3678 4.2
4.0
NOTES
1
When E is plotted against relative values4 of Asoln, our values of E do not fall on the Chaudhuri, et al., 45" line: their conclusion (that the difference between the free energy of solvation of an aromatic anion and that of the neutral molecule is constant over a range of hydrocarbons) therefore is not yet confirmed.
3.8 .
5
2
3.6
5z 3.4 3.2
3.0 2.8
1 '
t
1 2.0
2.1
2.2
2.3
2.4
2.5
10a/T.
Figure 1. Plots used to determine values of E ( I , current when an electron-capturing species is present in the capture chamber; A I , drop in current when an electron-capturing species enters the capture chamber): A, anthracene (linear in the range 182-210'); B, pyrene (linear in range 172-207'); C, naphthacene (linear in the range 152-170'). Of the five recorded curves for each substance only a typical curve is shown. The experimental conditions were similar to those in ref 2.
E . Wentworth, et al., have identified E with the molecular electron affinity, A,. A modified procedure2 has been used t o find further values of E for two key hydrocarbons and to test a predicted value for naphthacene. Average results (Figure 1) are: anthracene, 0.57 f 0.02 eV; pyrene, 0.50 f 0.03 eV; naphthacene, 0.88 =t 0.04 eV. These are "determined-intercept" results;a ie., they have been obtained from the slopes of the lines in Figure 1. The anthracene result agrees with Becker and Chen's3 but differs from the prediction of 0.74 eT7 by Chaudhuri, et al.4 The pyrene result is lower than Becker and Chen's by 0.07 eV, but our value is supported by the fact that we found E for anthracene was greater than E for pyrene by 0.07 eV, in agreement with the average difference of Asoln (affinity in solution) (0.09 f 0.04 eV)4-6 and that of A, (affinity in the gas) calculated theoretically (0.06 eV) E for naphthacene is lower than the predicted4 value 1.15 eV for A,. Relative to anthracene our value of E for naphthacene is that expected from the difference in A values for the two molecules (0.38 f 0.04 eV).4-6p8 This suggests that the electron-capture technique may yield E values higher than previously t h ~ u g h t . ~ If E values are identifiable with A , values, then the molecular electronegativity x = O.5(Ag I ) , where I is the ionization potential from photoionization,6,10,11 varies from 3.98 f 0.03 for anthracene t o 3.88 f 0.04 for naphthacene. A similar drift in molecular electronegativity follows also from Figures 1 and 2 of ref 3, where the plots of (i) I us. absorption maxima (hv) and (ii) A , us. hv have slopes differingby about 50%. The experimental results so far obtained do not support a constant x value for this series of molecules.
.'
+
The Journal of Physical Chemistry
Acknowledgment. This research was sponsored by the U. S. Air Force Office of Scientific Research, Office of Aerospace Research, Directorate of Chemical Sciences, under Grant KO. AF-AFOSR-863-65 and by the Australian Research Grants Committee. We thank the Commonwealth Scientific & Industrial Research organization (CSIRO) for a scholarship to 1,. J.
w.
(1) W. E. Wentworth, E. Chen, and J. E. Lovelock, J . Phys. Chem., 70,445 (1966). (2) L. E. Lyons, G. C. Morris, and L. J. Warren, Aust. J . Chem., 21, 853 (1968). (3) R. S. Becker and E. Chen, J . Chem. Phys., 45, 2403 (1966). (4) J. Chaudhuri, J. Jagur-Grodzinski, and M. Szwarc, J . Phys. Chem., 71, 3063 (1967). ( 5 ) I. Bergman, Trans. Faraday Soc., 50, 829 (1954). (6) M. Batley, Ph.D. Thesis, University of Sydney, 1967. (7) J. R. Hoyland and L. Goodman, J . Chem. Phys., 36,21 (1962). (8) M. A. Slifkin, Nature, 200, 877 (1963). (9) R. S. Becker and W. E. Wentworth, ibid., 203, 1268 (1964). (10) A. Terenin and S. Vilesov, Advan. Photochem., 2 (1964). (11) F. I. Vilesov, Dokl. Akad. Nauk SSSR, 132, 632 (1960).
Inhibition by
C302
of the Explosive
Combustion of CO by Jean Lebel, Pierre Michaud, and Cyrias Ouellet Dkpartment de Chimie, Universitd Laval, Qudbec, Canada (Received April SO, 1968)
I n the course of an investigation of the combustion of ( 2 3 0 2 above 560°, we have observed delayed explosions following accumulation of CO and COz in the system. These explosions took place only after C302 had been consumed, indicating that this compound inhibits the explosive combustion of CO. The possibility of such an inhibition has already been suggested by Harteck and Dondesl in connection with the slow combustion of CO. We therefore studied the explosive combustion of CO in the presence of added GO2, and we also tried t o detect the formation of this compound during the slow combustion of CO-rich mixtures. Carbon suboxide was prepared by dehydration of malonic acid following the technique of Long, Murfin, and Williams2 in the version described by Batchelor, (1) P. Harteck and S. Dondes, J . Chem. Phys., 27, 1419 (1957). (2) D. A. Long, F. S. Murfin, and R. S. Williams, Proe. Roy. Soc., A223, 251 (1954).
