Photolysis of ethyl chloride at 121.6 nm

J. Photochem. Photobiol. A: Chem., 64 (1992)

135-143

135

Photolysis of ethyl chloride at 121.6 nm Sung-Seen Choi and Dae-Gon

Oh

Center for Molecular Science and Department of Chem&t$, Korea Advanced Institute and Technology, P.O. Box 150 Chongyang, Seoul 130-650 (South Korea)

of Science

Seong Keun Kim Department of Chem&y,

Seoul National

University,

Seczul

151-742

(South Korea)

Kynng-Hoon Jungt Center for Molecular Science nnd Department of Chemistry, Korea Advanced Ik&ute and Technology, P.0. BCIX150 Chongyang, SeouI 130-650 (South Korea)

(Received June 12, 1991; accepted October

of Science

17, 1991)

Abstract The photolysis of ethyl chloride at 121.6 nm was studied over the pressure range 60-400 Pa at room temperature using a hydrogen atom resonance lamp. The pressure effect was investigated with CO* and SF,. The scavenger effect of the reaction was also observed by adding NO gas. The major products of the reaction were CH,, GH,, GH, and GH,. The branching ratios between the elimination and the radical processes were found to be 0.68-0.32. The fraction of hot ethylene (above the critical energy) was found to be about 30%.

1. Introduction

The photochemical behaviour of gas phase ethyl chloride was investigated at 121.6 nm using a hydrogen atom resonance lamp and at room temperature. The reaction modes of photodecomposition of alkyl halides in general have been known to be wavelength dependent and a fnnction of electronic ekited states [l, 23. The production of vibrationally excited molecules by thermal [3-5], chemical [6, 71 and photochemical [8-lo] activations has also been reviewed comprehensively. ‘The first absorption band of the alkyl halides occurs in the UV region; it has been attributed to the promotion of a non-bonding p orbital on the halogen atom to an antibonding a* orbital involving largely the carbon and halogen atoms. On the contrary, alkyl halide molecules, excited to Rydberg states in the vacuum UV region which correspond to an np -+ (n+ 1)s or an np-, (n -I-1)p transition, show a decomposition pattern of molecular HX eLimination. In the ethyl bromide system, these observations include the photolysis of GH5Br-acetaldehyde at 313 nm and at 310 “C [II], C$H5Br-tenfold cyclopentane-mercury at 210-260 nm over the temperature range 30-250 “C [12], the neat system at 253.7 nm and at 1.50-300 “C [ 131 and at room temperature [14]. Extensive photochemical studies of CzI-fsBr in the vacuum UV region, 104.8-193.1 nm [X5-22], +Author to whom corresponderlce

lOlO-6030/92,‘$S.OO

should be addressed. I

Q 1992 - EIsevier Sequoia. Al1 rights reserved

136 have also been reported. The observations have shown 58.7%, 60%, 50%, 20% and 18% molecular elimination at 121.6 nm, 123.6 nm, 147 nm, 163.3 nm, 174.3-174.5 nm and 191.3 nm respectively and confirmed a decreasing propensity of molecular elimination at longer wavelengths. In the case of &H&l, Cremieux and Herman [23] have proposed that the major primary mode of photochemical decomposition of &H&l at 123.6 nm is the molecular elimination of HCl from an electronically excited state. Other primary processes proposed are the elimination of Hz and the formation of C&H, radicals. Tiernan and Hughes [24] have studied the photolysis of C;H,Cl at 123.6 nm as part of an extensive investigation of the 50 keV radiolysis. The major primaIy process at 147 nm photolysis of GHsCl using a xenon resonance lamp has shown the molecular elimination of HCl and pressure dependence of the reaction mode [25], i.e. C-Cl bond fission has increased as the pressure of ethyl chloride rise. From these findings, they have suggested the involvement of multielectronically excited states in the primary processes. The photolysis of QH=JZl at 163.3 nm using the bromine atom resonance lamp has also been investigated in this laboratory [26]. In this work, we present a study of the branching ratio between the elimination and the radical processes in the photolysis of GH&l at 121.6 nm and the excess energy redistribution of hot ethylene produced from the photolysis as a continuing effort of the energy partitioning among the photofragments [27]. The observed average energies removed from ethylene per collision by CO2 and SF, are used to make a comparative study with the predicted values from a Rice-Ramsperger-Kussel-Marcus (RRKM) calculation.

