J . Phys. Chem. 1988, 92, 2821-2824
2821
Laser-Initiated Free-Radical Chlorination of Propane in Amorphous Thin Films: Temperature Dependence from 15 to 77 K Arthur J. Sedlacek and Charles A. Wight* Department of Chemistry, University of Utah, Salt Lake City, Utah 841 12 (Received: August 24, 1987)
The free-radical chlorination of propane, deposited as a thin amorphous film, has been investigated at temperatures ranging from 15 to 77 K. Reactions are initiated by pulsed laser photolysis of chlorine molecules at 308 nm. Product yields and branching ratios are characterized by Fourier transform infrared absorption spectroscopy. The inherent reactivity of chlorine atoms toward secondary versus primary hydrogens (when corrected for the greater number of primary sites) decreases from 17:l at 77 K to 2.3:lat 60 K and below. Over the same temperature range the quantum yield is observed to increase from 0.12& 0.02 to 0.70 f 0.06. These results reflect the onset of translationally excited (hot) chlorine atom reactions.
Introduction Most investigations of solid-state reactions have concentrated on how the 3-dimensional regularity (geometry) of the crystal determines the reactivity of the solid. From these studies have come detailed physical pictures of the controlling factors present in crystalline reactions. Recent progress in organic crystals has been reviewed by Ramamurthy and Venkatesan,' McBride, et al.? and Gavezzotti and S i m ~ n e t t a . Unfortunately, ~ many systems of practical interest lack this long-range order, so the applicability of what is learned from these ideal environments is limited. On the other hand, studies of reactions in amorphous solids suffer from complexities which render detailed mechanistic analysis very difficult. However, reactions in thin films can be monitored spectroscopically to gain insight into the topological effects of rigid semistructured environments on reaction mechanisms. It is from these fundamental studies that models can begin to be developed which relate macroscopic observables to solid-state chemistry on a molecular scale. We have been investigating the free-radical reaction between chlorine and small hydrocarbons deposited as thin amorphous films on a cryogenic substrate. One of the principal goals has been to determine the propensity of these reagents to participate in free-radical chain reaction^.^ Part of the motivation for choosing these systems is that the kinetics and reaction mechanisms in the gas phase and in solution have been well-characterized. In a fluid environment, a free-radical chain reaction C1' C3HB HCl + C3H7' (1)
+
C3H7.
-
+
+ Cl2
C3H7Cl
+ C1'
(2) forms 1-chloropropane, 2-chloropropane, and HCl as the major products. The chain length, defined as the number of product molecules produced for every chlorine atom in the initial photolysis, has been reported5 to be as high as 10'. However, work in our laboratory has demonstrated that at 77 K the restricted mobility in the solid state effectively prevents chain propagation from occurring, as evidenced by low product yield^.^ It was shown that the initial hydrogen atom abstraction (eq 1) occurs only 12 times for every 100 chlorine molecules photodissociated; the remaining 88% simply recombine to form Clz (no reaction). Following H atom abstraction, the nascent propyl radical recombines with the other unreacted chlorine atom (instead of attacking C12), effectively keeping the entire reaction confined to the site of initial UV photolysis. In the same study4 it was reported that the relative reactivity of secondary versus primary hydrogens, when corrected for the ( 1 ) Ramamurthy, V.; Venkatesan, K. Chem. Reu. 1987, 87, 433. (2) McBride, J. M.; Segmuller, B. E.; Hollingsworth, M. D.; Mills, D. E.; Weber, B.A. Science 1986, 234, 830. (3) Gavezzotti, A.; Simonetta, M. Chem. Reu. 1982, 82, 1. (4) Sedlacek, A. J.; Mansueto, E. S.; Wight, C. A. J . Am. Chem. SOC. 1987. 109, 6223. (5) Yuster, S.; Reyerson, L. H. J . Phys. Chem. 1935, 39, 859. Yuster, S . ; Reyerson, L. H. J . Phys. Chem. 1935, 39, 11 11.
greater number of primary sites, is 17:l. This result was found to be in remarkable agreement with an extrapolation of gas-phase Arrhenius p a r a m e t e d to 77 K. The agreement suggested that the chlorine atoms formed by dissociation of C12 are thermalized to the characteristic temperature of the environment (77 K) on a time scale which is short compared with H atom abstraction. In order to ascertain whether the agreement between Arrhenius prediction and our experimental results was real or simply fortuitous, and to further investigate this notion of chlorine atom thermalization, the present study was initiated which examines the temperature dependence of the reaction from 15 to 77 K. All reactions are initiated by pulsed laser photolysis of chlorine molecules at 308 nm. The product yields and branching ratios are characterized by Fourier transform infrared absorption spectroscopy. Our results show that as the photolysis temperature is lowered, the relative reactivity of secondary versus primary hydrogens drops from 17:l (77 K) to 2.3:l (60-15 K). In addition, the quantum yield is observed to increase from 0.12 f 0.02 to 0.70 f 0.06 over the same temperature range. These results are interpreted in terms of the onset of hot atom reactions at the lower temperatures.
