2405
NOTES 340 320
240
0'21 290
310
330
X,mp
-
350
!-
I
I 37*
Figure 1. Long wavelength absorption spectra of a-naphthylamine in I, n-heptane (A, 318 mp, e 3920) and in 11, mesitylene (A, 322 mp, e 8710).
Optical density measurements were made on mixtures of the proton donor and the different a bases at various concentrations of the latter at the shifted peak (322 mp). The results were then examined for possible 1:l complex formation. I n the hydrogen bonding of a-naphthylamine (B) and T bases (A)
A + B ~ A B when [A] >> [B], the equilibrium constant may be calculated using the following equation3,*
where [A] and [B] are the concentrations of the a base and the proton donor, respectively, €1 and €0 are the extinction coefficients of the complexed and free anaphthylamine, respectively, K is the equilibrium constant, ;is the formal extinction coefficient of the solution, given by ; = D/[B]Z, where D is the measured optical density of the solution containing an initial concentration of [B] in moles per liter, and 1 is the path length in centimeters.
0
2
4
[AI
6
8
10
Figure 2. Plots of [A]/(: - eo) vs. [A] for different bases: I, benzene (7.94): 11, toluene ( 8 . 5 5 ) ; 111, ethylbenzene (7.81); IV, p-xylene (8.33); V, cumene (7.16); VI, chlorobenzene (7.81); VII, mesitylene (7.15). Numbers in parentheses indicate values of z in the ordinate where z X lo3 = l / [ B ] .
The observed linear plots of [A]/; - eo us. [A] (Figure 2 ) ) according to eq 1, show the formation of 1: 1 hydrogen-bonded complex between a-naphthylamine and the a bases. The equilibrium constants (Table I) increase with the increase in the number of alkyl groups in the series of aromatics studied. Possible steric effects due to substituted groups in the alkylbenzenes2 are absent in the present system of hydrogen bonding. Our results can be explained on the basis of inductive effects alone, where alkyl substitution increases the basicity of the a bases, while halogen substitution, as in chlorobenzene, decreases the same. Our findings are similar to the observations on 0-Ha . a i n t e r a c t i ~ n . l , ~ - ~
-
Table I : Equilibrium Constants for 1:1 Hydrogen-Bonded Complex Formation between a-Xaphthylamine and the A Bases at 25"
Base
Chlorobenzene Benzene To1u en e Ethylbenzene Cumene p-Xylene Mesitylene
Equilibrium constant, K , M -1
0.109 + 0.040 0 . 1 4 7 3 ~0.030 0 , 2 5 4 * 0.040 0.244=tO0.O50 0.608=tO0.O80 1.3003Z 0,100 2 . 2 2 0 i 0.150
Low-Energy Electron Radiolysis of Methane
by C. D. Finney and H, C. Maser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66609 (Received JuLu 1.4, IQYO) Publication costs assisted b y U.S. Atomic Energy Commission
Several electron radiolysis studies have shown that information on mechanisms may result when the imThe Journal of Physical Chemistry, Vol, 76, N o . 16, 1971
2406
NOTES
pacting electron energy is defined.l-6 Melton and Rudolfa-5 have well established the relationship between impacting electron energies and product yields. It is the purpose of this note to report additional information, obtained by an independent experimental method, on the lowenergy electron radiolysis of methane. Some results pertaining to product thresholds differ from those previously reported.
1
0.7
pc'--o /' I /
I I
I
I
Experimental Section Photoelectrons as opposed to thermionic electrons were used so that static gas phase experiments could be performed without the complication of pyrolytic background products. The radiolysis cell is a modification of one developed by 1 V i l l i a r n ~ . A ~ ~positive-ion ~~~ measuring capability originated by Compton and van Voorhis*was also incorporated. The method is limited only by the fact that the gas under study (and the radiolysis products) must be transparent to the light which produces the photoelectrons. A schematic representation of the tetrode radiolysis cell and auxiliary equipment is shown in Figure 1. Experiments were performed in a glass vacuum system capable of attaining ultimate pressures of less than 5 X Torr. The potentials on the various electrodes were then applied. The grid and collector electrodes were always at ground potential. The cathode was set at the desired negative accelerating potential. The bias on the ion-collecting electrodes was always 1.5 V more negative than that on the photocathode. Voltages applied in this mannern esured that neither primary nor secondary electrons would be collected on the ion measuring system. The reaction period was 45 min. During runs the currents at the anode and ioncollecting wires were recorded. The currents typically decreased by a fern per cent during the reaction period. The current at the anode vias about 5 X lo-' A. Reactant consumption did not exceed 5%. After reaction the gases were quantitatively transferred to a sampler and analyses were performed on an F & RI Node1 810 chromatograph equipped with an HF1 detector.
