Thermal Electron Attachment to Some Aliphatic ... - ACS Publications

W. WENTWORTH, R. BECKER, AND R.

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TTJNQ

Thermal Electron Attachment to Some Aliphatic and Aromatic Chloro, Bromo, and Iodo Derivatives

by W. E. Wentworth, Ralph S. Becker, and Roberta Tung Department of Chemistry, University of Houston, Houston, Texas 77004 (Received August 18, 1066)

Thermal electron attachment to some aliphatic and aromatic halogen derivatives was investigated by the pulse-sampling technique as a function of temperature ( =30-200°). I n contrast to stable negative ion formation, the electron attachment increased with increasing temperature, indicating an energy of activation for the process despite the fact that the over-all process is exothermic. Two mechanisms are proposed to explain the observed results: (1) electron attachment to a thermally excited molecule followed by direct dissociation into a halide ion and a radical; (2) electron attachment to form a stable negative ion of the molecule which in turn becomes thermally activated to undergo dissociation into the halide ion and radical. Except for special cases, assignment between the two mechanisms is based upon comparison of the carbon-halogen bond dissociation energies with energies of activation along with estimated or known electron affinities of the aromatic derivatives and the aromatic radical. The aliphatic halides (CH2C12, CHC13, CCl,, n-C3H7Br,and CZHSI) apparently undergo dissociative electron attachment by the former mechanism, whereas the aromatic derivatives (bromobenzene, o-dichlorobenzene, m-dichlorobenzene, 1-chloronaphthalene, and 1-bromonaphthalene), which are expected to have a reasonable electron affinity, apparently follow the latter mechanism.

Introduction

It is generally considered that electron attachment to molecules occurs by either of two processes represented by the chemical

+ e- +ABAB + e - + A + BAB

The latter process, called dissociative electron capture or attachment, in contrast to the nondissociative process, will be the primary concern of this paper. Thus far, most research on dissociative electron attachment has been concerned with the rate or cross section for the process as a function of the electron energy. Generally, the work has been carried out a t ambient or near ambient temperatures on relatively small compounds. In contrast, the work described in this paper is restricted to thermal or near-thermal electron energies, and the temperature dependence of the dissociative electron-attachment process has been investigated where possible over the range 30-200”. Chloro, The JOUTTUZ~ of Physical Chemistry

bromo, and iodo organic derivatives have been studied, and in extreme cases the electron attachment can vary 1000-fold over this temperature range. Within the past 10 years or so, techniques for obtaining monoenergetic electrons have been developed and applied toward the study of dissociative electron attachment. FOX’observed a maximum cross section (1) No attempt will be made to cite all the original references on this subject. Only more general sources will be cited which within themselves contain most of the important original references. (2) R. H.Healy and J. W. Reed, “The Behavior of Slow Electrons in Gases,” Amalgamated Wireless Press (Australasia) Ltd., Sydney, 1941. (3) (a) H. S. W. Massey, “Negative Ions,” Cambridge University Press, New York, N. Y., 1950; (b) H. S. W. Massey and E. H. S. Burhop, “Electronic and Ionic Impact Phenomena,” Oxford University Press, New York, N. Y., 1952. (4) F. H. Field and J. L. Franklin, “Electron Impact Phenomena,” Academic Press Inc., New York, N. Y.,1957. (5) L. B. Loeb, “Basic Processes of Gaseous Electronics,” University of California Press, Berkeley, Calif., 1961. (6) E. W. McDaniel, “Collision Phenomena in Ionized Gases,” John Wiley and Sons, Inc., New York, N. Y., 1964. (7) R. E. Fox, J. Chem. Phys., 26, 1281 (1957).

THERMAL ELECTRON ATTACHMENT TO HALOGEN DERIVATIVES

for HCl at 0.66 ev. Frost and McDowelP investigated the remaining hydrogen halides and found a correlation between the electron energy a t the maximum cross section and the difference between electron affinity of the halide and bond dissociation energy. In general, a rough correlation of this type would be expected; however, a rigorously perfect correlation would not be necessary. Hickam and Berge studied a series of fluoro and chloro derivatives of methane and ethane and generally found one or more maxima at or above thermal energies. The only compound in their study common to those considered in this paper was CCL Some other compounds are similar to ours and these will be commented on later in the paper. In the case of CC4, Hickam and Berg reported a slight temperature dependence for the electron-attachment cross section. After taking into account the change in gas density, their variations are on the same order of magnitude as our measurements. Later, Fox and Curran’O reinvestigated the electron attachment of CC& as a function of source temperature in addition to electron energy and found no significant change in the shape of the curves. However, no mention was made concerning any change in magnitude of capture. Stockdale and Hurst,l’ using a swarm technique, found maximum cross section for electron attachment a t electron energies of 0.76 and 0.70 ev for chlorobenzene and bromobenzene, respectively. Dissociative electron a b tachment was confirmed by observing C1- and Br- in a negative-ion time-of-flight mass spectrometer. Bromobenzene was included in the present studies and the results relative to the above values will be discussed later. An attempt was made in this work to study chlorobenzene; however, the gas chromatographic columns employed did not give sufficient separation from the impurities to give reliable results. Dissociative electron capture can also occur in rigid media by y radiation of organic halides in hydrocarbons.l 2 Presumably, thermal electrons are produced by the y radiation of the solvent or other species present, followed by dissociative electron capture by the organic halide. Many of the radical products have been identified by their visible-ultraviolet absorption spectra and more recently by paramagnetic resonance.’* To the best of our knowledge, there has not been an extensive study of dissociative electron attachment in the gas phase using thermal electrons. In our work, the “pulse-sampling technique” was employed which has been described previously. 14~16 The details of the operational parameters have been reported recentlyls along with justification for electronmolecule interaction under a zero potential field. In a gas of argon-10% methane, presumably thermal

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or near-thermal electron energies are obtained within the relatively long 1000-psec pulse intervals. The kinetics of the process have been presented for the nondissociative electron-attachment process.l6 A similar investigation of the kinetic processes for dissociative capture will be presented in this paper. Further indirect support for thermal or near-thermal electrons exists from previous correlations of these measurements with other experimental and theoretical results. Wentworth and Becker first suggested that the electron-capture results could be used to calculate molecular electron affinities and applied the method to some polycyclic aromatic hydrocarbon^.^? They showed a positive correlation between the experimental electron affinities from electron-capture coefficients and polarographic half-wave reduction potentials. Also, good agreement with theoretical estimates of electron affinities was obtained. Later, Becker and Wentworth18 showed that the sum of the electron affinity and ionization potential for the polycyclic aromatic hydrocarbon was essentially constant as predicted by theory. Recent experimental electron affinities of the five-ring polycyclic aromatic hydroc a r b o n ~along , ~ ~ with the the more recent values for the three- and four-ring cornpounds,l6 also show good agreement with recent theoretical considerations and predictions.‘9 Finally, experimental electron affinities for some aromatic aldehydes and ketones correlate well with polarographic half-wave reduction potentia1s.m These electron affinities, along with a more recent investigation of fluoro-, methyl-, and trifluoromethyl-substituted derivatives of these compounds, agree reasonably well with molecular orbital calculations based on Hiickel approximations.*l I n total, 33 compounds which apparently undergo nondissociative ~~

