C1 atoms will have relaxed to the ground state before combining with I atoms. Reaction of A t o m i c Chlorine w i t h Iodine Monochloride. Christie. Roy, and studied the low-intensity, steady-state photolysis of iodine monochloride. From their initial photodecomposition rates and the available data a t that time relative to k 4 , they were able to derive values of k2 at 30 and 60", provided the assumption that k3 = k5 = Mk4 was valid. Inspection of Table I shows that this assumption is not a useful one in this system. We have, accordingly, recalculated k2 from their results24 utilizing the data shown in Table I and assuming that over the temperature range of 30-60" the recombination rate constants are temperature independent. The result of this recalculation is that k2 is about doubled at each temperature, being 3.2 X 10-16 cm3 molecule-I sec--l at 30" and 6.5 X 10-.IGcm3 molecule- sec .. a t 60". Knowledge of h 2 permits us to assess more quantitatively our conclusion that (2) is playing only a very minor role under our conditions. From mechanism (1)--(5),one may write for the probability of occurrence of' @), the relation P ( 2 ) = h,[lCll/(h,[ICl] = 1/{1
+
+
h,[MllIl
+ h,[M][Clj)
(~JMI[II/~~,[IC11) + (hslMl~Cll/h,[IC11)1 (20)
Substitution of the values of k2, k4, and k5 into (20) and consideration of the fact that -10% of IC1 is decomposed in the flash leads to the conclusion that for F'(2) to be as high as 0.1 some 95% of the original chlorine atoms and iodine atoms have disappeared. Thus (2) can become sig(33) 0. K. Rice. J. Chem. Phys., 9, 258 (1941). (34) G. Porter, Disc. FaradaySoc.. 33. 197 (1962) (35) H. B. Palmer and D. F. Hornig, J. Chem. Phys., 26, 98 (1957). (36) R. J. Donovan. D. Husain, A. M. Bass, W. Braun. and D. D. Davis, J. Chem. Phys., 50, 4115 (1969). The Journal of Physical Chemistry, Vol. 77. No. 4 , 1973
D, R . Dice and R. P. Steer
434
nificant only in the very last stages ( t > 300 psec) of the iodine and iodine monochloride formation shown in Figure 1.
served. These attempts failed because in all three cases a very rapid dark reaction occurred, consuming the iodine monochloride.
Effect of Additives. We attempted to examine the effects of nitric oxide, oxygen, and ethylene on the flash photolysis to see if atom-scavenging reactions could be ob-
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
Primary and Secondary Photolytic Processes in the Photodecomposition of Thietane Vapor
. Dice and R. P. Steer* Besartment of Chemistry and Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada (ReceivedJuly 14, 1972) Publication costs assisted by the University of Saskatchewan
The photolysis of thietane vapor a t 254 and 313 nm and a t temperatures between 22 and 200" has been examined. The only primary photodecomposition forms ethylene and thioformaldehyde. Thioformaldehyde monomer is long lived and undergoes secondary photolysis to produce propylene in a sequence of reactions involving both the photoexcited monomer and a diradical. Photolysis of thioformaldehyde yields Hz with a quantum yield 50.08 a t temperatures less than 200" and is therefore much more resistant to photodecomposition than its oxygen analog.
Extensive investigations have yielded valuable information concerning the mechanism of the primary processes involved in the pklOtQlySiSof the cyclic ketones,l the pyrazolines,2 the cyclic ethers,3 and the cyclic sulfides.4 In particular, the quesl ion of &radical us. molecular decomposition pathways has been thoroughly studied and, in the cases of the cyclic ketones and the pyrazolines, tentative identifications of the excited states involved have been made. For the cyclic ethers and sulfides, however, the nature of the excited states and the photodecomposition pathways are much less certain. In part this is due to the added complication that one or more of the primary products may absorb light strongly a t the photolysis wavelength. For the particular case of the cyclic ethers, some evidence that secondary photolysis may be important has been obtained ~3 The carbonyl-containing fragment produced in the primary step apparently undergoes secondary photodecomposition with the production of several additional products. revious reports of the photolysis of thietane4b vapor indicated tbac ethylene is a major product, likely arising via the molecular scission of an excited molecule. (1)
Other hydrocarbon products, cyclopropane and propylene, were thought to arise from the reaction of unidentified intermediates with the substrate. It was necessary to postulate the existence of a t least four different excited states and three additional intermediates to explain the results The Journal o f Physical Chemistry, Vol. 77, No. 4, 1973
obtained. No secondary photolysis processes were reported. In this paper evidence that propylene is not a primary product of the photolysis of thietane vapor in its lowest energy absorption hand is presented. ather, it is demonstrated that propylene is very likely produced as a result of the secondary photolysis of monomeric thioformaldehyde. Conclusions regarding the nature of the likely primary processes in the photolysis of thioformaldehyde are also presented. Experimental Section Materials. Thietane (Eastman) was purified by preparative gas chromatography on a 10-ft dinonyl phthalate column. NO (Matheson) was purified by passing it through a 5-ft silica gel column a t -78" followed by degassing under vacuum at -210". It was distilled from (1) (a) H. A. J. Carless and E. K. C. Lee, J. Amer. Chem. Soc.. 94, 1 (1972);(b) J. Metcalfe and E. K . C. Lee, ibid.. 94, 7 (1972);(c)J. C. Hemminger, C. F. Rusbult, and E. K. C. Lee, ibid.. 93, 1867 (1971);(d) T. F. Thomas and H. J. Rodriguez, ibid., 93, 5918 (1971);(e) W. C.Agostaand A. B. Smith, ibid., 93,5513 (1971). (2) (a) E. 6 . Klunder and R. W. Carr, Jr., Chem. Commun.. 742 (1971);(b) 5 . S. Nowacki, P. B. Do, and F. H. Dorer. J. Chem. Soc., Chem. Commun., 273 (1972);( c ) J. J. Gajewski, A. Yeshurun, and E. J. Bair. J. Arner. Chem. Soc.. 94,2138 (1972). (3) J. 5 . Margerum, J. N. Pitts, Jr., J. G. Rutgers, end S. Searles, J. Amer. Chem. Soc.. 61,1549 (1959). (4) (a) A. Padwa and R. Gruber, J . Org. Chem.. 35, '1781(1970);(b) H. A. Wiebe and J. Heicklen. J . Amer. Chem, Soc.. 92. 7031 (1970);( c ) S.Braslavsky and J: Heicklen, Can. J. Chem.. 49, 1317 (1971);(d) W, E. Haines, 6. L. Cook, and J. S. Ball, J. Amer. Chem. Soc.. 78,5213 (1956).
