NOTES
4094
demonstrated by the appearance of CD3H. The reactions leading to methane formation are
Table 11: Mass Spectrum of Vapor above Li3P04 at 1400’K Re1 intensity
Ion
PP Lip02 Li + P+ PO Lip0 + LiP08+ +
+
+
100
CDa
12 12 7
(1)
C3Ds +
(2)
+ C3Hs
--j
+ C3Ds CH3 + C3Hs CD3
1.7 0.7 0.3
CH3
-LiN03, Li~(N03)2, Li2(N02)2, NaN03, Na(N03)2, NaN02, and Na2(NO&-are typical of the vapor species associated with the vaporization behavior of halides and pseudohalides; the dimerization energy of LiNOz derived above also is typical of such compounds. To that extent, the present work supports Hardy and Field’s suggestion2as to the ionic nature of the gaseous alkali nitrates and nitrites. Against this must be placed the results of Butkov and T~chassowenny~ and the recent electron-diffraction work of Khodchenkov, Spiridonov, and Akishin,”which suggest a covalent structure. Further work would seem to be necessary to classify the nature of bonding in the alkali nitrates and nitrites. In particular, vibrational spectra, which permit a clear differentiation between ionic nitrate and covalent nitrato groups,lo would be highly desirable. (9) A. N. Khodchenkov, V. P. Spiridonov, and P. A. Akishin, Zh. Strukt. Khim., 6 , 765 (1965). (10) C. C. Addison and N. Logan, Advan. Inorg. Chem. Radiochem., 6 , 72 (1964).
The Chain Decomposition of Propane Initiated by Vacuum Ultraviolet Photolysis
Institute for Basic Standards, National Bureau of Standards, Washington, D . C. (Received June 20, 1966)
One of the more striking features of the photolysis of mixtures of C3Hs and C3Ds at 1470 A and 25” is that CD4.I the methane product is more than 90% CH4 This certainly signifies that the dominant mode of methane formation is molecular elimination from excited propane rather than abstraction of hydrogen from propane by methyl radicals. At 25”, a minor contribution of methyl radicals to methane formation is
+
CD3H
+ C3H7
+ C3D7 CH4 + C3H7
(3)
+CD,
(4)
+
(5)
+ C3Ds -+- CH3D + C3D7
(6)
where C3D7 and C3H7 may be n-propyl or isopropyl. It will be seen in the Results section that about 80% of the methanes formed by mechanisms other than molecular elimination come from abstraction reactions 3-6 and only 20% from disproportionation of methyl and ethyl. The latter mechanism has, therefore, been neglected. The ratios CH4’/CH3D’ and CD,H’/ CD,’, where the prime signifies the rate of formation, are given by 0lIl -CH4’ CH3D’ ka[CH31[C3Dsl k3
CDIH’ CD4’
k4
+-ks[C3Hsl ka[C3DsI
[CaHsl [CaDa]
0212
(7)
(8)
+1 h[CD31 [C~DSI
where I, and I2are the rates at which light is absorbed by C3H8 and C3D8, respectively, and 01 and 0 2 are the quantum yields for molecular elimination of methane and methane-&. Under conditions where kg [CH,]. [GDs] >> 0111and k4[CDa][C3Da] >> 9 2 1 2 , eq 7 and 8 become
CHI’ - ks [C3Hsl CH3D’ k g [CsDs] CD3H’ 04’
by A. H. Laufer and J. R. McNesby
The Journal of Physical Chemistry
+ CzH4 CD4 + CzD4
C3Hs --+ CH4
k3
kd
[C3Hs] [C,Dsl
(9)
(10)
It is known that the activation energies corresponding to k4 and k6 are in the range 11.1-12.5 kcal/mole depending upon whether the secondary or primary D atom is abstracted.2 If the quantities @.J1 and @*I2 are assumed to be essentially independent of temperature, the conditions required for eq 9 and 10 to be valid will be approached by increasing the temperature. The chain is propagated by the decomposition of npropyl formed in reactions 3-6. (1) H.Okabe and J. R. McNesby, J . Chem. Phys., 37, 1340 (1962). (2) W.M. Jackson, J. R. McNesby, and B. deB. Darwent, ibid., 37, 1610 (1962).
NOTES
4095
n-C3H7 +CHa
+ CzH4
‘11)
The objective of this work is to estimate the chain length of propane decomposition well below pyrolysis temperatures. Experimental Section
The vacuum system was of the conventional type with the exception that all valves in the gas-handling system were of Viton and Pyrex and no absorption of hydrocarbon products or reactants was experienced. The Xe resonance lamp was ring-sealed into the reaction cell and the LiF window was sealed onto the lamp envelope by means of a silver-silver chloride graded seal as previously described. Absorbed light intensities were about 1014quanta cc-’ sec-’ at 1470 A. The reaction mixture was circulated continuously to prevent accumulation of reaction products near the window. After photolysis the reaction mixture was sampled for chromatographic analysis (10 m of 30% Squalane on Celite and flame ionization detection), and finally the condensibles were frozen out and the volatile methane-hydrogen fraction was allowed to expand into an evacuated sample flask. This fraction was analyzed by mass spectrometry. Control experiments were done to ensure the absence of any mechanism in the procedure for fractionating reaction products. Cracking patterns of authentic samples of the isotopic methanes were measured in separate experiments. The maximum impurity in the propane and in the C3D8was 0.02% and no detectable methane or hydrogen was present. Photolysis of C3Ds alone was done at all temperatures to assess the contribution to CDaH of incompletely deuterated propane impurities. Corrections were made for this contribution prior to entering the data in Table I. Temperatures were controlled automatically to *3”.
