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The temperature dependence of electron attachment to CCl4, CHCl3 and C6H5CH2Cl
Znt. J. Radiat. Phys. Chem. 1971,
Vol. 3, pp. 273-282. Pergamon Press. Ptinted in Great Britain
THE TEMPERATURE DEPENDENCE OF ELECTRON ATTACHMENT TO Ccl,, CHCl, AND C,H,CH,Cl* JOHN M. WARMANtand MYRANC. SAUER,JR. Chemistry
Division, Argonne
National Laboratory,
Argonne,
Illinois
(Received 23 December 1970) Abstract-The temperature dependences and absolute values of the rate constants for the reaction of electrons with Ccl,, CHCl, and CeHsCH,Cl (benzyl chloride) have been measured from 20°C to 200°C. The rate constants are found to be quite well described by the Arrhenius equation k = A exp (- E*/RT). The values of A and E* found are 1.55 x IO-‘, 8.6 x lo-*, 2.0 x lo-’ cm3 s-l and -@57,2*2,2*9 kcal/mol (- 2.4,9.2,12*1 kJ/mol) for Ccl,, CHCl, and C,H,CH,Cl respectively. The pressure dependence of the rate constants have also been investigated. INTRODUCTION
As FORthe reaction kinetics of neutral free radicals the temperature dependence of the rate constant for electron reactions is of considerable interest. The determination of the temperature dependence of dissociative electron attachment to a large number of halogen-containing organic compounds has been carried out by Wentworth et uZ.(13) using the pulse-sampling technique. These studies have shown that the rate constants for such reactions can be well described by the Arrhenius expression k = Aexp(-E*/RT)
found for chemical reactions in general. Since absolute values of the rate constants were not measured in the work of Wentworth et al. values of the pre-exponential factor A could not be obtained. For several halogen-containing compounds absolute rate constants are, however, available from drift tube studies though usually only at one temperature (z 300K). From a combination of the results of these two methods one should then be able to evaluate both the activation energy and pre-exponential factor for a given reaction. Measurement of the absolute value and temperature dependence of the rate constant using a single experimental technique would, however, be preferable. This has been carried out in the present work using a combination of pulse radiolysis and the microwave conductivity method of electron detection for compounds also studied by the pulse-sampling and drift-tube methods. EXPERIMENTAL The gas mixtures to be irradiated were contained in a cylindrical, brass microwave cavity (volume approx. 60 ems) which was coupled to the waveguide detection circuitry by a &in. (6.35 mm) diameter hole, see Fig. 1. The iris coupling hole was sealed with a sandwich of R.T.V. silicone rubber cement (General Electric Co.) between two 5 mil (O-13 mm) sheets of mica. This allowed transmission of microwave power into the cavity while enabling it to be evacuated to less than lo--’ torr (130 pNm-2). To facilitate cleaning and gold plating of the cavity one of the sides consisted of a removable plate which was attached by a flange assembly and sealed with a gold O-ring. The cavity was connected to the vacuum line via &in. (6.35 mm) internally gold-plated copper tubing. * Work performed under the auspices of the U.S. Atomic Energy Commission. t Present address:
Interuniversitair
Reactor Instituut, 273
Delft, Nederland.
214
JOHN M. WARMANand MYRAN C. SAUER, JR.
FIG. 1. Cavity design: A, &in. (0.8 mm) diameter gold O-ring; B, 5 mil (0.13 mm) mica with RTV cement between; C, eight equally spaced bolts (only two shown); D, 1 in. x 4 in. (25.4 x 12-7 mm) waveguide; E, $ in. (6.35 mm) copper tubing (internally gold plated); F, gold-plated cylindrical brass cavity [&in. (0.8 mm) walls].
Gold plating of the cavity was carried out in order to minimize absorption onto and desorption from the walls, the effects of which were apparent in initial experiments using an untreated brass vessel. The cavity was pretreated by scrubbing with scouring powder, treating with 30% nitric acid and rinsing several times with water, ethanol and hexane. Plating was achieved by immersion in Atomex (Engelhard Industries Inc.) gold solution buffered to pH 5.15 with citric acid. The vessel was then rinsed with water, connected to the vacuum line and pumped at 200°C for several hours. Effects of absorption of materials on the walls was further minimized by having the vessel permanently in place in front of the beam exit window and connected to the vacuum line. This allowed samples to be irradiated within a minute of being introduced into the vessel. The electron decay rate was in fact found to vary negligibly even when a mixture containing less than 1O-5 torr (1.3 mNmW2) Ccl, was allowed to stand in the cavity for 10 min. To prevent mercury contamination a liquid nitrogen trap was placed between the mercury diffusion pump and the main vacuum manifold and pressures were measured using a Baratron (MKS Instruments) capacitance manometer. Apart from a gold-plated brass Hoke valve which was connected to the cavity, all stopcocks were Pyrex with Kel-F bores and viton-A O-rings (Kontes). Glass-to-glass and glass-to-metal joints were achieved using Cajon stainless steel ultra-torr connectors. All gas mixtures were initially prepared as a solution of the compound of interest in liquid hexane. The concentration required was obtained by accurate dilution of more concentrated solutions. This was thought to be a much more reliable method of sample preparation than conventional gas-mixing techniques since trace amounts of material lost by absorption on the vacuum manifold could have introduced large errors in mixture composition. A known amount of the solution was introduced
The temperature
dependence
of electron attachment
to CCL, CHCls and GH,CH,Cl
275
into the evacuated cavity by syringe injection through a septum, H in Fig. 2. It was found unnecessary to deaerate the liquid samples since the amount of dissolved air was insufficient to appreciably effect the electron decay rate. The decay rate in hexane alone was frequently determined as a check on the condition of the vessel and purity of the solvent. The vessel was enclosed in an oven as shown in Fig. 2 and the temperature was controlled using a Fisher proportional temperature controller with the probe of the controller in contact with the vessel. The gas temperature was measured with a thermocouple which was attached to the surface of the vessel with asbestos paste.