2. Experimental details The light source used in this work was a sealed hydrogen atom resonance lamp which was operated by a microwave generator, KIVA model MPG-4M. Details of the construction, the test of spectral data and the calibration of spectral lines of the lamp (Ha:argon ratio, 1:9) have been described elsewhere [26, 281. The typical intensity of the lamp was 4x lCP4 quanta s-l. The recovery of the original transmittance 1261 of the window from a colour centre formation on MgFz crystal and from the deposit of photolytic products on the outside surface was performed by the technique reported in our earlier work [26, 273. A standard vacuum line was used for sample handling; a Penning gauge (CVC GPH-320C) for system vacuum monitoring, a thermocouple gauge (CVC GTC-360) as an auxiliary gauge, and an oil manometer and the Wallace-Tiernan pressure gauge for sample transfer. The reaction vessel used in this study was a 340 ml cylinder with 20 cm of light path length and equipped with an all-glass gas-circulating pump to prevent secondary photoreactions of the reaction products. The experimental details have also been described in our earlier reports [26, 271. In brief, the photolysis of sample was carried out at constant irradiation time, i.e. 10 min. The major reaction products, i.e. CI& C&H,, C$I& and GH2, were obtained and plotted as a function of total pressure as well as a function of additive pressure, eg. CO2 and SF,. Actinometry was on the basis of the production of acetylene from ethylene actinometer under the same irradiation condition as the main reaction. By adding NO gas as a radical scavenger (about 60 Pa) the radical processes were inhibited and the elimination processes were observed.

137

Product analysis was carried out using HP 584OA gas chromatograph equipped with twin hydrogen flame ionization detectors. The total conversion of GH&I to products was kept to no more than 1% so that high concentration of the reaction product could not complicate the reaction_ The reaction products were separated and identified using an 4 in x3.5 m nickel 80/100 mesh chromosorb 108 column. Ethyl chloride obtained from Matheson Co. was used after trap-to-trap distillation using a dry ice-acetone slush bath and its purity was checked with gas chromatography (GC) to be better than 99.9%. The C&, GH2, &H4, &H6 and NO obtained from Matheson Co., with stated purities of 99.6%, 99.6%, 99.6%, 99.6% and 99.0% minimum respectively, were also purified by the same technique except at 77 K and their purities after the processes were checked to be better than 99.9% by GC analyses. Carbon dioxide obtained from Matheson Co., with a stated purity of 99.995% minimum, and SF, obtained from Takachiho Co. were used after several freeze-pump-thaw cycles by using a dry ice-acetone slush bath. Nitrogen, hydrogen and compressed air obtained from Sin-Yang Gas Co. were found to be suitable for GC work.

3. Results

and discussion

The major reaction products were found to be C&, GH2, C&L, and QH,. The product quantum yields of minor products, CH&l, C;H3Cl, C;H, and n-C4HIo, were always less than 0.01. The quantum yields of GH2 and C& increased from 0.060 and 0.210 to 0.170 and 0.630 respectively over the entire pressure range used. CH, and GH, have shown increasing quantum yields up to 470 Pa, followed by a slight decrease with increasing reactant pressure. The product quantum yields of major reaction products, i.e. CH4, C&Ha, CH, and C;H6, reached a plateau at total pressure of about 2.4 kPa and had values of 0.040, 0.170, 0.630 and 0.075 as listed in TabIe 1 and displayed in Fig. l(a). The scavenger effect on GH.$Zl photolysis by NO gas is presented in Table 2 and illustrated in Fig. l(b). The quantum yields of &HZ and GH4 were increased from 0.060 and 0.140 to 0.150 and 0.525 with the increase in total pressure at 60 Pa TABLE

1

Pressure dependence of quantum yields of ethyl chloride Run

P(EtC1)

C-J%

c2H6

C&

Irradiation time (min)

0.285 0.349 0.428 0.502 0.586 0.611 0.608 0.596 0.637 0.655 0.637

0.054 0.069 0.055 0.091 0.072 0.063 0.072 0.089 0.074 0.069 0.075

0.086 0.110 0.096 0.128 0.142 0.174 0.160 0.176 0.169 0.163 0.174

10 10 10 10 10 10 10 10 10 10 10

Quantum yield

(Pa)