~~

(8) D. C. Frost and C . A. McDowell, J. C h . Phys., 29,503 (1958). (9) W. H. Hickam and D. Berg, ibid., 29, 517 (1958). (10) R. E. Fox and R. K. Curran, ibid., 34, 1595 (1961). (11) J. A. Stockdale and G. 8. Hurst, ibid., 41, 255 (1964). (12) E. P. Berth and W. H. Hamill, J . Am. C h a . Soc., 86, 1301 (1964). (13) D. W. Skelly, R. G. Hayes, and W. H. Hamill, J . C h . Phys., 43, 2795 (1965). (14) J. E. Lovelock, Nature, 189, 729 (1961). (15) J. E. Lovelock and N. L. Gregory, “Gas Chromatography,” N. Brenner, Ed., Academic Press Inc., New York, N. Y., 1962, p 219. (16) W. E. Wentworth, E. Chen, and J. E. Lovelock, J. Phye. Chem., 70, 445 (1966). (17) W. E. Wentworth and R. 8. Becker, J . Ant. Chem. Soc., 84, 4263 (1962). (18) R. 9. Becker and W. E. Wentworth, ibid., 85, 2210 (1963). (19) R. 5. Becker and E. Chen, J . C h . Phye., 45, 2403 (1966). (20) W. E. Wentworth and E. Chen, J. Phys. Chem.,71, 1929 (1967). (21) R. 9.Becker, W. E. Wentworth, and L. Wang, in preparation.

Volume 71, Number 6 May 1067

W. WENTWORTH, R. BECKER,AND R. TUNG

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electron attachment have been studied by the pulsesampling technique in our laboratories to date. I n all cases, the experimental electron affinities appear reasonable with respect to other experimental and theoretical estimates of the electron affinities as well as by correlation with other parameters. The pulse-sampling technique, in addition to its simplicity in operation and design, has another distinct advantage since it can be used in conjunction with a gas chromatograph. At present, gas chromatography is probably the most effective technique to separate and/or purify organic compounds. For this reason, with a relatively impure sample, a satisfactory separation of the major constituent from its impurities can frequently be made with proper gas chromatographic procedures. The use of a gas chromatograph is extremely important when very weakly capturing species are being studied, as will become evident later.

Kinetic Model In the electron-capture d e t e ~ t o r , l ~argon , * ~ atoms are either ionized or excited to metastable states upon collisions with p particles emitted from a tritium foil. The electrons ejected from the argon atoms will then gradually lose their kinetic energy through numerous collisions with other particles in the cell. Since an argon atom does not have the vibrational and/or rotational levels which a polyatomic molecule possesses, it is not very effective in removing energy from an electron through mutual collisions. When a small amount of methane is mixed with the argon, however, the electrons can soon lose their excess energy to the low-lying excited vibrational and/or rotational levels of methane molecules and come to thermal equilibrium with the gas.16 A constant supply of thermal electrons can thus be obtained. The gas being added to the argon to permit "cooling" of the electrons to thermal energies will be called the moderating gas. Furthermore, the moderating gas (at = 10% concentration by volume) is effective in quenching the metastable argon atoms produced. If now a thermal electron is captured by a molecule, AB, which can undergo dissociative capture, two processes may occur. In one process, an unstable negative ion, AB-, is first formed which then either dissociates to give a free radical A' and a negative ion Bor releases the electron through collision with a third particle. In the other case, the molecule, after capturing the electron, is promoted to a repulsive dissociative state and dissociation proceeds immediat,ely. Since various radicals and positive ions resulting from ionization and dissociation are present in the cell, it is also possible that electrons and negative ions may react with The Journal of Physical Chemistry

these species. In a general model, all of these possible reactions will be considered. Different situations may arise for different types of compounds and lead to cancellation of certain reactions, but the general model should still be applicable. Before proceeding to the reactions, it is appropriate to define some terms and names that are to be used: e- is the thermal electron; AB is the capturing molecule; AB- is a negative ion; A and B- are products of dissociation; P+ is a symbol to designate any of the positive ions in the cell, e.g., Ar+, ArH+, ArCHf, ArCH2+, ArCH3+, ArCH4+, CH4+, CH3+, CH2+, H+, etc.; R' is a symbol to designate any of the radicals in the cell, e.g., H', CHD',CH2', etc. The concentration of each species is represented by brackets containing the name of the species, for example, [AB-]. The possible reactions are listed below, with reaction rate constants attached to the corresponding reactions.

P-

+ Ar + CH4 3 P+ + e- + P-

(1)

(This reaction also includes cooling of the electrons.) AB

+ e--%A'+

B-

(3)

AB+e--%AB-

+ M "1:AB + e- + M AB- + M krl A- + B- + M

AB-

+ P+ -% R* + R*-% R-

ee-

kN1#

AB-

/+

AB-

+ R'

,/

A-

(5)

(7)

/- AB + R'

(8)

ABR-

+ AB

+ R- -Zneutrals

\-

B-

(4)

(6)

+ P+ \+A' + B' + R' kRf'

(2)

B'

(9)

+ R'

+ R' -%BR + e-

(11)

A* + e- k", A-

(12)

+ P+ "Ntl AP or A' + R' A- + R'-%AR + e-

(13) (14)

THERMAL ELECTRON ATTACHMENT TO HALOGENDERIVATIVES

A’

+A

+

B - ~ A B A-

(15)

The following assumptions are made in this model: (a) the electrons are produced by the /3 particles at a constant rate; (b) the diffusion loss of electrons can be neglected; (c) the concentration of the negative ions is negligible compared to the capturing species; (d) the positive species and radicals are present in large excess compared to the negative ion concentration; (e) a steady state is reached by the intermediate negative species e-, AB-, and B- during the time when the pulse is off; (f) essentially no reaction occurs during the application of the pulse necessary to collect electrons. At this point, certain arguments will be given so that some of the reactions listed can be neglected. First of all, comparing reactions 2, 3, 6, 7, and 12 and using the assumptions made in (c) and (d) above, it can be seen that since [A’] is probably much smaller than either [R’] or [AB], eq 12 can be neglected. Since reactions 13, 14, and 15 are dependent on (12), elimination of (12) also eliminates these reactions. Therefore, only reactions 1-11 will be involved in the kinetic expressions. The steady-state equations of the negative species are

k,Rp

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- (kl + k12)a[e-l - ( k +~ k~ILe-1 + k-i[AB-]

kla[e-]

k~2[B-]

