Chemical Reaction Rates of Quasifree Electrons
25 (13) See, e.g., P. J. Wagner, "Energy Transfer Kinetics in Solution" in "Creation and Detection of the Excited State," Vol. 1, Part A, A. A. Lamola, Ed., Marcel Dekker, New York, N.Y., 1971, p 173. (14) G. Favaro, G. Bartocci, and P. Bortolus, in press. (15) The possibility that a nonrelaxed CT collision complex undergoes a very fast ISC cannot be excluded. (16) J. Sa!tiel and E. D. Megarity, J. Amer. Chem. SOC.,94, 2742 (1972).
(9) F. Masetti and U. Mazzucato, Ann. Cbim. (Rome), 62, 519 (1972). (10) K. H. Grellmann, A. R. Watkins, and A. Weller, J. Phys. Chem., 76, 469 (1972). (11) See, e.g., J. B. Birks, "Photophysics of Aromatic Molecules," Wiley-Interscience, London, 1970, p 436. (12) J. Chodkowski and T. Giovanoli-Jakubczak, Rocz. Chem., 41, 373 (1967).
Chemical Reaction Rates of Quasifree Electrons in Nonpolar Liquids. Ill Augustine 0. Allen," Thomas E. Gangwer, and Richard A. Holroyd Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973
(Received July 15, 1974)
Publication costs assisted by Brookhaven National Laboratory
Reaction rates of the electron, generated by X-ray ionization in various solvents, with a number of compounds which are known as good electron scavengers (e.g., SF6, N20, CC4, C2HsBr, and trichloroethylene) have been determined as a function of temperature. The results are correlated with the quantity Vo, the energy of the electron in its mobile state, previously determined by photoelectric threshold measurements in the various solvents. The measured rate constants exhibit maxima for characteristic values of V o , reminiscent of the resonant energy maxima in the dissociative attachment cross sections for electron reaction shown by the same molecules in the gas phase. Since Vo shifts toward more positive values with decreasing temperature, it is found that, for a particular solute, in a solvent having Vo lower than the value for the maximum electron-solute reaction rate, the reaction has a negative temperature coefficient. In solvents with higher Vo than corresponding to the maximum rate, the temperature coefficient of the reaction rate is positive. Thus the kinetics of these reactions appear quite different from those of ordinary chemical reactions. New data are presented on electron mobilities in various solvents, obtained incidentally in the course of the measurement of the rate constants. The relative reaction rates for quasifree electrons with various solutes in cyclohexane are in good general agreement with Schuler and Klein's values for relative reaction rates with transient "geminate" electrons.
Introduction The reactions of electrons formed by ionization in liquid or amorphous materials are little understood, and yet are of considerable importance in a number of applications ranging from radiobiology to dielectric breakdown in insulators. The present paper extends our earlier work' on the interaction rates of electrons with scavenger molecules in liquid hydrocarbons and tetramethylsilane (TMS). The present work has resulted in some clarification of the nature of the reactions occurring and their relation to the dissociative attachment reactions of molecules with electrons in the gas phase. Experimental Section Our method for determining rate constants has been described in detail.2 The mobility of electrons in the purified solvent is first determined by measuring the rate of approach of the electron current to a steady-state value (Hudson method); the total steady-state current is then determined and the deviation of the ratio of the electron current to the total current from the value one-half provides a measure of the amount of electron-trapping impurities in the solvent preparation. The measurements are made over a range of temperatures. Small amounts of solute are then added to the liquid in the cell and new measurements are
then made of the electron current. The amount by which the electron current is reduced by the presence of scavenger provides a measure of the ratio @ / l e of )the rate of reaction of the electron with the solute to the mobility of the electron in the solvent, All hydrocarbon solvents (Phillips Research Grade) and tetramethylsilane (nmr grade) were passed through activated silica gel, deaerated by bubbling with Nz, degassed and stirred over NaK, and finally the solvent vapor was bubbled through NaK at 200'. All solutes (CP grade) were degassed prior to use. The Biomation Model 802 recorder used in the previous work was employed for all h / p measurements. For some of the mobility measurements the faster Biomation Model 8100, which can record 2048 data points over a time range which can be as small as 20 ysec, was used. This recorder was on the output of a Keithley Model 105 pulse amplifier; the terminating resistance on the amplifier input provided a time constant of less than 0.2 hsec. Temperature control was improved by the use of a Thermo Electric Co. Model 32422 indicating proportional temperature controller.