Experimental Section A closed cycle refrigerator (Air Products Model CS-202) is used to cool the cryogenic substrate (CsI) to 15 K whereupon approximately 0.06 mmol of the reagents (1: 1 ratio) is deposited (typically 0.1-0.3 mmol/h). All depositions are carried out in the dark to prevent any gas-phase chain reactions from occurring. The vacuum shroud, mounted with a rotatable flange, is equipped with one fused silica window through which samples are irradiated and two CsI windows for collecting transmission infrared absorption spectra of the films. By regulating the voltage across a strip heater (located on the cryogenic finger), we obtain various photolysis temperatures. The temperature of the film is monitored with a 4% Fe doped Au versus chrome1 thermocouple. Following deposition, the films are irradiated with 308-nm light from an excimer laser (Questek Model 2200). In most experiments the laser output was attenuated to 1.O mJ/cm2, as measured with an absorbing disk calorimeter (Scientech Model 38-01). Bulk heating of the film by each laser pulse can be a significant effect. The optical density at 308 nm is typically 0.1, so the absorbed energy is uniformly distributed through the thickness of the solid. The transient temperature rise is estimated by using a Debye model for the heat capacity of the solid. A crude estimate for the Debye temperature is taken to be 80 K, and the contribution from intramolecular vibrational modes is assumed to be negligible. Under our experimental conditions a 1.O mJ/cm2 laser pulse should raise the temperature of a 77 K film by less than 3 K. The temperature of a 15 K film should rise by about 6 K due to its lower heat capacity. Estimates of the thermal conductivity of the films show that transfer of the heat into the CsI substrate should return the ( 6 ) Knox, J. H.; Nelson, R. L. Trans. Faraday SOC.1959, 55, 937.
0022-3654/88/2092-2821$01.50/00 1988 American Chemical Society
2822 The Journal of Physical Chemistry, Vol. 92, No. 10, 1988
Sedlacek and Wight
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-
1
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Before photolysis
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I
2600 900 800 700 600 Wovenumbers ( c m - ' ) Figure 1. Infrared spectra of an equimolar mixture of propane and chlorine prior to and after laser photolysis at 308 nm. The sample was subjected to 1000 pulses with an average pulse fluence of 12 mJ/cm2. Each division on the vertical scale represents 0.4 absorbance unit on the left-hand side and 0.04 on the right side. 3000
2800
temperature of the solid to its original value in about 4-10 ms. Reaction yields and branching ratios were determined by integrating the infrared absorption bands of reactants and products before and after photolysis. Spectra were obtained with a Biorad/Digilab Model FTS-40 Fourier transform infrared absorption spectrometer. Quantitative evaluation of the product yield requires that the number of molecules per unit area be known. From our earlier investigation4 it was estimated that 11% of the total sample is deposited in 1 cm2 at the center of the CsI window. All reagent gases were obtained from Matheson and have a minimum stated punty of 99.5%. Both propane and chlorine were subjected to repeated freeze/pump/thaw cycles to remove noncondensable impurities. Samples of 1-chloropropane and 2chloropropane, obtained from Aldrich (gold label, 99%), were used without further purification.
Results The infrared spectra for a mixture of propane and chlorine frozen on a CsI window are presented in Figure 1. Prior to irradiation all of the absorption bands were found to be consistent with the IR spectrum of pro pane.'^^ Each of the absorption bands is found to be slightly red-shifted (5-10 cm-I) relative to the gas-phase values. Molecular chlorine is transparent in the infrared spectrum. After the film is irradiated at 308 nm the reactant peaks are diminished in intensity, and a concomitant appearance of new absorption bands characteristic of the major products 1-chloropropane, 2-chloropropane, and HC1 is observed (Figure 1). The product peaks are identified with the aid of spectral assignments in the literature9-I4 and verified by comparison with infrared spectra of the authentic compounds deposited directly onto the CsI substrate. The absorption bands at 737 and 650 cm-l are attributed to the C-Cl stretch of the anti and gauche conformers of 1-chloropropane, respectively, while the product peak at 605 cm-I is due to 2-chloropropane, which has only one conformer. The spectra of the authentic compounds are also used to determine the relationship between intensity and concentration so that acc u r a t e q u a n t u m yields c a n be determined.