FIG. 2. Heating and injection system: A, Heaters; B, Q in. ( zz 10 mm) asbestos walls; C, microwave cavity (irradiation vessel); D, Hoke valve; E, 4.5 mm i.d. copper; F, 4 mm i.d. Pyrex; G, Cajon ultra-torr fittings (stainless steel); H, silicone rubber septum; I, Kontes greaseless stopcock.
The gas was ionized by a 3 ns pulse of 600 kV X-rays from a Febetron 706 (Field Emission Corporation). Because of the high penetrating power of the X-rays no transmission windows in the oven or vessel were required. The electron concentration in the cavity was followed by measuring the change in resonant frequency of the cavity using microwave circuitry which has been described in full previously@). RESULTS
AND DISCUSSION
with CCL, CHCI, and C,H,CH,Cl in the gas phase is presumed to occur by dissociative attachment resulting in the formation of the chloride ion: The reaction
(1)
of thermal
electrons
e+RCl
kBcn’ R-t Cl-.
276
JOHNM. WARMANand MYRANC. SAWER,JR.
For these compounds reaction (1) is 15, 17 and 17 kcal/mol(63, 72 and 72 kJ/mol) exothermic respectively. In the present experiments the rate constants of reaction (1) have been measured by studying the decrease in electron concentration as a function of time following pulse ionization of n-hexane gas containing small amounts of RCI. Since the electron concentration, [e], was several orders of magnitude less than the concentration of attaching additive, [e] was found to decrease exponentially with time, t, following the pulse, in accordance with a pseudo-first-order reaction mechanism :
(4
kl = kl&v - hdRC~l+
0 21.
In equation (A) [e& is the electron concentration immediately after the pulse and D is a term introduced to take into account the first-order decay which occurs even in the absence of additive due to impurities present in the buffer gas. The exponential decay rate, kncl[RCl] + D, was obtained from the slope of a plot of In [e] vs. t for several additive concentrations. The attachment rate constant was then determined from the slope of a plot of the decay rate against [RCl]. Such plots are shown in Fig. 3 for C,H,CH,Cl at several different temperatures. As can be seen, good linear plots are obtained and even at the highest temperature the correction for the decay in the buffer gas alone is small.
[B&I]
lOi molecules
cm-3
FIG. 3. Dependence of the rate of electron decay on benzyl chloride concentration. Pressure of n-hexane = 22 torr (2.9 kN m-2) (at 273K): 0, 22°C. 0, 54°C; q ,108.5”C; a, 147°C.
If electron attachment occurs dissociatively, reaction (l), then the rate constant should be independent of the buffer gas pressure. Extensive experiments were carried out with Ccl, to determine the effect of total pressure, if any, on the rate constant.
The temperature
dependence
of electron attachment
to CC&, CHCI, and SHsCHsCl
277
The invariance of kc,, at 23”C, over a pressure range from 0.1 to 100 torr (13 to 1300 Nm-2) is shown in Fig. 4. The results of Fig. 4 also testify to the accuracy of the liquid dilution method of sample preparation since most of the results lie within 5 per cent of the mean value for kCCI,of 4.1 x lo-’ cm3s-l. In the case of CHCI, a slight increase in the rate constant with increasing pressure was found. At 23°C kCHClsincreased from 2.2 to 2.65 x 1O-9 cm3s-l in going from 33 to 80 torr (4.4 to I
I
I
I
IllIll
I
I
IllIll
I
I
111111~'
I
I
1111111
5-
% T
:: Z E
A.-_--_--_--__--_------------------~--~--~~ 4-
___-__-__--_-___-__-------------~-----~--~~~ 0
% 2 ?-
0 x 2 83 + 2
2
0.1
I
I
Illllll
I
I
Illrrll IO
I
100
Pn-hranr (tow) FIG. 4. Dependence of the rate constant for electron attachment to carbon tetrachloride on buffer gas pressure. Dashed lines represent + 5 per cent deviation from mean (solid line).
10.6 kNm-2) and at 130°C from 5.3 to 6.5 x 10” cm3s-l in going from 25 torr to 110 torr (3.3 to 14.6 kNm-2). The fact that experiments carried out in exactly the same way with CC& showed no pressure dependence suggests this to be a real effect and not an artifact of the experiment. A pressure dependence of the thermal attachment rate constant for certain alkyl bromides has recently been observed(6) using the drift-tube technique. This effect could be explained if the initially formed excited negative ion state of CHCl, was sufficiently long lived with respect to dissociation that collisional stabilization could also occur to an appreciable extent. e+CHCl 3_ CHCl,-* CHCl,-* + M -
CHCl,-*, CHCl, + Cl-, CHCl,-.
Since such pressure effects would invalidate the comparison of absolute values of rate constants determined in different pressure r&imes or with different buffer gases, this aspect of the present results obviously requires further investigation. At 135°C k CsHsCHnClincreases from 5.7 to 6.1 x lo-lo cm3s-l in going from 47 to 128 torr
278
JOHN M. WARMANand MYRAN C. SAUER, JR.