!O
APPLIED
0 POSITIVE ION %"e 0 Ce"4
I
A
%Y
POTENTIAL ( VOLTS)
Figure 2. Total ionic and neutral product yields per anode electron as a function of applied potential.
Results and Discussion The product yields are presented in Table I and are plotted in Figure 2. The units are molecules of product (ionic or neutral) per anode electron. From Figure 2 it is seen that the experimental threshold for ion formation occurred in the region of an applied potential of 20 V. This value is clearly much greater than the electron impact ionization energy of 12.7 eV recently reported by Lossing and S e m e l ~ k . ~The latter may be taken as the Table I: Product Yields" with Methane Reactant (0.025 Torr; Reacted for 45 min) Applied potential,
V
100 75
50 40 30 25
20 17.5
Positive ion
0,627 0.609 0,389 0.321 0.139 0.047
CzHz
CZH4
0.197 0.141 0.058 0.039
0.222 0.237 0.190 0.196 0.155 0.156
0.007 0.003
0.085 0.063
CZHB
0.087
0.098 0.106 0,142 0.148 0.122 0,089 0.066
The yields presented are in units of molecules of product per electron collected a t the plate. Q
QOLD CLiTHOOE WIRE
so08 QOLD FILY
(1) R. R. Williams, Jr., J . Phys. Chem., 63, 776 (1959); 66, 372
I IO00 WATT XENON MERCURY
LENS ASSEMBLY
FILTER CELL
PlRC
Figure 1. Schematic representation of the tetrode radiolysis cell and auxiliary equipment. The Journal of Physical Chemistry, Vol. 76, N o . 16,1971
(1962). (2) J. E. Manton and A. W. Tickner, Can. J . Chem., 38, 858 (1960). (3) C. E. Melton and P. S. Rudlof, J . Chem. Phys., 47, 1771 (1967). (4) P. S. Rudolf and C. E. Melton, J . Phys. Chem., 71, 4572 (1967). (5) P. S. Rudolf, Advan. Chem. Ser., No. 82, 101 (1968). (6) C. D. Finney, Ph.D. Thesis, Kansas State University, Manhattan, Kana., 1970. (7) J. L. Moruzzi, Rev. Sci. Instrum., 38, 1284 (1967). (8) K. T. Compton and C. C. van Voorhis, Phys. Rev., 26, 436 (1925). (9) F. P . Lossing and G. P. Semeluk, Int. J.Mass Spectrom. Ion Phys., 2 , 408 (1969).
NOTES
2407
effective upper limit of the electron energy spectrum at an applied potential of 20 V. The margin between the two must be due principally to the perturbation of the electric field caused by the ion-collecting electrodes in the collision region. Compton and van Voorhiss analyzed the effect of such a field on a beam of electrons and found that it caused a spread in impact energies ranging from about 0.6 to 1 times the applied potential and thus shifted the electron spectrum to lower energies. Two other effects in our system which caused the electron spectrum to be broadened less extensively were the initial spread in photoelectron energies and the inhomogeneity of the electric field between the cathode and grid. Figure 2 illustrates a major result of our investigation. Ethane and ethylene were produced a t energies below the threshold for ionization, while acetylene had a threshold about equal to that for ionization. I n physical studies of the direct scattering of low-energy electrons, Bowman and hIillerlO observed a threshold for methane a t 7.5 eV. Using higher energy electrons, Lassettre and Francis” reported an energy loss threshold a t 8.0 eV. I n chemical studies, however, Rudolf5 has reported product thresholds of 12.7, 12.7, and 12.9 eV for ethane, ethylene, and acetylene, respectively. The formation of significant yields of CzHsand C2H4 at applied potentials where no ions were detected is evidence for primary excitation being a principal contributor to these yields. However, some ethane and ethylene were probably formed from ionic precursors a t voltages where ionization was important. The appearance of acetylene at the threshold for the formation ions signifies that it was formed entirely from ionic and/or superexcited12precursors. Acknowledgments. Informative discussions with Professors A. G. Harrison, C. E. Melton, R. L. Platzman, and D. W. Setser were greatly appreciated. C. D. F. is indebted to the Phillips Petroleum Co. for a fellowship. The work was supported by the United States Atomic Energy Commission. (10) C. R . Bowman and W. D. Miller, J . Chem. Phys., 42, 681
(1965). (11) E. N. Lassettre and S.A. Francis, ibid., 40, 1208 (1960). (12) R . L. Plataman, Vorter, 23, 373 (1962).