Several experiments were conducted in which samples were deposited at 15 K but photolyzed at various temperatures. At ~~
(7) Flurry, R. J. J . Mol. Spectrosc. 1975, 56, 8 8 . (8) Comeford, J. J.; Gould, J. H. J . Mol. Spectrosc. 1960, 5 , 474. (9) Rasanen, M.; Bondybey, V. E. J . Phys. Chem. 1986, 90, 5038 and references therein. (10) Bently, F. F.; Smithson, L. D.; Rozek, A. L. Infrared Spectra and Characteristic Frequencies 700-300 cm-I; Wiley-Interscience: New York, 1968. (11) Shipman, J. J.; Folt, V. L.; Krim, S . Specfrochim. A c f a 1962, 18, 1603. (12) Mizushima, S.;Shimanouchi, J.; Nakamura, K.; Nayashi, N.; Tsuchiya, S . J . Chem. Phys. 1957, 26, 970. (13) Brown, J. K.; Sheppard, N. Trans. Faraday SOC.1954, 50, 1164. (14) Sheppard, N. Trans. Faraday SOC.1950, 46, 533.
650
600
550
Wovenumbers ( c m - ' ) Figure 2. Infrared spectra of the C-CI absorption characteristic of gauche 1-chloropropane (650 cm-I) and 2-chloropropane (605 cm-I) as a function of photolysis temperature. The satellite peak centered at 663 cm-' is assigned to 1-chloropropane (probably perturbed by a local site defect or stress field). Each division on the vertical scale represents 0.02 absorbance unit. Samples at 15 and 45 K were subjected to 35 000 laser pulses at 1.0 mJ/cm2 while at 77 K, 1000 pulses at 12 mJ/cm2 were used.
photolysis temperatures below 70 K a satellite peak is observed at 663 cm-I, which we attribute to 1-chloropropane. As shown in Figure 2, the new peak increases in intensity as the photolysis temperature is lowered. This is paralleled by a decrease in the absorption band characteristic of gauche 1-chloropropane (650 cm-I). In an experiment where a sample was irradiated at 15 K (conditions under which this satellite peak assumes greatest intensity) and then warmed to 85 K (the melting point of pure propane), it was observed that this new peak's intensity decreased while the absorption bands characteristic of trans/gauche 1chloropropane increased. When the laser fluence was increased from 1.0 to 12 mJ/cm2, for a given temperature the integrated area of the satellite peak was found to be significantly attenuated. At the higher fluence annealing of the solid can occur, allowing the relaxation of any energetically unfavorable configurations of the product molecule. These results coupled with the proximity of the new peak to the fundamental absorption band of gauche 1-chloropropane support the assignment of this satellite peak as 1-chloropropane; the small blue shift from the main peak at 650 cm-I is likely due to site perturbation^'^ or local stress fields.2J6 In all quantum yield and relative reactivity measurements the total integrated area of the bands at 663 and 650 cm-l was attributed to gauche 1-chloropropane. By examining the intensities of product absorption bands with the spectrum of a 1:l mixture of the authentic compounds, one can estimate the relative reaction yields for the photochlorination of propane. This calibration procedure corrects for the fact that C-C1 stretching vibrations may have different transition moments in I-chloropropane and 2-chloropropane. It is assumed that the 1-chloropropane bands at 650 and 663 cm-I have the same absorption strength. At 77 K, the reaction produces nearly 6 times as much 2-chloropropane as 1-chloropropane. However, as the photolysis temperature is lowered, this value drops to a nearly constant 0.8; that is, the inherent reactivity of the chlorine atoms toward secondary versus primary hydrogen atoms, when corrected for the greater number of primary sites, decreases from 17:l to 2 . 3 : l . This decrease in the reactivity of secondary sites as com(15) Jacox, M. E. J . Mol. Spectrosc. 1985, 113, 286. (16) McBride, J. M. Acc. Chem. Res. 1983, 16, 304.
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2823
Free-Radical Chlorination of Propane
01 0
1
IO
20 30 40 50 60 Photolysis Temperature ( K )
70
00
Figure 3. Relative reactivity of secondary versus primary hydrogens toward chlorine atoms as a function of photolysis temperature. The decrease in the relative reactivity at lower temperatures signals the onset of hot atom reactions (see text for details). 20
I
I
I
O = R e a c t a n t loss
A = Product formation
Photolysis Temperature ( K )
Figure 4. Product yield (1-chloropropane + 2-chloropropane) as a
function of photolysis temperature. Discrepancy between reactant loss and product formation curves is attributed to the stabilizing (trapping) of propyl radicals at low temperatures.
pared to the primary sites is clearly seen in Figure 3 where the relative reactivity is plotted as a function of the photolysis temperature. This result is somewhat surprising since the activation energy for abstraction of a secondary H atom is lower than that for a primary hydrogen. An Arrhenius extrapolation to these lower temperatures would predict a trend resulting in a preferential reaction at the secondary site. Figure 4 shows that the reaction yield increases dramatically as the photolysis temperature is lowered from 77 K. Experiments conducted under low conversion conditions (