1 2 3 4 5 6 7 8 9 10 11

96 147 324 429 933 1152 1933 2026 2799 3199 3732

0.042 0.049 0.044 0.065 0.051 0.047 0.042 0.038 0.035 0.044 0.045

138 TABLE

2

Radical

scavenger

effect

Run

by adding

NO gas Quantum

P{EtCl)

Irradiation time

yield

(Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13

32 89 209 220 324 544 742 977 1452 1600 2042 2410 2706

GJ%

C&Z

(min)

0.184 0.242 0.314 0.375 0.372 0.427 0.476 0.468 0.479 0.480 0.548 0.469 0.527

0.063 0.090 0.095 0.112 0.116 0.130 0.138 0.127 0.155 0.139 0.168 0.137 0.148

10 10 10 10 10 10 10 10 10 10 10 10 10

I

(b) 0

o

0

00

w

1

I

’

3

Pressure

1

3

2

v

2

1

(kPa)

c)

0

Pressure

B 3

(kPa)

Fig. 1. The product quantum yields @ of major reaction products as a function of (a) the total pressure and (b) the scavenger effect of the reaction products by NO gas at 60 Pa: 0, C&; 0, GJ322; D, Cd%; 0, Cfb. Fig. 2. Pressure effects on the quantum yields @ of major reaction products at a constant substrate pressure of 130 Pa (a) due to the additive CO* collider and (b) due to the additive

SF,: 0, W%;

l,

C&2;

D,

c;H,;

EI,

CIA.

of fixed NO partial pressure and then reached their plateau values of 0.150 and 0.525 respectively at 2.6 kPa. The quantum yield of the sum of total GH, and its deduced product &Hz was 0.800 and decreased to 0.675 by the scavenger effect of NO at 60 Pa. These observations suggest that there exist the non-scavengeable and the scavengeable portions of products by the ratio of 0.675 to 0.125. The pressure effect of CO2 and SF, as additives in the region of 1304000 Pa (Fig. 2) show the variation in the quantum yields of the major products with the additive pressure. The quantum yield of c;H, decreased while that of C;H, increased

139

with the additive pressure and then reached the plateau values at about 2.6. kPa of total pressure. These values of GH, and GH, at 2.6 kPa were 0.120 and 0.438 for COz, and 0.070 and 0.515 for SF6 respectively. The quantum yields of CI-& and GH, had increased up to 0.08 and then showed a decrease with further increase in additive pressure. The most important aspect of the observation in the present work was that the quantum yield of GH4 at 121.6 nm was affected by the presence of NO as shown in Tables 1 and 2. As can be seen from these tables, GH, exhibited a considerable change in quantum yields, a decrease of about 0.11, by addition of NO, which did not leave out the possibility of a radical process in C;H, production. One interpretation of this observation is that the precursor of the major portion of CJ& is an electronically excited state of G,HC1; subsequent decomposition of this precursor gives NC1 and C&k, by molecular elimination. The minor scavengeable portion of CJ& seems to originate either from the &H, radical, formed by C-Cl bond fission, or from the CH,CH,C!l radicals, formed by @hydrogen abstraction, of an electronically excited G,H,CL. As for the pressure effect studies illustrated in Fig. 2, the quantum yield of G,H increased while that of &Hz showed a decrease with increase in additive pressure. This observation demonstrated the stabilization of GH, by increased number of intermolecular collisions. The higher quantum yield of CJ& with SF, than with CO, may then be attributed to the property of SF6 as a better quencher than CO*. 3.1. Primary processes Since the chemical reaction in this system occurs only from electronically excited states, one can assume two electronically excited states, i.e. one for molecular elimination and the other for radical processes. It might be quite probable that the observation of the predominant molecular elimination products is due to a Rydberg transition [l, 281, the first allowed transition of a non-bonding electron on the chlorine atom. Another radical formation process is due to the n-u* transition from a non-bonding p electron on the chlorine atom to an antibonding (+* orbital. On the basis of the foregoing discussion, simultaneously excited two electronic states have been assumed [26]. One of these states, GH,Clt(l}, produces C&l_+ or C&H&l by the molecular HCI or H2 elimination reaction, or CH3 radical via radical formation process by C-C bond fission (M= 368 k.T mol-l). The other simultaneously excited state, GH,Clt(2), decomposes to scavengeable C;H, radical by C-Cl bond fission (fW=339 kJ mol-‘). The statistical model [29, 301 for the energy partitioning in the photodissociation states that an electronically excited molecule usually survives for a period of several vibrations before it undergoes dissociation during which time the excess energy acquired by the molecule may be equally partitioned among all vibrational degrees of freedom. Since the total energy of the photon at 121.6 nm is 984 kJ mole1 and only 71 kJ mol (M= 71 kJ mol-‘) of energy is consumed by the reaction G,H,Cl+ CJX++HCl, there are still 912 kI mol-* of excess energy to be distributed between GH, and HCl. Hence the statistical partitioning of excess energy into C&L, that will be about 12/18 of the total available excess energy exceeds 335 kJ mol-‘, the threshold energy for further elimination of HF Accordingly most of the hot C!-$& molecules with an excess energy of about 607 kJ mol -’ dissociate further into QH2 and Hz. Similar thermochemical considerations can be applied to the elimination of Hz to give vinyl chloride: GH,Cl+ cLH3Cl-t Hz; M= 146 W mol-‘. Since the excess energy, 837 kJ mol- ‘, in this case can be distributed between GH&l and Hz, the energy content of GH,Cl,

