228.8 nm Photolysis of 1,1,1-trichloroethane

Journal of Photochemistry

and Photobiology,

A: Chemistry, 48 (1989)

199

- 218

199

228.8 nm PHOTOLYSIS OF 1 ,l ,l-TRICHLOROETHANE GUI-YUNG

CHUNG

and ROBERT

W. CARR*

Department of Chemical Engineering Minneapolis, MN 55455 (U.S.A.) (Received

November

and Materials Science,

University of Minnesota,

29, 1988)

Summary Room temperature photolysis of up to 100 Torr of l,l,l-trichloroethane in the presence of iodine at 228.8 nm, and 30 Torr l,l,l-trichloroethane, also in the presence of iodine, and with added CF4 was investigated. The main photodecomposition pathways are molecular elimination of HCl and C-Cl bond scission, with $..iCI/@C--CIapproximately equal to 3 at low pressure. The yields of CClzCHz formed by HCl elimination decrease with increasing pressure, while the products of radical reactions resulting from C-Cl dissociation are independent of pressure. The results are consistent with Cl atom loss occurring from the u* state populated by 228.8 nm photoexcitation, while HCl elimination does not occur from either u* or the ground state, and must proceed through the intermediacy of another excited state, suggested to be a triplet state. Analysis of the pressure dependence of CHzCC12yields gave the average energy transferred per gas kinetic collision as 1’7 + 5 kcal mol-’ for self-deactivation and 16 f 6 kcal mol-’ for deactivation by CF4.

1. Introduction Two previous investigations [ 1, 21 of l,l,l-trichloroethene (l,l,l-TCE) photochemistry using broad band excitation in the far UV showed that two primary photochemical processes occur, C-Cl bond scission and four-center molecular elimination of HCl. At wavelengths between 185 nm and 220 nm C-Cl bond scission is the dominant process, while at wavelengths greater than 220 nm HCl elimination becomes more important. Furthermore, HCl elimination is pressure quenched, both studies reporting that yields of the elimination product 1,ldichlorethylene decrease with increasing pressure. l,l,l-TCE was therefore identified as a candidate for energy transfer studies by the photoactivation method. It is an attractive case since there is a scarcity of energy transfer data for molecules of only modest size. TAutbor

to whom

lOlO-6030/89/$3.50

correspondence

should

be addressed.

@ Elsevier

Sequoia/Printed

in The Netherlands

200

l,l,l-TCE is also of interest with respect to atmospheric chemistry. The only known atmospheric source of this species is the result of industrial activity, and the amounts in the atmosphere have been increasing dramatically in recent years. Although the principal route for removal in the troposphere is reaction with OH this sink does not remove all l,l,l-TCE. The 1982 average tropospheric concentration in northern midlatitudes was measured at 175 pptv, while in the stratosphere the mixing ratio decreases rapidly to less than 1 pptv at 30 km [3]. Since photolysis must make a contribution to the rate of stratospheric removal, further work on l,l,lTCE photochemistry is desirable. Here we report the results of a study of l,l,l-TCE photolysis at 228.8 nm, using a monochromatic source, in contrast with the earlier broadband excitation work, since control of the nascent energy distribution of photoactivated species is important for unambiguous interpretation of collisional deactivation. Also, using a wavelength longer than 220 nm seemed important to maximize the fraction of 1,ldichloroethylene product for the deactivation studies, and to document better HCl elimination us. Cl atom release since HCl is an effective stratospheric sink for chlorine, whereas Cl is active in ozone removal.

2. Experimental details l,l,l-TCE of 99.7% stated purity was obtained from the Aldrich Chemical Company, and after degassing was used directly. Iodine of 99.999% stated purity was also obtained from Aldrich Chemical Company. Matheson CF4 (99.7%) was used directly. The optical train consisted of a cadmium lamp, planoconvex lens, reactor, monochromator, and phototube, arranged in that order. A cylindrical 55.2 cm3 Pyrex reactor, of 10 cm length and 2.7 cm i.d., with quartz windows was used. For the deactivation with added CF4 the reactor temperature was kept constant at 65 “C by wrapping with a heating band and the temperature was measured with a thermocouple. The effective photolysis wavelength from the Philips 0.3107E25W cadmium lamp was 228.8 nm. Shorter wavelengths could be neglected owing to absorption by the quartz window and quartz lens, while the extremely weak absorption of 1,1,1-TCE at longer wavelengths rendered them of negligible importance. The monochromator was set at 228.8 nm to ensure that transmitted light detected by the phototube and recorded on a chart recorder would be confined to that wavelength. Gas handling was done with a conventional vacuum system evacuated to 10e6 Ton before each experiment. Photolysis time was 1 h in most experiments. Conversions of l,l,l-TCE were typically between 0.5% and 1%. All reaction product analyses were done on a Hewlett Packard 5730A gas chromatograph with flame ionization detector. Samples were taken directly from the reactor to the gas sampling valve of the gas chromatograph.