0 (19)

- (k-1 + h)[AB-] k~i[AB-l - ~ R ~ [ A B= - ] 0 (20)

k2[AB-]

+ klza[e-] - k~2[B-]- k~z[B-]= 0

(21)

respectively. An expression for [AB-] in terms of [e-] can be obtained from eq 20 and an expression for [B-] in terms of [AB-.] and [e-] can be obtained from eq 21. Thus from eq 20 and 21, [AB-] and [B-] can be expressed in terms of [e-] and substituted into eq 19 giving

kpRp - (kl

+ kda[e-l

+

- ( k +~ k ~ > [ e - l

where = k-1+ k2 4-~ N 4I kR1. In the absence of a capturing species, a = 0, [e-] = [e-],&, and eq 22 becomes

Equations 22 and 23 can be combined to eliminate k,RB,giving the single expression

= Ka

d [AB-] = 0 = kl[AB][e-] dt

k12[ABl[e-l

(24) This general equation shows that the quantity ( b - [e-])/ [e-] is a linear function of a, the concentration of the capturing species, the slope, K , being a combination of rate constants of reactions 2 to 11.

-

- (kNz’[P+] 4- k~2’[R’l)[B-l

(18)

According to assumption d above, [MI, [R’], and [P+] are large and can be considered constant. Thus, some of the reactions involving these particles can be considered as pseudo-first-order reactions. The primed second-order rate constant times the corresponding concentration can be replaced by an unprimed pseudofirst-order rate constant; e.g., k-l’[M] = kl and k ~ ’ [ p += ]k ~ . Based on assumption c, [AB-] is negligible compared t o [AB]. Therefore, a, the initial concentration of AB, can be used in these equations instead of [AB]. Equations 16,17, and 18 can be simplified to give

Experimental Section The construction of an electron-capture detector as well as the flow diagram of the equipment used along with the detector has been described in satisfactory detail previously. 14*15 The electron-capture cell used for this work was much the same and will not be described again. A tritium-imbedded titanium foil 1 was used in the cell. with an activity of ~ 0 . curie A square-wave potential of 40 v was applied across the cell by means of a Datapulse generator, Model 102. The pulse width, depending on the moderating gas used, ranged from 0.5 psec for methane to 4.0 psec for pure argon (Table I). The pulse interval was 1000 psec. During the applied pulse, free electrons were collected Volume 71, Number 6 May 1967


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on the grid, and the current was detected and magnified by a Cary 31 vibrating-reed electrometer. Preceding the electron-capture detector was a short section of tubing allowing scavenger gas and moderating gas to mix with the column effluent. Most of the columns used in this wopk were made of 0.25-in. packed Table I: Pulse Widths Required for Various Gas Mixtures (Pulse Interval 1000 psec, Pulse Voltage 40 v) Pulse width, Gas mixture

Ar-lO% CH4 Ar-1 .ti% COn Ar-2.0% Hz Ar

piec

0.5 2.0 2.5 4.0

columns 2-6 ft in length. The solid support for all packed columns was Analabs ABS 70-80 mesh. ccI4, CHCL, CH2C12,CzH& CaH,Br-1, and C4HgCl-1 were run on a 20% C-16 column a t room temperature, Iodobenzene, bromobenzene, and o- and m-dichlorobenzenes were run on a Carbowax 20A9 column a t 85-120". l-Bromonaphthalene was chromatographed on a 3% C-60 column, whereas a 250-ft1 0.03-in. i.d. (polyphenyl ether) column was required to obtain sufficient purity. Argon from the Big Three Welding Equipment Co. (purity >99.99%) was used without further purification but was passed through a molecular sieve (Type 5A) trap, the Illinois Institute Dri-pak, to remove traces of moisture in the gas. Methane (Matheson, 99.9%) was also passed through the Dri-pak before mixing with the scavenger argon. Flow rate was measured at the outlet of the electron-capture detector with a soap bubble flowmeter and a stopwatch. The flow rate was measured at room temperature while the temperakure in the cell was usually higher. I n order to obtain the true flow rate in the cell, a correction factor was needed. Assuming ideal gas behavior, in this temperature and pressure range, the true flow rate in the cell was obtained by multiplying the measured flow rate with the factor T/298. A random error of about 10% may be involved in the measurement of flow rate. The temperature of both the column and detector was controlled with Variacs and read with thermometers inserted into the ovens. For this work, the accuracy of column temperature is not very critical. On the other hand, an accurate cell or detector temperature reading is very important. Since the thermometer bulb was in the air instead of being directly in contact with the cell, a possible error was involved in cell The Journal of Phyaical Chemistry

temperature reading. The resulting bias error could be l o . Solutions of the tested compounds were prepared to give suitable peak sizes (about 60% capture) and linearity of response in the concentration ranges used. The solutions were prepared by volume measurements. Concentrations in moles per liter were then calculated with known densities and molecular weights. The concentration of a solution thus prepared has been checked by weighing the solute also. The error proved to be small. Especially when the amount of solute is as small as a few microliters, weights may not be any more accurate than volumes. When the compound was very high capture and a very dilute solution was needed, a second dilution of a diluted solution of the compound was made. The solvents used were all low-capture compounds like benzene and toluene. All samples were injected with a Hamilton 10-pl syringe. The injector was heated to a temperature comparable to column temperature so that the sample would not condense in the injector. Since the electron-capture ability has a very selective response to certain compounds, sometimes it became necessary to identify the major peak among several peaks in a sample. This is especially true for the low-capture compounds. A detector developed in this laboratory was used for this purpose22which shows roughly the same sensitivity for all compounds. Toward the end of this work, a 50-ft packed SE 30 column was available (HiPak purchased from F & M Scientific). This column was capable of giving much better resolution than the other columns used, so it was utilized to check some of the compounds that were suspected to have high-capture impurities. Chlorobenzene and chlorobutane were run at a column temperature of 100" and were found to have impurities very close to the major peak. For this reason, these compounds were not included in this study. A benzene solution of propyl bromide run at the same column temperature shows only the benzene peak and the sample peak. Bromobenzene was run a t 105". The nearest impurity was 6.5 min after bromobenzene, while the latter has a retention time of 17 min. It was thus concluded that the impurity would also be resolved, if the column given earlier was used. The o- and m-dichlorobenzenes were both run a t 150". Benzene, which was the solvent, was the only other peak seen besides the sample peak. For all these compounds, the column flow rate was approximately 30 ml/min. I n eq 24, a is the concentration of the tested com(22) W. E. Wentworth and W. Ristau, in preparation.