(6.3 to 17-OkNm-2) total pressure. However, this change is within the error of the experiment and therefore indicates that if there is any three-body component to electron attachment by benzyl chloride it is extremely small. The temperature dependences of the rate constants are shown in Figs. 5, 6 and 7 as plots of In kRcl vs. T-l in order to test the applicability of the Arrhenius expression : k = Aexp(-E*/RT).
09
Over the temperature
range studied (23-200°C) the Arrhenius plots are seen to
x * u 0 + 2
2-
’
2.4
I
I
I
I
2.6
2.8
3.0
3.2
3.4
T-’ x I03 (K-I) FIG. 5. Arrhenius
plot of the temperature dependence of the rate constant electron attachment to carbon tetrachloride.
for
approximate quite well to straight lines though a slight upward curvature for C,H,CH,Cl and CHCl, and downward curvature for Ccl, are apparent on closer inspection. The values of the pre-exponential factors, A, and activationenergies, E*, calculated from the straight lines in Figs. 5, 6 and 7 are given in the second and third columns of Table 1. Since the rate constant of 4.1 x lo-’ cm3s-l for electron attachment to Ccl, at 300 K is close to the theoretical limit? for thermal electron attachment t The maximum cross-section
for electron attachment
is given by
hZ (Tmax = G’ where h is the de Broglie wavelength maximum rate constant is then
of the electron
= h/me.
For an electron
of velocity u the
The mean value of the maximum rate constant for thermal electrons, km,, is obtained by averaging over the Maxwellian distribution of electron velocities.
h2
1 = @$
mP(kT)a
= 5.0 x lo-’
cm3 s-l.
T-’
x
I
IO3 (K-l)
2.8
I 3.0 3.2
3.4
FIG. 6. Arrhenius plot of the temperature dependence of the rate constant for electron attachment to chloroform for n-hexane pressures (at 273°C) of 17 torr (2.3 kN m-3, 0; 30 torr (4.0 kN m-a), 0; 52 torr (6.9 kN m-*), l ; 74 torr (9.6 kN m--8), A.
I
2.6
I
2.4
Fro.
7.
2.2
2.0
2.4
I T-’
x IO3
2.6
I
(K-l)
2.8
I
3.0
I
3.2
I
\ 3.4
Arrhenius plot of the temperature dependence of the rate constant for electron attachment to benzyl chloride.
I
I
I-
I-
I-
t-
JOHN M. WARMAN and MYRAN
280
C. SAUER, JR.
TABLE 1
Compound
A(cm3 s-l)
k~~l (cm3 s-l)
CCI,
1.55 x10-7
-0.57
-2.4
CHCl,*
8.6 x 10-S
-0.05 -0.6 2.2
-0.2 -2.5 9.2
CsH,CH,CI
2.0 x 10-7
3.1 2.9
13.0 12.1
3.6
15.1
T(K”)
Reference
4.1 10-F 2.9 10-7 2.9 10-T
300 300 300
Present work 6 8 1
2.2 10--B 3.9 10-Q 5.0 10-Q
300 300 300
5.5 lo-‘0 4 10-10
393 393
Presenf work 6 6 1 Present work 9 2
* Values are for 30 torr (4.0 kN m-2) n-hexane pressure.
That E*CC,a is 5 x lo-' cm3s-l) a very low activation energy would be expected. actually negative, - 0.57 kcal/mol (- 2.4 kJ/mol), is not surprising in view of the very marked decrease in k,,, 4 with increasing electron energy which has been found in drift-tube experiments@) and used by the present authors to detect epithermal electrons(7). A negative activation energy for this reaction has also been measured by Wentworth et al. who have published values of -0.05(l) and -06 kcal/mol(2) (-0.2 and - 2.5 kJ/mol), the latter being in very good agreement with the present value. The absolute value of kCCll at 300 K is approximately 40 per cent higher than the 2.9 x lo-’ cm3 s-r obtained in two independent drift-tube studies@ss). Because of the very rapid decrease of kC,-.i4with increasing electron energy this discrepancy may be due to a slight deviation of the mean electron energy from the thermal value even at the lowest reduced field strengths used in the drift-tube experiments. A mean energy only 10 per cent greater than the thermal value would be sufficient to explain the difference. Certainly impurities could not be responsible since the rate constant based on the total concentration is already very close to the theoretical limit. Also possible effects of adsorption in the present experiments would only serve to give an apparently lower rate constant. In view of the fact that one is dealing with very low concentrations (< 10e5 torr, or 1.3 mN m-“) a deviation of less than 20 per cent from the mean value of 3.5 x lo-’ cm3 s-r determined by two entirely different experimental methods might, however, be considered to be fairly satisfactory. Despite the pressure dependence of kCHC13 the activation energy is seen in Fig. 6 to be independent of the buffer gas pressure within the errors of the experiment. The value of 2.2 kcal/mol(9.2 kJ/mol) is considerably lower than the 3.1 kcal/mol (13.0 kJ/mol) reported by Wentworth et d(1) which is outside the maximum error limits of the present work, jO.2 kcal/mol(O.8 kJ/mol). No explanation of this rather serious discrepancy, which as discussed below is also found for benzyl chloride, is apparent at present. As mentioned previously, the comparison of absolute values of kcncis is complicated by the apparent pressure dependence of this rate constant. This may explain why the values of 3.9 x 1O-g cm3 s-l and 5.0 x 1O-g cm3 s-l measured using the drift-tube method”) (at 300 K) are higher than the value of 2.2 x lo-” cm3 s-i obtained in the present work for a n-hexane pressure of 30 torr (4.0 kN rnw2).