201

A column of Porapak Q (SO/l00 mesh) of 3 m length and 3 mm i.d. was used at 180 “C and with 30 cm3 min-’ helium carrier gas. Since l,l,l-TCE corrodes aluminum and aluminum alloys, a stainless steel column was used. The gas chromatographic analysis revealed the following substances to be present: CH2CC12, cisCHClCHC1, trans-CHClCHCl, CH3CClJ, CH,CCl,I, (CHsCC& CH3CC13, CH2CC12, cisCHClCHC1, and truns-CHClCHCl were identified with authentic samples, and CH3CC121 and (CH3CC1J2 were identified by GCMS. CF4 was not detected with the flame ionization detector. Sensitivities of CHzCClz, cis-CHClCHCl, and trunsCHClCHC1 with respect to CH3CC13 were 0.84, 0.90, and 0.90 respectively. The relative sensitivities of the rest of the products were assumed to be 1.0. Small amounts of lower molecular weight species were not identified. Two series of steady illumination photolyses were performed; one at up to 100 Torr of l,l,l-TCE with 0.15 Torr of iodine and another at 30 Torr of l,l,l-TCE with 0.15 Torr of iodine and up to 100 Torr of CF4 as bath gas.

3. Results 3.1. Mechanism The percentages of each product after 1 h photolysis of l,l,l-TCE in the presence of iodine are shown in Fig. 1. The major product, CH2CC12, is pressure dependent, and the minor products, CH,CC&I, (CH3CCl&, and c&CHClCHCl show no clear trend with pressure and are considered, within experimental error, to be pressure independent. Traces of trans-CHClCHCl

0

20

40

60

60

100

120

p [Torr] l,l,l-TCE Fig. 1. Changes in percentages of products with the pressure quenching photolysis of l,l,l-TCE with 0.15 Torr 12 at 228.8 0, CHzCClz; A, cbCHClCHC1; o, CH3CC121; A, (CH3CC12)2.

of l,l,l-TCE nm.

Reaction

in the selftime,

1 h.

202

were also found. Similar results were obtained from photolysis of mixtures consisting of 30 Torr l,l,l-TCE with 0.15 Torr I2 and added CF4 up to a total pressure of 100 Torr. Note that the low pressure data in Fig. 1 show scatter owing to the small amount of sample for analysis. Both Yuan and Wijnen [l] and Hautecloque [Z] observed pressure dependent formation of CHzCClz and attributed it to four-center molecular elimination of HCl from a photoactivated l,l,l-TCE intermediate that is capable of being collisionally deactivated. The CH&ClJ observed by both Yuan and Wijnen [l] and Hautecloque [2], and the (CH&Cl& reported by the latter were attributed to C-Cl bond scission followed by reaction of the 1,ldichlorethyl radical with I? and with itself, respectively. The formation of these three products are most readily explained in the present instance by the same mechanism. CHsCC13+ hv (228.8 nm) __f CH,CC13* -

CH,C!Cls*

(1)

CH&C12 + HCl

(2)

CH,CC13 + hv (228.8 nm) __+ CH,CC12 + Cl

(3)

CH,CCls* + M -

CHsCCls

(4)

CHsCC12*+ Iz -

CH3CC121+ I*

(5)

2CH,CC12* -

(CHsCClJz

(6)

The presence of 0.15 Torr Iz was expected to be sufficient to scavenge all of the CHsCC12 radicals, and since combination to form (CH&Cl& occurs, there must be another path for Iz consumption, most probably by reaction with Cl atoms formed in reaction (3). Reactions (2) and (3) are the two lowest energy channels of l,l,l-TCE, having threshold energies of 53 kcal mol-’ and 76 kcal mol-‘, respectively. Other paths accessible at 228.8 nm (127.5 kcal mol-‘) are: three-center H, elimination, ~86 kcal mol-‘; C-C bond scission, 88 kcal mol-‘; three-center Cl2 elimination, 092 kcal mol-‘; and C-H bond scission, 95 kcal mol-‘. Thermochemical data are given in Appendix A. The C-H and C-C scissions can be ruled out since the radicals formed would, upon scavenging by I*, be expected to form l,l,l-trichloro-2-iodoethane and iodomethanes, respectively, none of which were observed. Three-center elimination of Cl? can also be dismissed since the carbene:CClCH, would be expected to rearrange to vinyl chloride, which was not found. Similarly, three-center elimination of H, must not occur to an observable extent since it would result in trichloroethylene. The formation of cis-CHClCHCl may occur by a molecular mechanism since it is observed in the presence of Iz, whereas if radicals were involved in its formation an iodinated monochloroethylene or dichloroethylene might be expected to be formed. However, a molecular path, if it exists, must occur by the 12-shift of both Cl and H, which seems somewhat unlikely.