140

also exceeds its elimination energy of HCl, 251 kJ mol-’ [31]. However, since the observed quantities of GH4 were considerably larger than that of QH,CI, it may be interpreted that the reaction cH&l--, C-J&+ HCl is favoured over the reaction &.H5Cl + GH&l + Hz. Moreover, the actually observed quantity of C&H2 was

558 kJ mol-‘,

less than half the predicted amount. This finding is taken as strong evidence the statistical partitioning of energy in GHsC1 photodissociation.

against

3.2. Non-scavengeable C& Since the ratio S/D of the stabilization to decomposition processes increases with increasing additive pressure as shown in Fig. 3, it is more likely that the vibrationally excited C&H, becomes relaxed to the eollisionally quenched ground state than going through the decomposition process to give C&H2. The deviation from the linearity of slope is then strong evidence that hot GH, molecules are not monoenergetic but are distributed over a broad energy range. Hence, the elimination processes are summarized bY C&*+MG&*

G&+M

-

(1)

GJ32+H2

(2)

3.3. Formation of scavengeable products The complete disappearance of C&, C&He, c;Hs and n-C4Hlo in the presence of NO confirms that these products result from radical precursors. The GH5 radical formed by the fission of the C-Cl bond of QH<(2) may abstract the hydrogen atom from the reactant molecule according to G,H, + GH,CI

-

GH,

+ GH&l

or undergo further dissociation process to a possible a simple one step mechanism as follows:

‘3% -

6.0

-

0

by

0

0 q o m

B 4.0

&H4

a

0

e in

of scavengeable

(4)

G%+H

5.0

(3) source

-@

m

l n

m

n

8m 3.0.

3

2

1 Pressure

(kPo)

Fig. 3. The uncorrected and corrected ratios S/D of stabilization to dissociation as a function of additive pressure in the photolysis of ethyl chloride at 298 K and at a constant substrate pressure of 130 Pa: 0, 0, SF, collision partners; n, 0, CO2 collision partners, respectively; 0, I, uncorrected values; 0, a, corrected values.
141

However, reaction (4) has been excluded from the reaction mechanism for two reasons. First, reaction (4) must be unfavourably slow compared with radical reaction (3) since its activation energy is relatively too high, i.e. 170 kJ mol-‘. Secondly, because of its competitive nature with reaction (3), the product quantum yield of GH, should decrease with increasing qH&l pressure, while on the contrary our experimental finding has shown the opposite trend. The CH3 radical produced by C-C bond fission may undergo hydrogen abstraction and forms CM,. 3.4. Formation of scavengeable C,H, Among possible sources of the scavengeable GH,, the decomposition of cHs to C;H, and H had been excluded by the reason given above (vide supra). Another possible source of CzH, may be a disproportionation reaction of GH,. This possibility had to be also excluded from experimental as well as thermochemical considerations. The reported value of the ratio kdis/kambof the rate constants for the disproportionation to combination of GH, at 298 K is 0.11 [32] and hence the possible value of C$& from this reaction is about IO% of the recombination product, n-C4Hfo. However, our observed n-CsHlo quantity was only a trace amount. Consequently the most probable source of the scavengeable c;H, is then CH2CH2Cl produced by hydrogen abstraction from GH,Cl. The reaction pathways of CHzCHZCl can be described by the following reactions: CI&CHZC1 -

c2H4+Cl

CH,CHzCl + CH,CH&l

(5)