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pound in moles of compound per liter of gas. Since a is not available directly in this experiment, an integration is carried out on both sides of eq 24 to give

v

*

where v is the volume of gas flowing through the detector during the peak. The integral on the right-hand side readily gives n, the number of moles of the compound. To obtain the integral on the left-hand side, a transformation needs to be carried out; v can be expressed in terms of some measurable quantities as

v

Q

37.5'-c 6.5OC 24.5*C

Fr

= --w

cs

where Fr is the gas flow rate in the detector in liters per minute, Csis the chart speed in inches per minute, and w is the peak width in inches. Taking the differential of eq 26 and substituting into the left-hand side of eq 25, we have 0

or

K = -A Fr n E where A is the transformed peak area. I n an actual experiment, however, n is a product of two other quantities, namely, the microliters of solution injected and concentration of the solution in moles per microliter. Using S and C for these two quantities, respectively, we have

The peak areas are calculated with a digital computer program written for this purpose.2a A series of straight lines are drawn approximating the curve as closely as possible, and the coordinates of each point joining adjacent lines are taken with reference to the origin as the start of the peak. On the basis of this, the computer calculates the transformed response (b - [e-])/[e-] for each given point and then calculates the peak area with the transformed coordinates. For some compounds a Leeds and Northrup analog computer was used. This instrument converts the output signal of the electrometer to (b - [e-])/[e-] and integrates over the gas chromatographic peak. From five to ten samples were injected a t each temperature. The areas obtained were plotted against the corresponding sample sizes S. A straight line

5 IO SAMPLE SIZES (PI)

Figure 1. Integrated converted electron-capture response, f(b [e-])/[e-] dw, for CHCla as a function of concentration at various temperatures. The slope at 24.5" is lower than a t 6.5"; however, correction for the span, b, makes the true K value at 6.5' lower than at 24.5".

-

through the origin was drawn that would best fit the data points. As an example, Figure 1 shows the plotted data of chloroform at several temperatures. It is to be noted that the integrated response f$(b - [e-])/ [e-] dw does appear to be a linear function of the sample size-consistent with eq 27 and 28, derived earlier. The slope of the straight line, AIS, multiplied by the factor (F,/C,)/C, gives the constant K at this temperature. Resulting from bleed of the gas chromatographic column and/or a slowly emerging peak of a high boiler, the span b may not stay constant. This problem has been mentioned in an earlier publication and a correction factor of bo/b is applicable under certain conditions;16 bo is the span when pure carrier gas (no column bleed or impurity) is passed through the cell, and b'is the measured span when the column is used. As mentioned earlier,16this problem is under study, and, at present, data are taken only where bo/b is relatively close to unity. I n this work, the correction factor ranges from loo/40 to 1 o o / ~ 5 ~ , which is not actually (23) W. Hirsch, M.S. Thesis, University of Houston, 1965.

Volume 71,Number 6 May 1967

W. WENTWORTH, R. BECKER, AND R. TUNG

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large when one considers that the change in capture coefficient may be 1000 or so over a 200” range. Wentworth, et UZ.,‘~*’’ have shown that, for a nondissociative electron-capture reaction, the statistical thermodynamic expression of the equilibrium constant can be employed. This gives

K

=

ZT-%/~~-A.E/~T

(29)

where K is the electron-capture coefficient, Z is a preexponential factor independent of temperature, aE is the electron affinity, k is the Boltzmann constant, and T i s the cell temperature. Equation 29 was rearranged to give In KT”* = In z

AE - kT -

In general, a temperature region was observed where a graph of In KT”/’ us. 1/T was linear with the expected positive slope.16 A brief survey of reactions 1 through 11 reveals that there are several types of reactions, each involving very different charged or uncharged species. No satisfactory theoretical treatment as to the role of temperature in the preexponential factor has been

known for these reactions. It is thus impossible at the present time to derive an expression similar to eq 30 for the dissociative model suggested here. However, in order to simplify the comparison of the results of this study with those previously published, all data obtained for dissociative capture are plotted as In KT”/’ vs. 1/T. As can be seen in Figure 2, a linear relation is obtained for all the compounds studied. The slopes are related to the activation energies for the operative mechanism, as will be discussed later. A weighted least-squares adjustment of data to eq 30 was carried The solid lines drawn in Figure 2 are the least-squares estimate of the function. The errors involved in measuring the parameters were estimated as follows. The value of K , as expressed in eq 28, is calculated from five independent measurements, and its error will be a function of the errors in these measurements. The error in chart speed can be assumed to be negligible. Thus the error in K is expressed in terms of the other four parameters as

40

30 36

It follows that 34 32 30

Assuming a 10% error is involved in flow rate measurement and in solution preparation, eq 32 becomes

28

[gI2 [*!SI2 A/S +

(\I

&!.

f

=

26

24 22 20

18 16

1

I

3

1.0

I

2.0

I x 7

I

3.0

10” (OK-’)

Figure 2. Temperature dependence of electron-capture coefficients for various halogeuated organic compounde.

The Journal of Physical Chemistry

I

(0.1)2

+ (0.1)2 =

[%I2 +

0.02 (33)

As was described earlier, A / S is the slope obtained from the plot of peak areas vs. sample sizes at a certain temperature. A weighted least-squares adjustment of the linear relationship between A and S gives the error in this slope. The parameters resulting from the least-squares adjustment of In KT”/’ us. 1/T are given in Table 11. The column E* is the energy of activation one would (24) W. E. Deming, “Statistical Adjustment of Data,” Dover Publications, New York, N. Y . , 1964.


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Table I1 : Least-Squares Adjustment of the Temperature Dependency of Electron-Capture Coefficients AE Compound

Carbon tetrachloride Ethyl iodide Iodobenzene l-Bromonaphthalene Chloroform m-Dichlorobenzene Bromobenzene o-Dichlorobenzene Methylene chloride n-Propyl bromide l-Chloronaph thalene

Intercept

33.82f0.31 33.50f0.26 36.45 f 0.43 35.47f0.51 35.49f0.35 3 6 . 4 1 f 0.35 32.97f0.43 35.95 f 0.37 33.90f0.35 35.67 f 0.71 40.55f0.33

have obtained if In K vs. 1/T had been plotted. Without knowing the preexponential temperature-dependence term, one cannot precisely establish the energy of activation. E* would be the activation energy if the preexponential term were constant. The cext2 in Table I1 should be compared to uo2which was assigned a value of unity.24 In general, the are of the same order of magnitude, indicating the data can be adjusted to this function within the expected experimental error. If anything, the cext2are probably less than go2, suggesting that our error estimates in eq 33 were pessimistic. In that case, the quoted errors in the parameters are likewise pessimistic. The gas mixture used for the previous work was argon plus 10% methane. In order to study the importance of radical reactions, iodobenzene and bromobenzene were run using mixtures of argon and moderating gases other than methane. Hydrogen and carbon dioxide were used. The percentage of each moderating gas was such that additional moderating gas does not result in an appreciable increase in standing current. Different pulse widths were used in each case, as stated before, and are listed in Table I. The electron-capture data obtained were treated as discussed previously except that least-squares adjustment was not carried out. The data were graphed as In KT"/' vs. 1/T as before. No obvious difference in slope was observed; however, there was some increase in magnitude with COz and Hz for bromobenzene. Since the differences in slopes were not considered significant, the effect of different radicals as in eq 7, 9, and 11 apparently does not alter the energy of activation. The relative orders of magnitude of the capture coefficient of some of these compounds have been obtained before by L o v e l o ~ k . ~ ~ These , ' ~ ~seem ~ ~ to be in general agreement with the present work. Some differences occur (e.g., o- and m-dichlorobenzenes) prob-