203

An alternative mechanism for 1,ldichloroethylene (3) followed by CHsCCls + Cl. - CH&Cl, -

consists of reaction

- CH2CC13+ HCl

CHzCClz + Cl.

This possibility was shown to be unimportant by the following experiments. A mixture of Cl2 and 1,1,1-TCE was photolysed with a high pressure Hg arc lamp and 3.2 mm Pyrex filter to confine photolysis to wavelengths longer than 300 nm where Cl* is by far the dominant absorber. The atomic Cl so formed might then lead to 1,ldichloroethylene by the above sequence. l,l-Dichloroethylene was observed to be produced at the rate of 0.06% h-l. In a control experiment without Cl,, but otherwise identical conditions, 1,ldichloroethane was formed at the rate of 0.058% h-‘. Thus the Cl atom mechanism for 1,ldichloroethylene must be unimportant and we conclude that the primary mechanism for its formation must be HCl elimination. That atomic Cl is formed in 228.8 nm photolysis of l,l,l-TCE was shown in experiments with added CH4. Atomic Cl would be expected to react with CH4 to give CHs radicals which would compete with l,l-dichloroethyl radicals for I*, yielding CHJ. In the presence of a large excess of CH,, (50 Torr) this was found to be the case. The yield of CH,CClJ decreased while that of (CH,CCl& increased.

3.2. Quantum yields Relative quantum yields for HCl elimination can be obtained from the rate of change of 1,ldichloroethylene fractional yield at each pressure. If G is the quantum yield for HCl elimination, and a0 its value as pressure approaches zero,

cp =

.-

@O

p(df/dt)/( 1 - 10--EP”RT) p(df/dt)/(l

- 10--EP1’RT)lp+ o

(7)

where p is the pressure, E the extinction coefficient, 1 the length of the reactor, R the gas constant, T the temperature, t time and f the fractional yield of 1,ldichloroethylene. Denoting the probability of l,l,l-TCE deactivation as S and the probability of HCl elimination reaction as D, the quantum yield may also be expressed as @ D -_- = __ D+S @PO

(8)

Then D/S, which is required for later comparison with the theoretical model, is obtained as

204

It was found that the fractional yield of 1,ldichloroethylene is linear with time, within experimental error, for conversions of 1% or less. Thus df/dt was obtained by dividing the fractional yield of l,l-dichloroethylene by reaction time. The apparent extinction coefficient, E, for use in eqn. (7) was obtained from a Beer’s law plot incorporating measurements of transmitted intensity at 228.8 nm from the Cd arc-reaction vessel-phototube train of the normal experimental arrangement. The plot of -log(l/l,) us. l,l,l-TCE pressure was satisfactorily linear, yielding a 338 K value of E = 4.53 1 mol-’ cm-‘, in excellent agreement with the data of Hubrich and Stuhl [ 41. - 10-Ep”RT) was calculated at each With d/RT determined, p(df/dt)/(l pressure. Figure 2 shows this quantity plotted us. 1,1,1-TCE pressure. The upper line is a second-order least-squares orthogonal polynomial fit to all the data, from which the zero-pressure intercept can be obtained for use in eqn. (7). To investigate the uncertainties in the extrapolation to zero pressure, least-squares data fitting was applied to the six lowest data points, indicated in Fig. 2 as filled squares. To check the extrapolation further, df/dt us. p was plotted and the values of df/dt at zero pressure were obtained by least-squares fitting. Applying L’Hospital’s rule to the denominator on the right-hand side of eqn. (7) yields 1 p(dfldt) lim p+ ,, 1 - 10--EP”RT = 2.303(d/RT)

lim df (10) p-o dt Limiting values at zero pressure obtained from lim(p(df/dt)/(l - 10-EPl’RT)} and from l/(2.303 el/RT)lim(df/dt) respectively are compared in Table 1.

303 0

20

40

60

80

100

120

p [Torr] l,l,l-TCE Fig. 2. Least-squares polynomial data fitting of p(df/dt)(l - 10-“~oo23p)-1 in the selfquenching photolysis of l,l,l-TCE with 0.15 Torr 12. Upper line utilizes all data points, while lower line is fit to only the filled symbols, thereby giving a lowest limit estimate of the intercept.