The temperature
dependence
of electron attachment
to Ccl,, CHCI, and C,H,CH,Cl
281
In the case of benzyl chloride the activation energy of 2.9 kcal/mol(12~1 kJ/mol) is again considerably lower than that of Wentworth et al.@), 3.6 + 0.2 kcal/mol (15.1 i_O-8 kJ/mol). An approximate value of k4n1cnpCI = 4 x lo-lo cm3 s-l, at 12O”C, has been obtained@) by studying the competition between electron attachment to benzyl chloride and electron-ion recombination with an assumed rate constant of 2 x lo4 cm3 s-l. This is in quite good agreement with the present value of 5.5 x lo-lo cm3 s-l at 120°C. However, in the same work a value for kCoip/kCsH6cHtC1 = 180 was found by competitive electron scavenging as opposed to the value of 580 found in the present work. Unfortunately no value for ICo,,H6CH~CI is available from drift-tube experiments. In summary it may be said that while no serious disagreement has been found between the absolute rate constants determined using the present technique and other methods, large differences have been found in the values for the activation energies, in particular for CHCI, and C,H,CH,Cl. Since activation energies have been used, particularly by Wentworth et al., to evaluate molecular parameters relating the neutral and negative ion states of molecules, these discrepancies become of greater significance. Determination of the activation energies by a third independent method such as drift tube or the recently developed flowing afterglow technique may help to resolve these differences. In this respect it may be relevant to note that an activation energy of approximately zero has been obtained for electron attachment to SF, using the flowing afterglow method( whereas a value of 0.9 kcal/ mol(3.8 kJ/mol) is obtained using the pulse sampling method( REFERENCES W. E. WENTWORTH,R. S. BECKERand R. TUNG, J. phys. Chem. 1967,71, 1652. W. E. WENTWORTH,R. GEORGEand H. KEITH,J. them. Phys. 1969,51, 1791. J. C. STEELHAMMER and W. E. WENTWORTH,J. them. Phys. 1969,51,1802. R. W. FESSENDEN and J. M. WARMAN, Adv. Chem. Ser. 1968, 82, 222. L. G. CHRISTOPHOROU, private communication. 6. R. P. BLAUNSTEIN and L. G. CHRISTOPHOROU, J. them. Phys. 1968, 49, 1526. 7. J. M. WARMAN and M. C. SAUER,JR., J. them. Phys. 1970,52, 6428. 8. L. BOUBY,F. FIQUET-FAYARDand H. ABGRALL,C. r. hPbd Seam. Acad. Sci., Paris 1965,261,
1. 2. 3. 4. 5.
4059. 9. G. R. A. JOHNSON,M. C. SAUERand J. M. WARMAN, J. them. Phys. 1969, 50,4933. 10. F. C. FEHSENFELD, J. them. Phys. 1970, 53, 2000. 11. E. CHEN, R. D. GEORGEand W. E. WENTWORTH,J. them. Phys. 1968,49,1973.
R&urn&-On a mesure les valeurs absolues (ainsi que leur variation avec la temperature) des constantes de vitesse de la reaction des electrons avec CC&, CHC& et C,H&H,Cl (chlorure de benzyle) entre 20et 200°C. Les constantes de vitesse obeissent assez bien a la loi d’Arrhenius k = A exp (-E*/RT). Les valeurs trouvees pour A et E* sont respectivement: 1,55 1O-7, 8,6 1O-8, 2,0 lo-’ cm3 s-t, et - 0,57,2,2,2,9 kcal/mol (- 2,4,9,2,12,1 kJ/mol) pour CC&, CHCI, et C,H,CH,Cl. On a aussi CtudiC l’effet de la pression sur les constantes de vitesse. Zusammenfassung-Absolute Werte sowie die Temperaturabglngigkeit der Geschwindigkeitskontanten ftir die Reaktion von Elektronen mit Ccl,, CHCI, und C,H,CH,Cl (Benzylchlorid) im Bereich von 20 bis 200°C wurden bestimmt. Die Geschwindigkeitskonstanten gehorchen ziemlich gut der Arrhenius-Gleichung k = A exp (- E*/RT). Die gefundenen Werte von A und E* sind 1,55 lo-‘, 8,6 10es, 2,0 lo-’ cm8 s-l und -0,57, 2,2, 2,9 kcal/mol (- 2,4, 9,2, 12,l kJ/mol) fur Ccl,, CHCI, und C,H,CH,Cl bzw. Die Druckabhlngigkeit der Geschwindigkeitskonstanten wurde ebenfalls untersucht.