Results Electron Mobilities. New mobility data obtained in the course of this work are shown in Table I. The values for nhexane agree fairly well with previous studies (Figure 1). The Journalof PhysicalChemistry, Vol, 75, No. 1, 1575
A, 0. Allen, T. E. Gangwer, and R. A. Holroyd
26
TABLE I: Electron Mobilities
Temp, "C
Solvent n-Hexane
21
Cyclohexane
21 20 0 21
2,2,4-Trimethylpentane
68 50
50 35 20 20 20 - 25 - 25 Tetramethylsilane
- 60 - 83 19 0 - 25 - 50
Preparatione
pet
cm V-1 sec - 1
H1 H2 H3 H3
0.069 0.060 0.060 0.032 0.23
c2 c3 c4 c5 I1 I1 I2 I1 I1 I2 13 I2 I3
0.21
c1
I2
I3 T1 T1 T1 T1
0.20 0.21 0.23 8.1
6.5 6.9 5.6 5.1 5.3 5.6 3.6 4.2 2.6 1.85 97 96 99 103
a The use of different samples of each liquid is indicated by different numbers; where the number is the same it indicates the same sample of liquid was used in these experiments.
Figure 2. Electron rate constants at room temperature as a function C2HCI3, (0)eel4,(A) of Vo (in eV) for reaction with (V) SF6, (0) N20, and (0)C2H5Br:open symbols, this work; filled symbols, ref 2.
0.1 t
CSI:
0.C3I1
0
I
Mobilities as a function of 1 / T for tetramethylsilane, 2,2,4-trimethylpentane,and n-hexane: data for TMS and isooctane from this work; for hexane ( 0 )this work, (0)ref 4, (0) ref 2, (V)ref 5. Dotted lines are fit to eq 1. Solid curves are fit to eq 2. Figure 1.
The values for cyclohexane confirm our previous value, which was in disagreement with earlier determinations. The values obtained for TMS and for 2,2,4-trimethylpentane (isooctane) are graphed in Figure 1. The values for TMS a t low temperatures must be regarded as superseding the values that we published earlier. At the lowest temperature used in the earlier work, the ions were so slow that it was difficult to judge the correct value of the total steadystate current. The present results were also'improved by The Journal of Physicai Chemistry, Vol. 79, No. 1 , 7975
use of the faster Biomation recording equipment. It now appears that the electron mobility in TMS is independent of temperature over the range studied within the experimental accuracy. Since the electrical current measurements give a value of k / p , values of p, the electron mobility in each solvent a t each temperature used, had to be assumed in the light of available data to obtain the rate constants. For neopentane, the field-dependent values of ,u were taken from the recent work of Bakale and S ~ h m i d tFor . ~ TMS, where the field was always kept below 2000 V/cm, a value of 100 for was assumed a t all temperatures. For isooctane and n- hexane values were read from the lines shown in Figure 1 (see Table 11). The hexane points in Figure 1 include, besides values found in the present work, data previously published from our laboratory2,4 and that of Davis.5 Rate Constants. The new data on the second-order rate constants of reactions of electrons with trichloroethylene, sulfur hexafluoride, nitrous oxide, carbon tetrachloride, ethyl bromide, and methyl and ethyl chloride are assembled in Table 11. Rate constants obtained from successive additions of the same compound to a given preparation were in agreement Lo within 5-10%. Agreement is frequently but not always as good between results obtained at different times on different preparations of the solvent. In Figure 2 the available data for the reaction of electrons with five solutes at room temperature are presented as a function of the electron energy V o for the various solvents. (Values of V Oare discussed by Holroyd.G)A clear pattern emerges. The rate constant for reactions of electrons with each solute (with the exception of carbon tetrachloride) shows a pronounced maximum corresponding to some particular value of the energy Vo. For carbon tetrachloride no
27
Chemical Reaction Rates of Quasifree Electrons
TABLE 11: Rate C o n s t a n t s for Reactions of Electrons w i t h Various C o m p o u n d s in Different Solvents ~~
~
x
k/pL,(
Solvent and assumed
pce
n-Hexane pe = 160 exp( -2245/T)
Solute CzHC13
Temp, "C 50 41 22 19 0
- 20 - 25 - 52 - 55 CClr SFs
21 24 22 0
- 25 - 27 NzO
22 1
- 25
21 21 21 21 21 19
Cyclohexane pe = 0.24
2,2,4-Trimethylpentane pe = 40.8 exp( -585/T)
0
- 25 - 52 50
20 19 0 - 25 - 50
- 59 NzO
C2H6Br
19 0 - 25 -50 50
20 19 0
- 25
-51 - 59 - 85
Neopentane pe from ref 3 Tetramethylsilane pe = 100 at all tempo
19 21 21 21 21 0 - 25 - 50
21 0 - 25 - 50 21 0
- 25 - 50
20 0
- 25 -51
20
10 --12),a
V cm-2 M-1 36. 5b 47.3c 32.1* 52.80 48.9~ 28.6b 53. IC 23.3~ 22. l b 17.4 23.8 23.8 22.9 24.1 21.8 13.7 11.3 10.7 16.7 10.0
6.02 11.4 8.23 6.3 6.2 5.9 5.5 9.4 10.4 9.5 8.0 7.5 9.9 8.3 1.8 2.0
2.2 3.2 0.41 0.93 0.80 1.17 1.57 2.1 2.2 2.1