205

TABLE 1 Comparison of limit values at zero pressure obtained from dime p(df/dt)/(l and l/(2.303

cZ/RT) lilia df/dt for molecular elimination reaction in the self-quenching

photolysis of l,l,l-TCE

lim ,,+e

with 0.15 Torr IZ

p(df/dt) I _

~o-~P’/RT

lim df/dt p-0 2.303el/RF lim P+e

- 10fp”RT)

df/dt

40

Least-squares polynomial data fitting

Least-squares polynomial data fitting for some data points giving the lowest limit value

75.536

69.214

75.138

69.097

0.398

0.366

60

80

p [Torr] l,l,l-TCE Fig. 3. Stern-Volmer plot for molecular l,l,l-TCE with 0.15 Torr IZ.

elimination

in self-quenching

photolysis

of

Similar values were obtained by all methods, giving confidence in the extrapolations. A Stern-Volmer plot, with a,,/@ obtained via eqn. (7), is shown in Fig. 3. Stern-Volmer plots with the relative quantum yields determined by the lower limit intercept, or by use of eqn. (lo), were also satisfactory linear. For the photolysis quenched by CF4, quantum yields were obtained from

206 cp .-a 0 PA=30.PC

=-

a cp 0

W/WI,,

= so, pc ’

(df/dt)ipA=30,,c=o

pA=30,pC=0

(11)

where subscripts A and C denote l,l,l-TCE and CF4 respectively. Here, the quantum yield at 0 Torr CF4 and 30 Torr l,l,l-TCE was obtained as 0.775 from the self-quenching Stem-Volmer curve. The limiting value of df/dt at 0 Torr CF4 and 30 Torr l,l,l-TCE was obtained as 0.273 from a plot of (df/dt) us. p by the least-squares polynomial data fitting. In the same way as was done for self-quenching, some of the lower data points were connected and the lower limit value, 0.256, was obtained. Satisfactorily linear Stern-Volmer plots, similar to Fig. 3, were obtained with either intercept. Finally, eqn. (9) was used to prepare plots of D/S us. reciprocal collision frequency. There are shown in Figs. 4 - 7. The solid lines were calculated from a master equation model incorporating RRKM theory, described below. 3.3. Theoretical calculations The quantum yield, @‘/ao, was calculated from the master equation in matrix formulation

(12) where w is the collision frequency, k the diagonal rate coefficient matrix, I the unit matrix, P the transition probability matrix, and f the column 20

16 -

12 .aE>=lPkcal/

B

P

mol

8-

2

4

6

a

0 -1 [x10-9 set] Fig. 4. D/S of the molecular elimination reaction in the self-quenching photolysis of l,l,l-TCE with 0.15 Torr 12. Lines are calculated upper and lower hounds using RRKM theory (model I) in the master equation with
207 8

6-

cAE>=l

Okcal / mol

=lSkcal

0

1

/ mol

2

3

[x10-9 set] Fig. 5. D/S, obtained with quantum yields using the lowest limit value of the molecular elimination reaction in the self-quenching photolysis of l,l,l-TCE with 0.15 Torr 12. Lines are calculated upper and lower bounds using RRKM theory (model I) in the master equation with < AE > as a parameter. 0

-l

matrix of fractional population rate. The fraction of excited l,l,l-TCE entering the ith energy level, fi, was accurately approximated by a 6 function since excitation was by a line source of radiation. Microcanonical decomposition rate constants, /z.(E), for each energy level were computed using RRKM theory. For the sum and the density of quantum states counting of vibrational states was done by the direct counting method via the Beyer-Swinehart algorithm [5,6] with a grain size of 0.01 kcal mol-‘. The harmonic oscillator models for the activated 1,l ,l-TCE molecules in Appendix A were obtained from ref. 7. For the HCl elimination activated complex, one 205 cm-’ vibration was taken as the reaction coordinate. All other frequencies were adjusted to fit the high pressure A factor for the dissociation rate. However, there are two reported Iz, values: k,

-54

= 1014.0exp

kcal mol-’

(13)

RT

which was measured in a study of the thermal decomposition of l,l,l-TCE by Barton and Onyon [8], and k,

-49.5

= 1013*’exp -

i

kcal mol-’ RT

1

(14)

which was measured using sensitized laser pyrolysis by Dai et al. [9]. We will call the calculation which fits eqn. (13) model I and the calculation which fits eqn. (14) model II. The factors 0.909 (model I) and 1.073 (model

208

3-

9 0

~AE>=lOkcal/mol

2cAE>=22kcal/mol

0

4

0

12

16

20

24

0 -1 [xlO_lO set] Fig. 6. D/S of the molecular elimination reaction in the photolysis of 30 Torr l,l,l-TCE with 0.15 Torr 12 and CF4 quencher. Lines are calculated upper and lower bounds using RRKM theory (model I) in the master equation with as a parameter.