282
JOHN M. WARMAN and MYRAN
C. SAUER, JR.
Pe3IOMe-6bIJIa UCCJIeAOBaHa TeMIIepaTypHaR 3aBHCUMOCTb &OJIEOTHbIX BWIIi'iHHKOHCTaHT CKOpOCT& AJIR pesutI@ 3JIeKTpOHOB C CCL, CHCI, u C6H,CH,Cl B nmepsme TemepaTyp OT 20 A0 200 rpmCOB. fibIJI0 HdAeHO,'4TO KOHCTBHTbI CKOpOCTek AOBOlIbHO XOpOIIIO OIlECbIBFUOTCR ypaBHemeM Appemiyca k= A eXp(-E*/RT). BenwHHbI A H E* OKa3amcb paeHbrrm 1,55.10-‘; 8,6*10-‘; 2,0.10-7 CM3 CeK-l H -0,57; 2,2; 2,9KKan/MOn mm (-2,4; 9,2; 12,1 X_qH(/MOJl)AJIR cc14, CHQ, &&C&Cl COOTBeTCTBeHHO. 6bIna HCCneAOBaHa TaKxe 3aBHCHMOCTb KOHCTaHTbI CKOpOCTEiOTAaBlIeHU5L
Vol. 3, pp. 273-282. Pergamon Press. Ptinted in Great Britain
THE TEMPERATURE DEPENDENCE OF ELECTRON ATTACHMENT TO Ccl,, CHCl, AND C,H,CH,Cl* JOHN M. WARMANtand MYRANC. SAUER,JR. Chemistry
Division, Argonne
National Laboratory,
Argonne,
Illinois
(Received 23 December 1970) Abstract-The temperature dependences and absolute values of the rate constants for the reaction of electrons with Ccl,, CHCl, and CeHsCH,Cl (benzyl chloride) have been measured from 20°C to 200°C. The rate constants are found to be quite well described by the Arrhenius equation k = A exp (- E*/RT). The values of A and E* found are 1.55 x IO-‘, 8.6 x lo-*, 2.0 x lo-’ cm3 s-l and -@57,2*2,2*9 kcal/mol (- 2.4,9.2,12*1 kJ/mol) for Ccl,, CHCl, and C,H,CH,Cl respectively. The pressure dependence of the rate constants have also been investigated. INTRODUCTION
As FORthe reaction kinetics of neutral free radicals the temperature dependence of the rate constant for electron reactions is of considerable interest. The determination of the temperature dependence of dissociative electron attachment to a large number of halogen-containing organic compounds has been carried out by Wentworth et uZ.(13) using the pulse-sampling technique. These studies have shown that the rate constants for such reactions can be well described by the Arrhenius expression k = Aexp(-E*/RT)
found for chemical reactions in general. Since absolute values of the rate constants were not measured in the work of Wentworth et al. values of the pre-exponential factor A could not be obtained. For several halogen-containing compounds absolute rate constants are, however, available from drift tube studies though usually only at one temperature (z 300K). From a combination of the results of these two methods one should then be able to evaluate both the activation energy and pre-exponential factor for a given reaction. Measurement of the absolute value and temperature dependence of the rate constant using a single experimental technique would, however, be preferable. This has been carried out in the present work using a combination of pulse radiolysis and the microwave conductivity method of electron detection for compounds also studied by the pulse-sampling and drift-tube methods. EXPERIMENTAL The gas mixtures to be irradiated were contained in a cylindrical, brass microwave cavity (volume approx. 60 ems) which was coupled to the waveguide detection circuitry by a &in. (6.35 mm) diameter hole, see Fig. 1. The iris coupling hole was sealed with a sandwich of R.T.V. silicone rubber cement (General Electric Co.) between two 5 mil (O-13 mm) sheets of mica. This allowed transmission of microwave power into the cavity while enabling it to be evacuated to less than lo--’ torr (130 pNm-2). To facilitate cleaning and gold plating of the cavity one of the sides consisted of a removable plate which was attached by a flange assembly and sealed with a gold O-ring. The cavity was connected to the vacuum line via &in. (6.35 mm) internally gold-plated copper tubing. * Work performed under the auspices of the U.S. Atomic Energy Commission. t Present address:
Interuniversitair
Reactor Instituut, 273
Delft, Nederland.
214
JOHN M. WARMANand MYRAN C. SAUER, JR.
FIG. 1. Cavity design: A, &in. (0.8 mm) diameter gold O-ring; B, 5 mil (0.13 mm) mica with RTV cement between; C, eight equally spaced bolts (only two shown); D, 1 in. x 4 in. (25.4 x 12-7 mm) waveguide; E, $ in. (6.35 mm) copper tubing (internally gold plated); F, gold-plated cylindrical brass cavity [&in. (0.8 mm) walls].
Gold plating of the cavity was carried out in order to minimize absorption onto and desorption from the walls, the effects of which were apparent in initial experiments using an untreated brass vessel. The cavity was pretreated by scrubbing with scouring powder, treating with 30% nitric acid and rinsing several times with water, ethanol and hexane. Plating was achieved by immersion in Atomex (Engelhard Industries Inc.) gold solution buffered to pH 5.15 with citric acid. The vessel was then rinsed with water, connected to the vacuum line and pumped at 200°C for several hours. Effects of absorption of materials on the walls was further minimized by having the vessel permanently in place in front of the beam exit window and connected to the vacuum line. This allowed samples to be irradiated within a minute of being introduced into the vessel. The electron decay rate was in fact found to vary negligibly even when a mixture containing less than 1O-5 torr (1.3 mNmW2) Ccl, was allowed to stand in the cavity for 10 min. To prevent mercury contamination a liquid nitrogen trap was placed between the mercury diffusion pump and the main vacuum manifold and pressures were measured using a Baratron (MKS Instruments) capacitance manometer. Apart from a gold-plated brass Hoke valve which was connected to the cavity, all stopcocks were Pyrex with Kel-F bores and viton-A O-rings (Kontes). Glass-to-glass and glass-to-metal joints were achieved using Cajon stainless steel ultra-torr connectors. All gas mixtures were initially prepared as a solution of the compound of interest in liquid hexane. The concentration required was obtained by accurate dilution of more concentrated solutions. This was thought to be a much more reliable method of sample preparation than conventional gas-mixing techniques since trace amounts of material lost by absorption on the vacuum manifold could have introduced large errors in mixture composition. A known amount of the solution was introduced
The temperature
dependence
of electron attachment
to CCL, CHCls and GH,CH,Cl
275
into the evacuated cavity by syringe injection through a septum, H in Fig. 2. It was found unnecessary to deaerate the liquid samples since the amount of dissolved air was insufficient to appreciably effect the electron decay rate. The decay rate in hexane alone was frequently determined as a check on the condition of the vessel and purity of the solvent. The vessel was enclosed in an oven as shown in Fig. 2 and the temperature was controlled using a Fisher proportional temperature controller with the probe of the controller in contact with the vessel. The gas temperature was measured with a thermocouple which was attached to the surface of the vessel with asbestos paste.