-

-slope X R, kcal/mole

0.19 f 0.22 2.18 f 0.19 3.14 =!= 0.32 3.00 f 0.43 4.25f0.255 7.70 f 0.29 7.32 f 0.33 8.05 f 0.29 8.68 f 0.24 9.03 f 0.55 11.01 =k 0.29

E*, kcal/mole

--0.05 1.07 1.55 1.74 3.09 6.54 5.91 6.91 7.53 7.88 9.87

Uext'/bQ'

1.14 0.38 0.35 0.09 0.35 0.48 1.82 0.76 2.15 0.50 0.74

ably owing to the fact that he used short pulse intervals. Furthermore, he did not specify the cell temperature, and for compounds such as bromobenzene this can be most important. Lee,26using nitrogen as carrier gas in the swarm method, investigated electron attachment to a number of aliphatic halides as a function of electron energy. The energy range in this work was 0.0%1.2 ev. A comparison of Lee's values at E / p 0.2 and -0.032 with the present work shows general agreement on relative magnitudes. I n the case of CH2CI2, Lee's sample would have to be extremely pure in order for his results to be meaningful. For example, our value for CCI, is approximately lo4 greater than CH2Clzand hence a 0.01% impurity of CCL in CHzClzwould result in a 100% error. Other weaker capturing impurities, however, would be less critical.

Discussion A very striking feature about the data presented in Figure 2 is the completely different temperature dependence compared to the nondissociative cases. The capture coefficients either increase with increasing temperature (decreasing 1/T) or remain constant. This is probably characteristic of the dissociative compounds in general. Analogous to the nondissociative case, however, the extrapolation of the slopes in Figure 2 to infinite temperature (1/T = 0) shows that several compounds seem to have a common intercept. Thus iodobenzene, chloroform, o- and m-dichlorobenzene, l-bromonaphthalene, and n-propyl bromide all extrapolate to an intercept in the region of 35.9. Carbon tetrachloride, methylene chloride, and ethyl iodide extrapolate to approximately 33.5. Bromo(25) J. E. Lovelock, Anal. Chem., 35, 474 (1962). (26) T.G.Lee, J. Phys. Chem., 67, 360 (1963).

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benzene alone seems to have a lower intercept, whereas 1-chloronaphthalene is exceedingly high at 40.5. However, the least-squares adjustment does indicate that the errors in some intercepts seem to overlap the standard-error estimate of others. Off hand, it is difficult to assign any significance to the different intercepts. A conclusion as to whether there is a common intercept may be premature at the present stage. It is, however, proper to examine the fit of the proposed model in view of the observed temperature dependence. From eq 24 it can be seen that K is a sum of three terms representing three processes. The first term 0 1 2 3 4 INTERNUCLEAR D I S T A N C E represents the spontaneous dissociative process, eq Figure 3. Representative potential energy curves for 2; the second term, the intermediate negative ion dissociative electron attachment: A, klz mechanism; process, eq 3-5; and the third, the stable negative ion B, klkz/k-l mechanism (with a positive electron case, eq 3 and 4. Since klz appears only in the first affinity, Le., energy release); C, klkzlk-1 mechanism (with a negative electron affinity). term, it can be considered separately when the first term is assumed to predominate. For the x k in the second and third terms of eq 24, we can consider two limiting situations: (1) when k-1 >> (k2 k ~ l k ~ 1 ) ; which may represent the mechanisms proposed for djssociative electron attachment. The spontaneous disand (2) when k2 >> (kl k ~ 1 k ~ 1 ) . The situation sociative process represented by eq 2 is shown in when ( k ~ 1 ~ R I )>> (kz k-1) will not be considered 3A. The energy of activation would be that Figure here since this has been shown to correspond to the required to populate thermally the neutral molecule temperature-independent region of the stable negative in the ground electronic state to some higher vibrational ion case.16 Before considering the validity of each mode at an energy where the dissociative curve crosses limiting case, a simplifying assumption will be made the ground-state potential energy curve. Electron kR2) is very close to unity. This is that kN2/(k~2 attachment to such a vibrationally excited state should not an unreasonable assumption based on the two result in spontaneous dissociation. The experimentally following reasons. First, the attraction between two observed negative slope of In KT"/' vs. 1/T (Figure 2) unlike charged particles should be much larger than the can thus be explained by this mechanism. Two higher attraction between an uncharged and a charged particle; energy dissociative potential energy curves are inhence, kxz should be much larger than kR2. Second, cluded in Figure 3A for completeness to represent, the experiments done with different moderating gases in general, transitions which may be observed by have shown that radical reactions apparently are relaother experimental techniques. tively unimportant from the standpoint of regenerating neutral molecules and free electrons. The term k ~ ~ /When the latter two terms in eq 24 predominate, ie., (klkz/Ck) [kl(kN1 k R I ) / C k ] >> k12, the two (kN2 k ~ arises ~ ) only when free electrons are assumed possible limiting situations in the x k can be conto be formed in the radical reaction with a B- ion, sidered. First, if kW1 >> kz then the expression for K eq 11. For this reason, we will eliminate this term in becomes the subsequent simplified expressions for K . This does not mean that radical reactions as shown in eq 11 hk2 (kN1 kR1) kl K = do not occur and that they are not important in them(35) (kN kRlk-1 ( h kR) selves. In particular, only in the interpretation of the The second term in eq 35 is the stable negative ion results in this work they should not play an important case,16 and is certainly not applicable to the data obrole. tained in this study. The first term is the mechanism Considering first the situation when the first term in [ k l ( k ~ ~ for dissociative electron attachment which involves eq 24 predominates, Le., klz >> ( k l k z / ~ k ) the formation of an intermediate stable negative ion. k ~ ~ ) / x k The ] . expression for K is thus Typical potential energy diagrams for this case are shown in Figures 3B and 3C. The mechanistic path (34) followed is the formation of a stable negative ion Figure 3 contains various potential energy diagrams followed by thermal activation of the negative ion to I . ,