0

4

6

12

16

20

24

[x10-16 set] Fig. 7. D/S, obtained with quantum yields using the lowest limit value of the molecular elimination reaction in the photolysis of 30 Torr l,l,l-TCE with 0.15 Torr Ia and CFI, quencher, Lines are calculated upper and lower bounds using RRKM theory (model I) in the master equation with < AE > as a parameter. o-1

II) were multiplied by all the excited molecule vibrational frequencies except the reaction path coordinate, 205 cm-‘, to fit these Arrhenius parameters. The problem of the correct statistical factor, L*, for four-center elimination has been discussed by Hassler and Setser [lo]. Various values will

209

arise depending on whether there is free internal rotation or torsional vibration in the normal molecule, and depending on the geometry of the activated complex [ 111. The most likely complex seems to exist as optical isomers since the four-center ring is twisted. In this case, Ls is 9 for the free internal rotation model and 6 for the torsional vibrational model. Sudbo et al. [7] have used Ls = 9 for a free internal rotation model. In our calculation we used a torsional vibration model, i.e. L $ = 6, because of the uncertainty of the size of the moment of inertia in the rotational model. For RRKM calculations of the C-Cl dissociation path, it was necessary to estimate high pressure Arrhenius parameters because kinetic data are unavailable. Uncertainties introduced by the estimates are unimportant since only qualitative conclusions were drawn from the calculations. The value of A was estimated to be lo-l4 s-l by comparison with pre-exponential factors for other carbon-halogen bond dissociation reactions such as CH,Cl (A = 10’3.86 se’ [12]), CHBr, (A = 10 14.* s-l [13]) and CH3Br (A = 1014.* s-l [13]). The critical energy was taken to be the bond dissociation energy, 76 kcal mol-‘. For the transition state a 725 cm-’ vibration was assigned as the reaction coordinate, and with the reaction path degeneracy, L+ = 3, a fit to the estimated value of A was obtained by multiplying each of the activated molecule frequencies by 0.962. The values of h(E) are given in Table 2. The average total energy of excited molecules was calculated as the sum of the thermal energy and the photon energy. The thermal energy at 338 K was taken to be 2.4 kcal mol- ’ from 3.5kT which is the sum of average translational, rotational, and vibrational energies. Thus, after absorption of 228.8 nm photons, the energy of the excited molecules was 127.5 kcal are shown in Fig. 8. mol-‘. Calculated k(E) values for HCl elimination Comparison of k(E) values from model I, the torsional vibrational model, with those from the free internal rotation model of Subo et al. [7] shows

TABLE

2

Energy-dependent rate from RRKM theory

Energy of activated

constant,

molecule,

E*

k(E),

of

C-Cl

bond

dissociation

4-Q

(s-l)

1.719 2.896 5.552 3.135 2.335 8.791 2.667 6.892 1.572 6.193

x x x x x x x x x x

(kcal mol-‘) 77.5 82.5 87.5 91.5 97.5 102.5 107.5 112.5 117.5 127.5

10’ lo3 lo4 106 lo6 106 10’ 10’ lo* lo8

reaction

calculated

210

IO"-

109

-

'; ki &

107-

c z 105

-

103

0

I 60

10' I 40

I 80

I 100

I 120

140

Energy, E’ [kcal / mol] Fig. 8. Calculated energy dependent rate constant k(E) of the HCI elimination reaction in model I (0) and model II (A). l are the calculations by Sudbo et al. (71.

good agreement. Figure 8 clearly shows the effect of the different set of Arrhenius parameters on the calculated h(E). At 127.5 kcal mol-’ k(E) given by model II is 1.51 X 10” s-l, whereas the value obtained via model I is about five times larger at 7.83 X 10” s-l. The collision frequency, w, is expressed as the product of the collision number, 2, and pressure, p. For the calculation of the self-quenching collision number, ZAA, where A stands for l,l,l-TCE, molecular parameters are given in Appendix A. The collision diameter was calculated by multiplying the Stockmayer constant, u, by the square root of the reduced collision integral, L?*(2-2). According to Mom-its and Rumnens [14], if Pan, which is pi,’ /(03e) where pi, is the dipole moment and u and E are the Stockmayer potential parameters, is greater than 0.3, the molecule is a polar molecule. Since jade of l,l,l-TCE is 0.353, the polar molecule calculation was done. The calculated value of 2 AA is 1.49 X lo7 TOIY’ se1 at 338 K. For the collision of l,l,l-TCE with iodine or tetrafluoromethane, the . . collision numbers, ZAB or ZAC where subscript B denotes iodine and subscript C tetrafluoromethane, were obtained using the equations (15) toA + 0,) (JAI =

2

and

eAI = (eAei)1’2

(16)

The molecular parameters are given in Appendix A. The calculated values Of ZAB and zAc are 1.31 X lo7 Torr-’ s-l and 1.12 X lo7 Ton--’ s-l, respectively at 338 K. Hence, the collision frequency of l,l,l-TCE selfquenching is

211

0 = Z‘&‘$A + O.l5Z*n

(17)

and the collision frequency of l,l,l-TCE 0 = 30Z**

+ O.l5Z*s

+ ZAcpc

quenched by tetrafluoromethane

is (13)