FIG. 2. Heating and injection system: A, Heaters; B, Q in. ( zz 10 mm) asbestos walls; C, microwave cavity (irradiation vessel); D, Hoke valve; E, 4.5 mm i.d. copper; F, 4 mm i.d. Pyrex; G, Cajon ultra-torr fittings (stainless steel); H, silicone rubber septum; I, Kontes greaseless stopcock.
The gas was ionized by a 3 ns pulse of 600 kV X-rays from a Febetron 706 (Field Emission Corporation). Because of the high penetrating power of the X-rays no transmission windows in the oven or vessel were required. The electron concentration in the cavity was followed by measuring the change in resonant frequency of the cavity using microwave circuitry which has been described in full previously@). RESULTS
AND DISCUSSION
with CCL, CHCI, and C,H,CH,Cl in the gas phase is presumed to occur by dissociative attachment resulting in the formation of the chloride ion: The reaction
(1)
of thermal
electrons
e+RCl
kBcn’ R-t Cl-.
276
JOHNM. WARMANand MYRANC. SAWER,JR.
For these compounds reaction (1) is 15, 17 and 17 kcal/mol(63, 72 and 72 kJ/mol) exothermic respectively. In the present experiments the rate constants of reaction (1) have been measured by studying the decrease in electron concentration as a function of time following pulse ionization of n-hexane gas containing small amounts of RCI. Since the electron concentration, [e], was several orders of magnitude less than the concentration of attaching additive, [e] was found to decrease exponentially with time, t, following the pulse, in accordance with a pseudo-first-order reaction mechanism :
(4
kl = kl&v - hdRC~l+
0 21.
In equation (A) [e& is the electron concentration immediately after the pulse and D is a term introduced to take into account the first-order decay which occurs even in the absence of additive due to impurities present in the buffer gas. The exponential decay rate, kncl[RCl] + D, was obtained from the slope of a plot of In [e] vs. t for several additive concentrations. The attachment rate constant was then determined from the slope of a plot of the decay rate against [RCl]. Such plots are shown in Fig. 3 for C,H,CH,Cl at several different temperatures. As can be seen, good linear plots are obtained and even at the highest temperature the correction for the decay in the buffer gas alone is small.
[B&I]
lOi molecules
cm-3
FIG. 3. Dependence of the rate of electron decay on benzyl chloride concentration. Pressure of n-hexane = 22 torr (2.9 kN m-2) (at 273K): 0, 22°C. 0, 54°C; q ,108.5”C; a, 147°C.
If electron attachment occurs dissociatively, reaction (l), then the rate constant should be independent of the buffer gas pressure. Extensive experiments were carried out with Ccl, to determine the effect of total pressure, if any, on the rate constant.
The temperature
dependence
of electron attachment
to CC&, CHCI, and SHsCHsCl
277
The invariance of kc,, at 23”C, over a pressure range from 0.1 to 100 torr (13 to 1300 Nm-2) is shown in Fig. 4. The results of Fig. 4 also testify to the accuracy of the liquid dilution method of sample preparation since most of the results lie within 5 per cent of the mean value for kCCI,of 4.1 x lo-’ cm3s-l. In the case of CHCI, a slight increase in the rate constant with increasing pressure was found. At 23°C kCHClsincreased from 2.2 to 2.65 x 1O-9 cm3s-l in going from 33 to 80 torr (4.4 to I
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I
I
IllIll
I
I
IllIll
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I
111111~'
I
I
1111111
5-
% T
:: Z E
A.-_--_--_--__--_------------------~--~--~~ 4-
___-__-__--_-___-__-------------~-----~--~~~ 0
% 2 ?-
0 x 2 83 + 2
2
0.1
I
I
Illllll
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I
Illrrll IO
I
100
Pn-hranr (tow) FIG. 4. Dependence of the rate constant for electron attachment to carbon tetrachloride on buffer gas pressure. Dashed lines represent + 5 per cent deviation from mean (solid line).
10.6 kNm-2) and at 130°C from 5.3 to 6.5 x 10” cm3s-l in going from 25 torr to 110 torr (3.3 to 14.6 kNm-2). The fact that experiments carried out in exactly the same way with CC& showed no pressure dependence suggests this to be a real effect and not an artifact of the experiment. A pressure dependence of the thermal attachment rate constant for certain alkyl bromides has recently been observed(6) using the drift-tube technique. This effect could be explained if the initially formed excited negative ion state of CHCl, was sufficiently long lived with respect to dissociation that collisional stabilization could also occur to an appreciable extent. e+CHCl 3_ CHCl,-* CHCl,-* + M -
CHCl,-*, CHCl, + Cl-, CHCl,-.
Since such pressure effects would invalidate the comparison of absolute values of rate constants determined in different pressure r&imes or with different buffer gases, this aspect of the present results obviously requires further investigation. At 135°C k CsHsCHnClincreases from 5.7 to 6.1 x lo-lo cm3s-l in going from 47 to 128 torr
278
JOHN M. WARMANand MYRAN C. SAUER, JR.
(6.3 to 17-OkNm-2) total pressure. However, this change is within the error of the experiment and therefore indicates that if there is any three-body component to electron attachment by benzyl chloride it is extremely small. The temperature dependences of the rate constants are shown in Figs. 5, 6 and 7 as plots of In kRcl vs. T-l in order to test the applicability of the Arrhenius expression : k = Aexp(-E*/RT).