Reactions of Fast Tz Molecules with l-Butenel
by J. W. Beatty, L. G. Pobo, and S. Wexler* Argonne National Laboratory, Argonne, Illinois (Received February 33, 1971)
60439
Publication costs assisted by Argonne National Laboratory
Chemical reactions in the gas phase of species with hyperthermal energies have mainly been studied by
the nuclear recoil technique.2alb I n particular, recoil tritium atoms are observed to react with 1-butene by addition to the double bond followed by decomposition of the excited butyl radical, by direct displacement of a hydrogen atom or a radical, and by abstraction of a hydrogen at0m.~-6 Atomic tritium with thermal energy distributions also insert into the double bond of 1-butene with the eventual formation of tritiated decomposition products of the transient butyl radical.’ The reactions of molecular tritium have not been studied, but the addition of Hz to a double bond is forbidden by molecular orbital symmetry rulesls and therefore, the reaction should require a rather high activation energy. The only presently known method of causing molecular hydrogen to react with unsaturated hydrocarbons is by exposing them to freshly prepared catalysts. Under proper conditions, hydrogen exchange, addition t o the double bond, and isomerization are prominent reactions. We wish t o report some observations on the reactions of accelerated T Pmolecules with 1-butene molecules, the experiments being conducted with a “chemical accelerator.” Tz+ions are produced in a lowvoltage arc, extracted and accelerated, separated from other species by a zero deflection mass filter, decelerated to a well-defined energy between 5 and 100 eV, and then neutralized by near resonance charge exchange. A beam of the accelerated T Pmolecules collides with a crossed beam (actually a broad sheath) of 1-butene molecules, and the products are collected and later analyzed by radiogas chromatography. A detailed description of the apparatus has been given.’O I n Figure 1 are the yields of the major products formed by reactions of fast Tz molecules with 1-butene molecules as functions of the initial kinetic energy of tritium. Kote that the products are 1-butene, n-bu(1) Work performed under the auspices of the U. S. Atomic Energy Commission. One of us (J. W. B.) received partial support from the Kational Science Foundation under NSF Grant No. GY8428. (2) (a) F. S.Rowland in “Molecular Beams and Reaction Kinetics,” Ch. Schlier, Ed., Academic Press, New York, N. Y., 1970, pp 108116; (b) A. G. Maddock and R. Wolfgang in “Nuclear Chemistry,” Vol. 11, L. Yaffe, Ed., Academic Press, New York, If.Y., 1968, pp 186-248. (3) E. K. C. Lee and F. S.Rowland, J . A m e r . Chem. Sac., 74, 439 (1970). (4) D. Urch and R . Wolfgang, ibid., 83, 292 (1961); Chem. E$. Nucl. Transform. Proc. S y m p . , 2, 99 (1961). (5) J. K. Lee, B. Musgrave, and R. S. Rowland, J . Arner. Chem. Sac., 82, 3545 (1960); R. S. Rowland, J. K . Lee, B. Musgrave, and R. M. White, Chem. E$. Arucl. Transform. Proc. Symp., 2, 67 (1961). (6) F. Schmidt-Bleek and F. S. Rowland, Angew. Chem. I n t . E d . Engl., 3 , 769 (1964). (7) W-.IM. Ollison and J. Dubrin, Radiochim. Acta, 14, 111 (1970). (8) R. G. Woodward and R. Hoffman, “The Conservation of Orbital Symmetry,” Academic Press, New York, N. Y., 1969; for a popular review, see R. G. Pearson, Chem. E n g . A-ews, 48 (41), 66 (1970). (9) See the following reviews for examples: G. C. Bond and P . B. Wells, Advan. Catal., 15, 92 (1964); J. J. Rooney, Chem. Brit.,2, 242 (1966); A. Clark, “The Theory of Adsorption and Catalysis,” Academic Press, Kew York, N. Y., 1970. (10) J. W. Beatty and S.Wexler, J . P h y s . Chem., in press. The Journal of Physical Chemistry, Vol. 75, No. 15, 1971