4. Discussion The data clearly show that the 228.8 nm photolysis of l,l,l-TCE proceeds via two major paths. One, a pressure-independent (up to 100 Torr) C-Cl bond scission, and the other a pressure-dependent HCl elimination, are both consistent with the earlier photochemical studies and with recent findings for other haloalkanes. Both Yuan and Wijnen [l] and Hautecloque [2] observed C-Cl bond rupture in their broad-band photolysis investigations of l,l,l-TCE. This primary photochemical process, common to many halogen-containing compounds, is readily understood by considering the electronic transition involved. The lowest unfilled molecular orbitals in the haloalkanes are formed from the overlap of an sp3 orbital of the carbon atom and a p orbital of the halogen atom. They are antibonding o* orbitals. The electronic transitions of lowest energy are npn o*(C-X) arising from excitation of nonbonding p electrons associated with the halogen atom into the lowest u* orbital [15,16]. The absorption in this lowest energy band, called the Aband, is continuous, and results in fast dissociation to yield an alkyl or haloakyl radical and a halogen atom. Recent work on photodissociation dynamics of halocarbons confirms this picture. Jackson and Okabe summarized the results of several studies where recoil velocities of halogen atoms produced by R-X photodissociation have been directly measured [17]. In all cases the results were consistent with that expected from repulsive upper states with subpicosecond lifetimes. The pressure independence of C-Cl bond rupture found for l,l,l-TCE here requires a short-lived or at least a pressure-insensitive precursor electronic state. It is almost certainly inconsistent with a statistical model, since the value of h(E) calculated by RRKM theory at 127.5 kcal mol-’ is 6.2 X 10-s s-l, and the 100 Torr collision frequency is 1.49 X 10-s s-l. Thus if statistical C-Cl dissociation were to occur out of the ground electronic state, up to 20% of the reaction might be quenched at 100 Torr, and much greater quenching would be expected if the dissociation occurred from any other accessible electronic state, since the excess energy and hence k(E) would be smaller. The evidence from these experiments is consistent with the results of photodissociation dynamics studies, and we conclude that C-Cl photodissociation goes through a repulsive state. The molecular elimination of HCl is the lowest energy reaction channel of l,l,l-TCE and it is readily accessed in the ground electronic state since in thermal decomposition, both conventional [8] and laser powered [9],

212

it is the dominant reaction observed. It is also the major reaction channel reported in a collisionless (done in a molecular beam) IR multiple-photon excitation study [7]. In that study, Sudbo et al. [ 71 found that HCl elimination from the ground state is well described by RRKM theory. However, it seems highly unlikely that HCl elimination observed here occurs from high vibrational levels of the ground electronic state that might be expected to be formed by internal conversion of l,l,l-TCE following 228.8 nm excitation. Since the /Z.(E) values given by RRKM calculations at the excitation energy (7.83 X 10” s-l (model I) and 1.51 X 10” s-i (model II)) are about two orders of magnitude larger than the collision frequency at the highest experimental pressure, HCl elimination would be expected to be independent of pressure, contrary to experiment. Thus HCl elimination must not originate from the ground state, but from some other excited electronic state. The e* state cannot be responsible since it is much too short-lived to suffer collisions at these pressures. Molecular H-X elimination has been directly observed at 193 nm in a photodissociation dynamics study of some unsaturated CZ and C3 chlorides and bromides [18]. It has been attributed to excitation of the lowest R* state, followed by intersystem crossing to a triplet state which eliminates H-X. While a K* state is not available in 228.8 nm photolysis of l,l,l-TCE, it seems reasonable that the state from which HCl originates is a triplet state, since both the ground state and the u* state have been ruled out as HCl precursors. This would require a very fast intersystem crossing from u* to be competitive with the picosecond or subpicosecond lifetime for C-Cl bond rupture. This is not unreasonable, since at excitation energies higher than 228.8 nm, where the repulsive potential in (T* is undoubtedly steeper, C-Cl bond rupture dominates HCl elimination. Kawasaki et al. [18] also observed HCl with a translational energy distribution peaked around zero from &H&l photodissociation at 193 nm. Knowledge of the Cl/HCl branching ratio in l,l,l-TCE photolysis is important when assessing the contribution of this species to stratospheric ozone depletion. While Cl is particularly deleterious in this respect, since it directly enters the long-chain destruction of ozone via the reaction Cl + O3 + Cl0 + 02, HCl is much less reactive and behaves as a chlorine reservoir, ultimately being transported to the troposphere where it is removed by rainout. Even though it is known that HCl formation from l,l,l-TCE becomes less important below 220 nm, and decreases with increasing pressure, the data that presently exist would permit only a crude estimate of the contribution from each channel in the stratosphere. Further uncertainty exists since the rate of release of chlorine from the other primary photodissociation products, CH&Cl, radicals and CHZCC12 is largely unknown. Further work is needed before the effectiveness of l,l,l-TCE toward ozone depletion can be properly evaluated. Since HCl elimination cannot originate from a vibrationally hot molecule at 127.5 kcal mol-‘, effectively ruling out the ground electronic state, the question of the excess energy, E*, of the HCl precursor electronic