09
Over the temperature
range studied (23-200°C) the Arrhenius plots are seen to
x * u 0 + 2
2-
’
2.4
I
I
I
I
2.6
2.8
3.0
3.2
3.4
T-’ x I03 (K-I) FIG. 5. Arrhenius
plot of the temperature dependence of the rate constant electron attachment to carbon tetrachloride.
for
approximate quite well to straight lines though a slight upward curvature for C,H,CH,Cl and CHCl, and downward curvature for Ccl, are apparent on closer inspection. The values of the pre-exponential factors, A, and activationenergies, E*, calculated from the straight lines in Figs. 5, 6 and 7 are given in the second and third columns of Table 1. Since the rate constant of 4.1 x lo-’ cm3s-l for electron attachment to Ccl, at 300 K is close to the theoretical limit? for thermal electron attachment t The maximum cross-section
for electron attachment
is given by
hZ (Tmax = G’ where h is the de Broglie wavelength maximum rate constant is then
of the electron
= h/me.
For an electron
of velocity u the
The mean value of the maximum rate constant for thermal electrons, km,, is obtained by averaging over the Maxwellian distribution of electron velocities.
h2
1 = @$
mP(kT)a
= 5.0 x lo-’
cm3 s-l.
T-’
x
I
IO3 (K-l)
2.8
I 3.0 3.2
3.4
FIG. 6. Arrhenius plot of the temperature dependence of the rate constant for electron attachment to chloroform for n-hexane pressures (at 273°C) of 17 torr (2.3 kN m-3, 0; 30 torr (4.0 kN m-a), 0; 52 torr (6.9 kN m-*), l ; 74 torr (9.6 kN m--8), A.
I
2.6
I
2.4
Fro.
7.
2.2
2.0
2.4
I T-’
x IO3
2.6
I
(K-l)
2.8
I
3.0
I
3.2
I
\ 3.4
Arrhenius plot of the temperature dependence of the rate constant for electron attachment to benzyl chloride.
I
I
I-
I-
I-
t-
JOHN M. WARMAN and MYRAN
280
C. SAUER, JR.
TABLE 1
Compound
A(cm3 s-l)
k~~l (cm3 s-l)
CCI,
1.55 x10-7
-0.57
-2.4
CHCl,*
8.6 x 10-S
-0.05 -0.6 2.2
-0.2 -2.5 9.2
CsH,CH,CI
2.0 x 10-7
3.1 2.9
13.0 12.1
3.6
15.1
T(K”)
Reference
4.1 10-F 2.9 10-7 2.9 10-T
300 300 300
Present work 6 8 1
2.2 10--B 3.9 10-Q 5.0 10-Q
300 300 300
5.5 lo-‘0 4 10-10
393 393
Presenf work 6 6 1 Present work 9 2
* Values are for 30 torr (4.0 kN m-2) n-hexane pressure.
That E*CC,a is 5 x lo-' cm3s-l) a very low activation energy would be expected. actually negative, - 0.57 kcal/mol (- 2.4 kJ/mol), is not surprising in view of the very marked decrease in k,,, 4 with increasing electron energy which has been found in drift-tube experiments@) and used by the present authors to detect epithermal electrons(7). A negative activation energy for this reaction has also been measured by Wentworth et al. who have published values of -0.05(l) and -06 kcal/mol(2) (-0.2 and - 2.5 kJ/mol), the latter being in very good agreement with the present value. The absolute value of kCCll at 300 K is approximately 40 per cent higher than the 2.9 x lo-’ cm3 s-r obtained in two independent drift-tube studies@ss). Because of the very rapid decrease of kC,-.i4with increasing electron energy this discrepancy may be due to a slight deviation of the mean electron energy from the thermal value even at the lowest reduced field strengths used in the drift-tube experiments. A mean energy only 10 per cent greater than the thermal value would be sufficient to explain the difference. Certainly impurities could not be responsible since the rate constant based on the total concentration is already very close to the theoretical limit. Also possible effects of adsorption in the present experiments would only serve to give an apparently lower rate constant. In view of the fact that one is dealing with very low concentrations (< 10e5 torr, or 1.3 mN m-“) a deviation of less than 20 per cent from the mean value of 3.5 x lo-’ cm3 s-r determined by two entirely different experimental methods might, however, be considered to be fairly satisfactory. Despite the pressure dependence of kCHC13 the activation energy is seen in Fig. 6 to be independent of the buffer gas pressure within the errors of the experiment. The value of 2.2 kcal/mol(9.2 kJ/mol) is considerably lower than the 3.1 kcal/mol (13.0 kJ/mol) reported by Wentworth et d(1) which is outside the maximum error limits of the present work, jO.2 kcal/mol(O.8 kJ/mol). No explanation of this rather serious discrepancy, which as discussed below is also found for benzyl chloride, is apparent at present. As mentioned previously, the comparison of absolute values of kcncis is complicated by the apparent pressure dependence of this rate constant. This may explain why the values of 3.9 x 1O-g cm3 s-l and 5.0 x 1O-g cm3 s-l measured using the drift-tube method”) (at 300 K) are higher than the value of 2.2 x lo-” cm3 s-i obtained in the present work for a n-hexane pressure of 30 torr (4.0 kN rnw2).