3679
NOTES
70
c 0 0
4
8
12
% CSOl. 0
1 2 Time, min.
3
Figure 1. Pressure variation a t 620°, showing inhibition period and explosion. Initial partial pressures: 40 mm of eo, 20 mm of 0 2 , 4 mm of c&.
Furby, and Wilkinson. Its infrared spectrum, checked against that published by Diallo and bar chew it^,^ revealed no impurities. Matheson C P grade CO and Ohio Medical O2 were used. A fresh mixture of CO, 02,and C302was prepared before each series of runs. The reaction took place in a Vycor cylinder 30 cm long and 4 cm in diameter, the temperature of which was uniform by 1" or better. The pressure was recorded by means of a Pace transducer. When cold 2CO: 1 0 2 mixtures were rushed into the reaction vessel at pressures above 30 mm and temperatures above 560", explosion took place within less than 1sec. However, in the presence of GO2,the same mixtures exploded only after considerable delay. Figure 1 shows a typical pressure record obtained at 620' with a mixture in which the initial partial pressures were 40 mm of CO, 20 mm of 0 2 , and 4 mm of C302. After introduction of the gases, a slow pressure rise precedes the explosion. I n all our experiments the pressure increased during the inhibition period by an amount equal to the partial pressure of C302initially present in the mixture. Such a pressure increase corresponds to 0 2 = COz 2C0, after the over-all change C3Oz completion of which explosion occurs. Figure 2 shows how the inhibition period varies with the initial c302concentration, expressed in volume per cent, in a mixture of 40 mm of CO and 20 mm of 0 2 at 620". Under these conditions, a threshold concentration of about 1% ( 2 3 0 2 was necessary to cause measurable inhibition. The shape of the curve in Figure 2 can be accounted for provided the rate of disappearance of C302 follows an order higher than zero. If so, the inhibition period, i.e., the time it takes 'for the C302 concentration to fall from the initial to the threshold value, will increase less rapidly than the initial concentration. To test the hypothesis that the slow combustion at CO-rich mixtures might be self-inhibited by formation of some C302, we did several runs with 3CO: lo2mixtures in the ranges 560-590' and 40-180 mm and withdrew samples after about 30% conversion. No C302 was detected by infrared analysis, showing at least that its concentration was well below the value of 1% nec-
+
+
Figure 2. Inhibition periods a t 620' for various percentages of C302 added. Partial pressures: 40 mm of CO, 20 mm of 0 2 .
essary to inhibit explosion in our experiments with stoichiometric mixtures at 620". The reactions of 0 and 0 2 with C302 and with the C20 radical have been the object of a number of investigations, especially those of von Weyssenhoff, Dondes, and Harteck,S of Williarnson and Bayes,6 of Kunz, Dondes, and Harteck,' and of Liuti, Kunz, and Dondes.8 There is little doubt that C302 competes with CO for the 0 atoms (and perhaps for some excited species) responsible for the branching reaction that leads to explosion. This is not necessarily linked to the main mode of destruction of GO2, which may begin with pyrolysis followed by reaction of C 2 0 with 02. This, in our view, is the reaction responsible for the pressure increase during the inhibition period.
Acknowledgment. This work was supported by National Research Council of Canada Grant A-113 and by a Canadian Government Scholarship to P. M. (3) J. 8. P. Batchelor, E. Furby, and K. L. Wilkinson, Atomic Energy Research Establishment Report R-3942, Harwell, Great Britain, 1962. (4) A. 0. Diallo and P. Barchewitz, J . Chim. Phys., 61, 1296 (1964). (5) H. yon Weyssenhoff, S. Dondes, and P. Harteck, J . Amer. Chem. Soc., 84, 1526 (1962). ( 6 ) D. G. Williamson and R. D. Bayes, ibid., 89, 3390 (1967); 90, 1957 (1968). (7) C. Kunz, 8. Dondes, and P. Harteck, J . Chem. Phys., 46, 4157 (1967). (8) G. Liuti, C. Kunz, and 8. Dondes, J . Amer. Chem. Soc., 89, 5542 (1967).
The Photochemical Decomposition of Methanethiol.
Hot Hydrogen Atom
Reaction with Deuterium'
by G. P. Sturm, Jr:,2 and John M. White Department of Chemistry, The University of Texas at Austin, Austin, Texas 78719 (Received May 8, 1968)
The gas-phase photochemical decomposition of methanethiol has been investigated extensively by Volume 72, Number 10 October 1968