213

state must be addressed before energy transfer can be analyzed. For the stepladder model, D/S can be expressed as

(19)

=cfj i

fi i=

1+

1

i

z1 -1

The high pressure (o % hi) limiting value of eqn. (19) is

In this experiment, the fraction entering jth energy level at the initial excitation, fi, is unity at the excitation energy and zero at all other energy levels. Thus, eqn. (20) becomes

(21)

where , the average unimolecular reaction rate, can be associated with k(E) at the excitation energy. The value of can be obtained from the limiting high pressure slope of D/S us. we1 and was found to be 1.4 X lo9 s-l. From Fig. 8, k(E) = 1.4 X lo9 s-l occurs at E* = 87.9 kcal mall’ (model I) or 95.7 kcal mol-’ (model II). These are the excess internal energies of l,l,l-TCE undergoing HCl elimination according to the two RRKM models. Subtracting the E* values from the excitation energy, 127.5 kcal mol-‘, implies that the electronic origin of the state from which HCl elimination occurs is either 39.6 kcal mol-’ (model I) or 31.8 kcal mol-’ (model II). The average energy transferred per gas kinetic collision,
214 TABLE 3 Values of p(df/dt)/(l - 10-E~z~RT)lp= e obtained by two different methods and their effect on for the HCl elimination reaction in the photolysis of l,l,l-TCE Intercept value atp= 0

a WY (Torr-‘)

75.5 51.6d

0.0097 0.0073

69.2 48.4d

0.0076 0.0058

wb

c

(kcal mol-‘)

Model Z

Model ZZ

1.42 x lo9 1.45 x 109

17 f 5 16 f6

20+ 5 21 f 6

1.85 x 109 1.68 x lo9

12.5 + 2.5 12 +4

17 + 2 16 f4

From the least-squares orthogonal polynomial data fitting TCE self-quenching CF4 quenching From the lowest data points TCE self-quenching CF4 quenching

aStern-Volmer constants. bAverage unimolecular rate constants at high pressure. CValues obtained using torsional vibration model and adiabatic overall rotation in models I and II. (Model I: k, = 1Ol4 eWs4iRT; Ea = 53.1 kcal mol-‘. Model II: k, = 1013*’ e-49.5/RT; Ea = 50.06 kcal mol-‘.) dIntercepta at TCE 30 Torr, CF4 0 Torr.

data. The same fitting was done for the data with the lowest limit value. Figures 6 and 7 show fits similarly obtained for the CF4 collider. Values of and are listed in Table 3. It can be seen that uncertainties in the intercept cause only small changes in
215

TABLE 4 Values of p(df/dt)/(l - lo- enz/RT)(p= a obtained by two different methods and their effects on <&> for all the reactions in the photolysis of l,l,l-TCE Intercept value atp=O

= (MY (Torr-‘)

84.2 66.2d

0.0056 0.0065

80.7 60.7d

0.0045 0.0047

mb

C (kcal mol-‘) Model I

Model II

2.36 x lo9 1.99 x 109

17.5 f 3.5 17.5 + 6.5

21.8 f 4.3 19.8 + 6.3

3.09 x 109 2.54 x lo9

12.5 f 2.5 13 _+5

16.5 + 3.5 15.3 _+4.8

From the least-squares orthogonal polynomial data fitting TCE self-quenching CF4 quenching From the lowest data points TCE self-quenching CF4 quenching

aStern-Volmer constants. bAverage unimolecular rate constants at high pressure. CValues obtained using torsional vibration model and adiabatic overall rotation in model I and model II. dIntercepts at TCE 30 Torr, CF4 0 Torr.