The temperature
dependence
of electron attachment
to Ccl,, CHCI, and C,H,CH,Cl
281
In the case of benzyl chloride the activation energy of 2.9 kcal/mol(12~1 kJ/mol) is again considerably lower than that of Wentworth et al.@), 3.6 + 0.2 kcal/mol (15.1 i_O-8 kJ/mol). An approximate value of k4n1cnpCI = 4 x lo-lo cm3 s-l, at 12O”C, has been obtained@) by studying the competition between electron attachment to benzyl chloride and electron-ion recombination with an assumed rate constant of 2 x lo4 cm3 s-l. This is in quite good agreement with the present value of 5.5 x lo-lo cm3 s-l at 120°C. However, in the same work a value for kCoip/kCsH6cHtC1 = 180 was found by competitive electron scavenging as opposed to the value of 580 found in the present work. Unfortunately no value for ICo,,H6CH~CI is available from drift-tube experiments. In summary it may be said that while no serious disagreement has been found between the absolute rate constants determined using the present technique and other methods, large differences have been found in the values for the activation energies, in particular for CHCI, and C,H,CH,Cl. Since activation energies have been used, particularly by Wentworth et al., to evaluate molecular parameters relating the neutral and negative ion states of molecules, these discrepancies become of greater significance. Determination of the activation energies by a third independent method such as drift tube or the recently developed flowing afterglow technique may help to resolve these differences. In this respect it may be relevant to note that an activation energy of approximately zero has been obtained for electron attachment to SF, using the flowing afterglow method( whereas a value of 0.9 kcal/ mol(3.8 kJ/mol) is obtained using the pulse sampling method( REFERENCES W. E. WENTWORTH,R. S. BECKERand R. TUNG, J. phys. Chem. 1967,71, 1652. W. E. WENTWORTH,R. GEORGEand H. KEITH,J. them. Phys. 1969,51, 1791. J. C. STEELHAMMER and W. E. WENTWORTH,J. them. Phys. 1969,51,1802. R. W. FESSENDEN and J. M. WARMAN, Adv. Chem. Ser. 1968, 82, 222. L. G. CHRISTOPHOROU, private communication. 6. R. P. BLAUNSTEIN and L. G. CHRISTOPHOROU, J. them. Phys. 1968, 49, 1526. 7. J. M. WARMAN and M. C. SAUER,JR., J. them. Phys. 1970,52, 6428. 8. L. BOUBY,F. FIQUET-FAYARDand H. ABGRALL,C. r. hPbd Seam. Acad. Sci., Paris 1965,261,
1. 2. 3. 4. 5.
4059. 9. G. R. A. JOHNSON,M. C. SAUERand J. M. WARMAN, J. them. Phys. 1969, 50,4933. 10. F. C. FEHSENFELD, J. them. Phys. 1970, 53, 2000. 11. E. CHEN, R. D. GEORGEand W. E. WENTWORTH,J. them. Phys. 1968,49,1973.
R&urn&-On a mesure les valeurs absolues (ainsi que leur variation avec la temperature) des constantes de vitesse de la reaction des electrons avec CC&, CHC& et C,H&H,Cl (chlorure de benzyle) entre 20et 200°C. Les constantes de vitesse obeissent assez bien a la loi d’Arrhenius k = A exp (-E*/RT). Les valeurs trouvees pour A et E* sont respectivement: 1,55 1O-7, 8,6 1O-8, 2,0 lo-’ cm3 s-t, et - 0,57,2,2,2,9 kcal/mol (- 2,4,9,2,12,1 kJ/mol) pour CC&, CHCI, et C,H,CH,Cl. On a aussi CtudiC l’effet de la pression sur les constantes de vitesse. Zusammenfassung-Absolute Werte sowie die Temperaturabglngigkeit der Geschwindigkeitskontanten ftir die Reaktion von Elektronen mit Ccl,, CHCI, und C,H,CH,Cl (Benzylchlorid) im Bereich von 20 bis 200°C wurden bestimmt. Die Geschwindigkeitskonstanten gehorchen ziemlich gut der Arrhenius-Gleichung k = A exp (- E*/RT). Die gefundenen Werte von A und E* sind 1,55 lo-‘, 8,6 10es, 2,0 lo-’ cm8 s-l und -0,57, 2,2, 2,9 kcal/mol (- 2,4, 9,2, 12,l kJ/mol) fur Ccl,, CHCI, und C,H,CH,Cl bzw. Die Druckabhlngigkeit der Geschwindigkeitskonstanten wurde ebenfalls untersucht.
282
JOHN M. WARMAN and MYRAN
C. SAUER, JR.
Pe3IOMe-6bIJIa UCCJIeAOBaHa TeMIIepaTypHaR 3aBHCUMOCTb &OJIEOTHbIX BWIIi'iHHKOHCTaHT CKOpOCT& AJIR pesutI@ 3JIeKTpOHOB C CCL, CHCI, u C6H,CH,Cl B nmepsme TemepaTyp OT 20 A0 200 rpmCOB. fibIJI0 HdAeHO,'4TO KOHCTBHTbI CKOpOCTek AOBOlIbHO XOpOIIIO OIlECbIBFUOTCR ypaBHemeM Appemiyca k= A eXp(-E*/RT). BenwHHbI A H E* OKa3amcb paeHbrrm 1,55.10-‘; 8,6*10-‘; 2,0.10-7 CM3 CeK-l H -0,57; 2,2; 2,9KKan/MOn mm (-2,4; 9,2; 12,1 X_qH(/MOJl)AJIR cc14, CHQ, &&C&Cl COOTBeTCTBeHHO. 6bIna HCCneAOBaHa TaKxe 3aBHCHMOCTb KOHCTaHTbI CKOpOCTEiOTAaBlIeHU5L