+ + +

+

I

I

+ +

+

+

+

+

+

+

The Journal of Physic& Chemistry

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IC-1


a vibrational mode which crosses the lowest energy dissociation curve. We refer to this dissociative process as the klk2/k-1 mechanism. The over-all energy of activation is shown as E* = (E2 - E-1) where the E2 and E-l are the corresponding energies of activation for the rate constants k2 and k-1. Figure 3 B is the more general case where the electron affinity of the molecule is positive (Le., exothermic in going from the neutral molecule to the stable negative ion). There is the possibility, depicted in Figure 3C,where a molecule has a negative electron affinity. However, this situation is probably unlikely, but may explain the data for some of the halogen-substituted benzenes. This will be discussed later in detail. If ( ~ N I ~ R I )in eq 35 is not negligible compared to k2, we have a sum of two terms, one for the intermediate negative ion, the other for the stable negative ion. It is possible to 0bserve.a transition region for such a case. The first term would appear a t higher temperatures and decrease with decreasing temperatures. As the temperature gets low enough, the second term should predominate and K starts to increase with decreasing temperature. The only compound investigated in this study that does this is 1chloronaphthalene. The situation where k2 >> L1does not really add much to the explanation of the temperature dependency for dissociative electron attachment. Under these assumptions, eq 24 will reduce to

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dependence will appear to be the same as the case when k-1 is much greater than k2. The only difference is that the former case, k-I >> kz, probably has a steeper slope. Finally, there is the possibility that the first two terms of eq 24 are both of comparable magnitude. However, this is probably unlikely since it would be coincidental that the potential energy curves would cross with approximately the same energy of activation for both the k12 and the klk2/k-l mechanisms. I n conclusion, there are two possible mechanisms which can explain the temperature dependence of K observed experimentally and shown in Figure 2. These are the k12 mechanism as given by eq 34 and the klkz/k-I mechanism as given by eq 35. On simply the information of the temperature dependence, it is impossible to decide which mechanism is operative. However, as will be discussed shortly, comparison of results for different compounds is suggestive that the k ~ k ~ / mechanism k-~ is operative in some compounds. Possible potential energy curves for bromobenzene are shown in Figure 4. The available data for the bond dissociation energy, the electron affinity of the bromine atom, and the electron affinity of the phenyl radical2’ are used and drawn to scale. A potential

0-

The first term has to be very large compared to the second term, the former term being the maximum value for the intermediate negative ion term.16 Since the second term has k2 in the denominator and k2 >> ( k ~ 1 k ~ ~it) is , necessarily smaller than the first term where there is a high-capture coefficient ICl with little or no temperature dependence. Of the compounds invest,igated, only carbon tetrachloride could fit this model. However, one could also explain carbon tetrachloride by the k12 mechanism if there was no energy of activation; ie., E12 = 0 . One should also consider the situation where k2 gets larger and becomes comparable to k-1. One may think that a curvature will be observed owing to the factor k2/(kt L1) in the first term of eq 35 which can be E21kT A-le-E-l/kT), expressed as A2e-E21k~/(A2eHowever, actual calculation of the factor k2/(kz k-$ using different values for E2 and E-1 shows that the curvature is hardly perceivable even if the preexponential factors are different by a factor of 10 in either sense. In short, if this is the case, the temperature

+

+

+

+

10

-

20

-

>0

K W

t

-

,30

W

4

5 40IW 2

$

50-

0

60

-

70

-

80

I

I

90

r

e

t

I.

2

3

RELATIVE INTERNUCLEAR DISTANCE(A0)

Figure 4. Possible potential energy curves for bromobenzene and bromonaphthalene: Ar., aromatic radical; - - - -, stable negative ion potential energy curve for bromonaphthalene.



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energy curve resulting in dissociation to Phthe naphthyl radical would be expected to have a comparable electron affinity to that of the phenyl radical. Br' is shown. It represents a stable negative ion formation, but with a negative electron affinity. As I n Figure 4,the electron affinities for the aromatic radimentioned previously, in general this is probably a cals have been assumed to be identical. The electron very unlikely situation, but it appears to be a reasonaffinity for 1-bromonaphthalene has been estimated by able possibility for bromobenzene. This will become adding the effect of a Br substituent to the electron more convincing after comparison with propyl bromide affinity of naphthalene. From half-wave reduction and 1-bromonaphthalene. The experimentally obpotentials and charge-transfer spectra, the effect of served 5.9 kcal/mole energy of activation is shown for bromo substituent on quinone increased the electron the klkz/k-l mechanism. affinity by 0.2 ev.ao This value of 0.2 ev was added to Stockdale and Hurst" measured the electron-capture the electron affinity of naphthalene to give an estimated cross section of bromobenzene and chlorobenzene with electron affinity of 0.35 ev for 1-bromonaphthalene. a swarm method. Using argon, nitrogen, and ethylThis estimate of electron affinity was used to draw ene as carrier gases, they were able to measure the the potential energy curve for the stable negative ion cross section with a distribution at thermal electron of 1-bromonaphthalene in Figure 4 (dashed curve). energies to distributions with maxima at about 10 These curves in Figure 4 suggests that l-bromonaphev. The maximum cross section occurs at 0.76 ev thalene may go through the klkz/L1 mechanism with a (17.5 kcal/niole) and 0.7 ev for bromobenzene and lower energy of activation and a higher capture of chlorobenzene, respectively. The present data give electrons compared to bromobenzene. This interpre5.9 kcal/mole for the energy required for the over-all tation is supported by the observations shown in process for bromobenzene. The discrepancy is apFigure 2 and Table 111. In bromobenzene, the klkz/ parently due to the different modes of electron attachL1or the klz mechanism or both may be important. ment. In the swarm method, a high-energy electron Shortly, it will be shown why we favor the former collides with a ground-state molecule; hence, a vertical mechanism. transition from the neutral molecule to the dissociative state can occur.28 However, in the electron-capture cell, the molecules are heated and, thereby, low-lying Table III : Bond Energies for Some Bromobenzenes vibrational levels become occupied to the point where the potential energy curve for the neutral molecule D(C-Br), Compound kcal/mole crosses with the dissociative state. I n Figure 4, the vertical transition is represented by 17.5 kcal/mole. Bromobenzene 71," 7O.gb p-ClCeH4Br 70.3" It is obvious from this figure that a higher value for rn-ClCCHIBr 69.9' the energy of the over-all process will be obtained if o-C1C6H4Br 69.7" the swarm method is used. Because of the difficulty p-CHaCsHdBr 70.7' in resolving the impurity in our work, no data on rn-CHaCsH4Br 70.7" chlorobenzeneare available for comparison. o-CH&eHdBr 70. le As stated earlier, although we cannot distinguish P. Smith, J. Chem. Phys., 29, 681 (1958). M. Szwarc, between the k12 and klk2/k-1 mechanism in all cases, ibid., 20, 1170 (1952). M. Szwarc and D. Williams, Proc. Roy. evidence can be given which suggests the klkz/k-l SOC.(London), A219, 353 (1953). mechanism is the primary process for some compounds. In comparing bromobenzene and 1-bromonaphthalene, the C-Br bond-dissociation energy in the two com(27) A. F. Gaines and F. M. Page, Trans. Faraday SOC.,59, 1266 pounds is quoted as the same value, 70.9 k c a l / m ~ l e . ~ ~(1963). (28) In a more recent paper, Christophorou, et al. (L. G. ChristoTherefore, since the electron affinity for the halide phorou, R. N. Compton, G. S. Hurst, and P. W. Reinhardt. J. Chem. atom is in both the dissociative state Phys., 43, 4273 (1965)), describe a procedure which combines electron-swarm data with electron-beam measurements t o obtain absolute giving Ar Br- for 1-bromonaphthalene should be cross sections for electron attachment as a function of electron similar to that for bromobenzene as given in -pigure 4. energy. The maximum for o-chlorotoluene differs from the maximum obtained from electron-swarm or electron beam measurements. However, since naphthalene itself has a measurable A similar investigation of bromobenzene should be made and the affinity Of ev'6 and benzene is expected energy of the true maximum should be used in place of the 0.76 ev (17.5kcal/mole) quoted above. to have a negligible or, more likely, a negative electron (29) T. L. Cottrell, "The Strength of Chemical Bonds," Academic affinity,18*19the potential energy curve for the stable Press Ino., New York, N. Y.,1958. negative ion be much lower for l-bromonaph(30) K. RI. C. Davis, P. R. Hammond, and M. E. Peover, Trans. thalene compared to bromobenzene. Furthermore, Faraday SOC.,61, 1560 (1965).