The preferred values of
and

216

References 1 T. S. Yuan and M. H. J. Wijnen, Ber. Bunsenges. Phys. Chem., 81(3) (1977) 310. 2 S. Hautecloque, J. Photochem., 12 (1980) 187. 3 Atmospheric Ozone 1985, World Meterological Organization Global Ozone Research and Monitoring Project, Report No. 16, p. 645. 4 C. Hubrich and F. Stuhl, J. Pkotochem., 12 (1980) 93. 5 T. Beyer and D. F. Swinehart, Commun. ACM, 16 (1973) 379. 6 S. E. Stein and B. S. Rabinovitch, J. Chem. Phys., 58 (1973) 2438. 7 A. S. Sudbo, P. A. Schulz, Y. R. Shen and Y. T. Lee, J. Chem. Phys., 69(6) (1978) 2312. 8 D. H. R. Barton and P. F. Onyon, J. Am. Chem. SOC., 72 (1950) 988. 9 H. L. Dai, E. Specht, M. R. Berman and C. B. Moore, J. Chem. Phys., 77(9) (1982) 4494. 10 J. C. Hassler and D. W. Setser, J. Chem. Phys., 45 (1966) 3246. Reactions, Wiley-Interscience, 11 P. J. Robinson and K. A. Holbook, Unimolecular London, 1972, 1st edn., p. 253. 12 0. Kondo, K. Saito and I. Murakami, Bull. Chem. Sot. Jpn., 53 (1980) 2133. 13 J. A. Dean (ed.), Lange’s Handbook of Chemistry, McGraw-Hill, New York, 1979, 12th edn., pp. 9-l-9-101 and 10-115. 14 F. N. Mourits and H. A. Rumnens, Can. J. Chem., 55 (1977) 3007. 15 M. Ito, P. C. Kuang and E. M. Kosower, Trans. Faraday Sot., 57 (1961) 1662. 16 J. R. Majer and J. P. Simons, Adu. Photochem., 2 (1963) 137. 17 W. M. Jackson and H. Okabe, Photodissociation dynamics of small molecules, Adu. Photochem., 13 (1986) 1. 18 M. Kawasaki, K. Kasatani, H. Sato, H. Shinohara and N. Nishi, J. Chem. Phys., 88 (1984) 135. 19 D. C. Tardy and B. S. Rabinovitch, Chem. Rev., 77 (1977) 369. 20 J. R. Barker, J. Phys. Chem., 88 (1984) 11. H. Hippler, Ber. Bunsenges. Phys. Chem., 89 (1985) 303.

Appendix A A.l. Molecularparameters of l,l,l-TCE Activated molecule frequencies in cm-’ [Al]: 2954, 1383,1069,526, 344, 205, 3017(2), 1456, 1427, 1089(2), 725(2), 301(2), 241(2). I (amu A2) = 214,214,289. I,$ = 6 (reaction path degeneracy). Stockmayer collision parameters from the boiling point and critical data correlation of 1 ,l ,l-TCE. pP (dipole moment) = 1.79 debyes T,, = 347.1 K T, = 544.7 K, pc = 47.27 atm (by Lydersen’s method) [A21 V, = 249.9 cm3 gmol-’ (by Lydersen’s group contribution method) [A21 u = 5.123 A, e/k = 488.6 K (by Stiel-Thodos Method) [A31

217

GJ*(*,*)= 1.9364 at 338 K pC = M/(0.34 + ZAP)*; V, = = T,,{O.567 + XAT - @A,)*}-‘; + (Z$fAv) 1.029. A quantities were evaluated by summing contributions

from T, 33.04

for various atoms and groups of atoms in Tables 2-1 and 2-4 in ref. A2. (I = 0.785I.7, 1’3, e/k = 0.897T, SF***) = reading from the Table 9-l in ref. [ A21 by Monchick et al. [A41 A.2. Lennard-Jones

collision parameters

of iodine

u = 4.982 A, e/k = 550 K [A51 0 AB=

5.053 A, e,&k

= 518.4 K

sZAr,*(***)= 1.9958 at 338 K = 1.16145/( T*“.‘4874) + 0.52487/exp(0.7732T*) + 2.16178/exp(2.4387T*) [A21 Here, subscript A denotes l,l,l-TCE A.3. LennardJones

and subscript B iodine.

collision parameters

of tetrafluoromethane

u = 4.486 A, e/k = 167.3 K (TAc= 4.805 A, EAC/k = 285.9

K

aAc*(*‘*) = 1.4644 at 338 K = 1.16145/(T*0.‘4874 ) + 0.52487/exp(0.7732T*) + 2.16178/exp(2.43787T*) [A21 Here, subscript C denotes tetrafluoromethane. A.4. Heat of formation

and bond dissociation

energy

At standard state (25 “C) in kcal mol-‘. AHf”(CH3CC13) = -34.01, AH,“(Cl*) = 28.99, AH,“(H. ) = 52.10, AH,“(I,) = 14.92, AH,“(I*) = 25.535, AHHf”(CH31)= 3.29, AH,“(HI) = 6.3, AHf”(CH2= 34.0, AH&CCls) = 19.0 Ccl*) = 0.3, AHH,“(HC1)= -22.06, AHJCH,) [A61

AHHf”(CH21CC1s)= -13.3 (using additivity of group properties in Table 7.4 of ref. A7 by Benson [A81 AHfo(CC131) = -8.1, AHf”(CHsCClJ) = -9.46 (using additivity of bond properties in Table 7.1 of ref. A2 by Benson [A81

218 D(CH,-CCl,) D(CH,CCl-Cl,) D(CH&Cl,--H) D(CH&Cl,-HCl)

= 88 [A61 = 92, D(CHCCl,-H,) = 95 [Al] = 53.1 or 50.1

= 86, D(CHC,CCl,-Cl)

= 76,