-

GH4 +- CJKl,

(6a)

-

C,H&l,

(6b)

The reported kinetic data of reactions (6) are favoured over reaction (5) [33], e.g. Es=99 kJ mol-‘, E6=0 kJ mol-‘, log&= 13.9 and log&= 10.1. Further, the large ratio of the rate constants [34] for reactions (6a) to (6b}, k6Jk6b = kdiJk_mb = 50, predicts reaction (6a) as the major channel for C&I,, production. Nevertheless, our observed results did not show any measurable quantities of C&I&Cl2 or C4H8C12. Therefore it may be assumed that the low steady state concentration of P-chloroethyl radical follows reaction (5) predominantly over reactions (6). 3.5. Energy distribution among non-scavengeable C& molecules The energy partitioning to non-scavengeable Gw molecules between above and below the threshold energy of dissociation can be obtained by observing the collisionfree population distributions of QH4 molecules in these two regimes. Since all Q?& molecules above the threshold energy dissociate into QH, without any stabilization process under this circumstance, the fraction of conversion can be obtained by observing C&H, at zero pressure, whereas the remaining WI., molecules at zero pressure are a direct measure of the population distribution below the threshold energy. The fraction of “hot” ethylene is then given by the relationship 1 - lower population/total population. This fraction in terms of “zero-pressure” quantum yields was obtained by extrapolating each quantum yields to zero pressure and from the relationship, i.e. the fraction of o + 4-O) = 1 - 0.14/(0.06 +0.14) = 0.3 and illustrated “hot” ethylene = 1 - +-“/(&2H2 in Fig. l(a). The hot ethylene molecules then participate competitively between the stabilization and decomposition reaction. In order to test our suggestion the observed competitive reaction of hot molecules was compared with the RRKM prediction for collision effect and the results are shown

142

20000

30000 Energy

40000 (cm-‘)

Fig. 4. The input energy distribution for the RRKM calculation of S/D. The shaded area depicts 30% of the total population of ethylene above the threshold energy of the dissociation reaction, and the unshaded area 70% below the threshold energy.

in Fig. 3. The

observed dissociation behaviour of hot ethylene molecules have shown excellent agreement with the prediction by the RRKM theory. For the theoretical prediction, the algorithm of Tardy and Rabinovitch [35] was used to compute the ratio of S/D, i.e. the ratio of the fraction of stabilization to the fraction of dissociation. The computational details have been reported in our earlier work [27]. In brief, the ratio of SD in terms of the RRKM formulation is given by ;

=FEXEE& 1 1

2

W/X_

I

&hrn

I

where Pii are the transition probabilities from the ith to the jth energy level and Pji = 0 for i<_i, w is the collision frequency, ki is the microscopic rate constant at the ith energy level, and fim is the steady state population_ The calculated average energies (AE),,, removed from vibrationally excited && per collision utilizing the exponential model were 810 cm-’ for SF, and 600 cm-’ for CO2 collision partners. The total energy distribution of ethylene is shown in Fig. 4.

4. Conclusion

The vacuum photolysis of ethyl chloride at 121.6 nm shows two markedly different decomposition modes, i.e. H2 and HCl molecular eliminations, and C-C and C-Cl bond fissions. The competitive decomposition modes were 68% molecular elimination and 32% radical formation. Two electronically excited states were suggested to participate simultaneously in photoactivation reactions. The sequential unirnolecular reaction technique was used to measure the collisional energy transfer of ethylene molecules formed by HCl elimination from photoactivated ethyl chloride. The fraction of hot ethylene was found to be 30% and the average energy removed (AE&,_ by collision in the dissociation reaction were 810 cm-* by SF, and 600 cm-’ by CO2 respectively.

143

Acknowledgments This work was done with financial assistance from the Korea Science and Engineering Foundation and the Korea Standards Research Institute which are gratefully acknowledged.

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