+

The Journal of Physical Chemistry


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~~

Table IV: Activation Energy for Dissociative Electron Attachment Related to Energy Change for the Process (kcal/mole)" Formula

Compound

Carbon tetrachloride Chloroform Methylene chloride m-Dichlorobenzene o-Dichlorobenzene l-Chloronaphthalene n-Propyl bromide Bromobenzene l-Bromonaphthalene Ethyl iodide Iodobenzene

E*

DA-B

-0.05f0.22 3.10 f 0.26 7.54 f 0.24 6.54 f 0.29 6.91 f 0.29 9.87 i0.29 7.89 f 0.55 5.92 f 0.33 1.75 f 0.43 1.07 f 0.20 1.55 f 0.31

67.9 f 3 O 71.4 f 2" 75 2" 87' 87' 87" 64 zk 3b 70.9 f 1.5d 70. gd 52.9 f:1.d 57 f 2.!jds8

EAB~

83.16 83.16 83.16 83.16 83.16 83.16 77.55 77.55 77.55 70.63 70.63

*

DAB- EAB

-15.3 -11.8 -8.2 $3.8 f3.8 +3.8 -13.6 -6.6 -6.6 -17.7 -13.6

f3 f2 f2

f3

f 1.5 f 1.0 f 2.5

The literature values quoted in this paper are, in the author's opinion, the best presently available. No attempt has been made Bond dissociation energies for CHa-Br and CzHs-Br are 67 and 65 kcal/mole, respectively, according to T. L. Cottrell.*g This difference is in line with the change from CHe-I P. Goldfinger and G. Martens, Trans.Faraday SOC.,57, to CzHs-I. On this basis, we have estimated the value for propyl bromide. 2220 (1961). M. Szwarc and D. Williams, J . Chem.Phys., 20,1171 (1952). M. Szwarc, Chem. Rev., 47,75 (1950). D. B. Hartley R. S. Berry and C.W. Riemann, and S. W. Benson, J . Chem. Phys., 39, 132 (1963). J. B. Farmer, et al., ibid., 24, 348 (1956). ibid., 38, 1540 (1963). C-Cl bond dissociation energy is taken to be that of chlorobenzene on the basis of previous discussion of the data in Table 111. Chlorobenzene bond dissociation from S. W. Benson, LLFoundations of Chemical Kinetics." a

to give recognition of possibly earlier or more recent estimates with similar or less reliabdity.

'

'

'

If the upper limit for the electron-capture coefficient of chlorobenzene is correct, then the data for the disubstituted o- and m-dichlorobenzenes also suggest that the intermediat,e stable negative ion mechanism is operative. The C-C1 bond dissociation energy for chlorobenzene is probably close to that of the C-Cl bond for the o- and m-dichlorosubstituted benzenes. Support for this supposition exists in the comparison of C-Br bond dissociation energies for several substituted bromobenzenes with that for unsubstituted bromobenzene as shown in Table 111. The electron-capture coefficient for the disubstituted compounds is well above even any upper limit estimate for the chlorobenzene, and again according to the proposed mechanisms one must assume that the intermediate stable negative ion plays an important role in the dichlorobenzenes. Apparently, the presence of two chloro substituents increases the electron affinity sufficiently to be effective in determining the mechanism of the over-all dissociative electron-attachment process. A similar situation occurs with l-chloronaphthalene, where it is expected to have an electron affinity greater than that of naphthalene. In fact, the positive slope at lower temperatures for this compound offers positive proof that a negative molecule ion forms. Since there are no low-lying vacant ?r orbitals in the aliphatic halides, the electron affinities for these compounds should be negligible, and dissociative electron attachment probably goes by the klz mechanism. I n contrast to the previously mentioned differences be-

tween the monosubstituted benzenes and the monosubstituted naphthalenes or disubstituted benzenes, comparison of iodobenzene with ethyl iodide and bromobenzene with n-propyl bromide reveals a much smaller difference in activation energy for dissociative electron attachment. However, in making these comparisons in activation energies, one must also consider the differences in bond dissociation energy. As can be seen from the data in Table I V relative to the activation energies, the bond energies for the aromatic halides are significantly larger than the corresponding aliphatic halides. Taking this into consideration, bromo- and iodobenzenes probably undergo dissociative electron attachment by the klkz/k-l mechanism. The electron affinities of bromo- and iodobenzenes are small and possibly even negative as illustrated in Figure 4. Mass spectrometric analysis of the negative ions would be required to confirm the existence of the intermediate molecular negative ion. The concentration of the molecular negative ion would probably be very low considering the expected low electron affinity. The potential energy curves in Figure 4 are drawn so that the klkz/k-l mechanism for bromobenzene requires the 5.9-kcal/mole energy of activation. The klz mechanism according to Figure 4 would have a higher energy of activation. If bromobenzene dissociates according to the klkz/k-l mechanism, then most certainly bromonaphthalene with a greater electron affinity would go by the same mechanism. As mentioned earlier, Frost and McDowelP found a correlation between the electron energy at the maxiVolume 71,Number 6 May 1067

1664

mum cross section for hydrogen halides and the difference between the electron affinity of the halogen atom and the bond dissociation energy. Their mechanism for dissociative electron attachment is the same as our k12 mechanism; however, they apparently measure the vertical transition to the dissociative potential energy curve. We interpret our energies of activation as the difference in energy between the crossing point of this potential energy curve with the groundstate potential energy curve for the neutral molecule and the zero-point vibrational level of the ground state for the neutral molecule (Elz in Figure 3A). In a similar manner, we would expect a rough correlation between our energies of activation and the difference between the electron affinity of the halogen atom and the bond dissociation energy for those molecules which we expect, go through the k12 mechanism. The correlation would not be expected to be perfect even in the absence of experimental errors. Such data are tabulated in Table IV along with similar data for compounds which apparently go by the klkzlk-1 mechanism. Hickam and Berg have investigated the dissociative electron attachment to cc14, CCbF, CC12F2, CHC12F, and CC1F3 as a function of energy, observing C1- appearance on the mass spectrometer. The appearance of SFs- from electron attachment to SFe was also run as a reference along with each compound. SF6 apparently captures electrons over a very narrow energy band with the maximum at very low energies. The difference in electron-accelerating voltage a t the peak maximum from that for SFe- for these compounds is as f o l l o ~ ~ sCCl,, : 0.1 V; CC13F, 0.2 V; CC12F2, 0.8 V; CHCl,F, 0.8 v. We have not investigated these same compounds, except for CCI,. However, we have studied some of the series with F replaced by H and one will note with reference to Table IV that the above order is identical with that of the activation energies for CC14, CClaH, and CC12H2. Again, it appears that we are probably measuring the same transition to a dissociative state, except that we are promoting the transition by thermal means. In Hickam and Berg’s work, and others using this technique, the transition occurs by increasing the electron energy only. A more recent paper by Christophorou, et U Z . , ~ ~describes dissociative electron attachment by some chloro-, bromo-, and nitrobenzene derivatives. It is stateda1 that the results contradict a previously reported interpretation by Wentworth and Becker” that this group of molecules captures low-energy electrons through a nondissociative resonance process. However, Wentworth and Becker” make no mention or inference that halogenated benzenes form stable negative ions. In a more recent publication,16it was The Journal of Phyeical Chemistry


suggested that dissociative and nondissociative electron attachment can be differentiated by the temperature dependence of the electron-attachment process a t thermal energies. It was stated that chloro, bromo, and iodo compounds probably dissociate upon electron attachment. The interpretation in the present investigation is in full accord with not only our previous publication^,'^^^' but also with the work of Christophorou, et aL31

Conclusions 1. The electron-attachment coefficients with thermal electrons increase with increasing temperature (except for CC14)for some aliphatic and aromatic chloro, bromo, iodo compounds. Apparently, in these compounds the molecular negative ion is not stable with respect to dissociation into another ion and radical. This temperature dependency is opposite to that previously observed for compounds which are expected to form stable negative ions. It is suggested that the temperature dependency can be used to distinguish between electron attachment forming a stable negative ion and dissociative electron attachment in most cases. 2. Two possible mechanisms are proposed for dissociative electron attachment, both of which can require an energy of activation which can be observed experimentally. One mechanism is the direct dissociation of the molecule upon electron attachment in which the molecule and electron possess the necessary activation energy for dissociation. The energy of activation in this case is the energy above the zero-point energy of the molecule a t which the potential energy of a dissociative state crosses that of the ground state of the molecule. This mechanism has been proposed before by other investigators. The other mechanism involves the formation of an intermediate stable negative ion which in turn can either lose the electron or undergo dissociation into the halide and ion and a radical. I n this latter case, the energy of activation is the energy above the zero-point energy of the molecule a t which the potential energy curve of a dissociative state crosses that of the ground state of the intermediate negative ion. 3. Comparison of results suggests that the aliphatic chloro, bromo, and iodo derivatives probably undergo dissociative electron attachment by the direct mechanism whereas the l-chloro- and l-bromonaphthalenes and o- and m-dichlorobenzenes probably go by the intermediate negative ion mechanism. Bromo- and iodobenzenes appear to undergo dissociative electron attach(31) L. G. Christophorou, R. N. Compton, G. S. Hurst, and P. Reinhardt, J . Chem. Phys., 45, 536 (1966).

W.

KINETICS OF ION EXCHANGE ACCOMPANIED BY IRREVERSIBLE REACTION

ment by the latter mechanism, but the electron affinity of the molecule is most likely quite small and probably negative. Acknowledgments. The authors wish to express their gratitude to the Robert A. Welch Foundation for

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financial assistance, to Dr. Edward Chen for assistance in the experimental work and specifically for the data on 1-bromonaphthalene,to Miss Elsie Bryan for assistance in preparing the manuscript, and to Mr. Joe Steelhammer for the data on 1-chloronaphthalene.

The Kinetics of Ion Exchange Accompanied by Irreversible Reaction. I. Film Diffusion Controlled Neutralization of a Strong Acid Exchanger by Strong Bases

by R. A. Blickenstaff, J. D. Wagner, and J. S. Dranoff Department of Chemical Engineering, Northwestern University, Evanston, Illinois

(Received August 81, 1966)

An experimental study has been made of the neutralization of a strong-acid ion exchanger by strong-base solutions. The kinetic data obtained in a well-stirred batch reactor were found to be self-consistent and t o agree closely vith the model proposed by Helfferich for the conditions of film diffusion controlled reaction.

Introduction

Theory

The rates of ion exchange coupled with reaction have recently been explored in depth by Helfferich.’ He has developed the first detailed theoretical analysis of four different exchange-reaction systems and has presented a collection of rate laws for each system under conditions of intraparticle and external film diffusion rate control. HelfTerich’s analysis rests on the customary simplifying assumptions used in describing the ion-exchange process, including the coupled effects of concentration and electrical potential gradients on the diffusion of ionic species. The present work was undertaken to provide an experimental test of the first of the processes enumerated by Helfferich, the irreversible consumption by chemical reaction of counterions released from the solid exchanger during the exchange process. The results presented below indicate the analysis to be valid and furnish numerical values for the model parameters.

The basic reaction scheme under consideration is illustrated by -

H+

+ M + + OH-

M++ H ~ O

(I) where the barred quantities represent species in the resin phase. It is assumed that both the acid-form resin and the nelitralizing base are completely dissociated, while the water produced by the reaction is dissociated only to its usual slight extent. It is also assumed that the resin particles are uniform spheres, that the system remains isothermal, and that physical properties of the various species remain constant. The observable rate of the process which occurs when acid-form resin particles and basic solution are contacted in a batch reaction vessel will be controlled by diffusion of the various reactant species, ie., dif----f

(1) F.Helfferich, J. Phys. Chem., 69, 1178 (1965).


Thermal Electron Attachment to Some Aliphatic ... - ACS Publications

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