Table I: Product Analysis in the Photolysis of 1 : 1 Mixtures“ of Propane and Propane-& Analysis
Hz HD Dz CH4 CHsD CHzDz CDsH CD4 Ethane Ethylene Acetylene Propylene
25O
150’
248‘
320’
100.0 20.4 57.5 10.40 0.11 0.62 1.33 6.32 14.5 30.5 7.4 61.0
100.0 30.3 42.0 9.68 0.82 0.13 3.14 4.92
100.0
100.0 31.0 28.6 54.0 15.0 0.05 17.5 9.95 14.5 96.9
...
34.2 38.6 24.3 3.07 1.56 8.06 8.86 15.3 51.0 5.1
0
Total pressure, 22.0 torr.
Results and Discussion
It has been shown2 that within experimental error CH3 and CD3 abstraction reactions exhibit the same kinetic parameters and ks/ks = k3/k4 = 1.04 exp (1400/RT). It follows that when the conditions for the validity of eq 9 and 10 are met CH,’/CH,D’
=
CD3H’/CD4’ =
Figure 1. Variation of methane isotope analysis with temperature (below the figure). The straight line is the relationship predicted for a pure methyl radical mechanism.
mental points approaching the expected straight line defined bv ea 12. The assumptions made in the following argument result in a lower limit for the chain length. The figures used are relative numbers of molecules. From experiments on the photolysis of CH3CD2CH3at room temperature and 1470 A, it can be estimated that an “
1.04exP(14W/RT) (12) Thus, the Arrhenius plot of eq 12 should be approached asymptotically both by CH4”CH3D’ and CD3”/ CD4’ as the temperature is raised. Temperatures sufficiently high to demonstrate this point were not achieved because Of Of the LiF-Ag-AgC1 around 400’. However, Figure 1 shows the experi-
A
3) R. F. Hampson, Jr., and J. R. McNesby, J. Chem. Phys., 4 2 , (2200 (1965).
Volume 70,Number 19 December 1966
4096
upper limit of one-third of the ethane is formed by association of methyl radicals.' Thus, reference to Table I (25" run) shows that of the 14.5 ethanes formed, a maximum of 5 ethanes arises from association of 10 methyl radicals. Since equal amounts of methyl and ethyl are formed in primary processes 13 and 14 and assuming the ethyl radical does not decompose (up to 350", hydrogen atom formation from ethyl radical decomposition has been shown3a4to be
NOTES
Electron-Acceptor Properties of Mellitic Trianhydride
by H. M. Rosenberg, E. Eimutis, and D. Hale Air Force Materials Laboratory, Research and Technology Division, WrTVright-Patterson Air Force Base, Ohio (Received July 25, 1966)
The following equation, proposed by McConnell, Ham, and Platt,' is generally applicable for describing
E, a relatively minor process), random association of methyl and ethyl requires that 10 additional methyl radicals must have disappeared by association with ethyl. Thus 20 methyl radicals disappear by association. Disproportionation of methyl and ethyl is only 6% as important as association5p6and only 0.6 methane is formed by disproportionation and 2.4 methanes are formed by abstraction of H and D. This includes CD3H and CH3D as well as that part of the CH4 and CD4 arising from methyl radical abstraction (obtained from eq 12). Thus a total of 23 methyl radicals must have been formed. Since D atom formation up to 350" is relatively u n i m p ~ r t a n t ,it~ ?is~ safe to assume that at 320" the D2 product is still almost entirely attributable to molecular elimination. The Dz is chosen as the monitor rather than H2 since the fraction of D2 arising from D atom abstraction reactions must be much smaller than the fraction of H2 arising from H atom reactions because of the kinetic isotope effect. The number of methyl radicals (based on D2 = 57.5) equals (57.5/28.6) X 90 S 181. In this calculation it is estimated (again on the basis of eq 12) that 90 of the 96 methane molecules arise from methyl radicals. If the average chain length is defined as the number of methyl radicals that form methane divided by the number formed in the primary process, the result is that the average chain length is 181/23 = 7.9 at 320". The chain lengths at 25, 150, 248, and 320" are approximately 0.07, 0.54, 2.2, and 7.9. Since the chain termination is effected by free-radical association reactions, the chain length is a function of the light intensity, and the present conclusions are valid for light intensity ol about lou quanta/cc-' sec-l.
(4) 9. Bywater and E. W. R. Steacie, J.Chem. Phys., 19,326 (1951). (5) P.Auslooa and E. W. R. Steacie, Can. J. Chem., 33, 1062 (1955). (6) C. A. Heller, J. Chem. Phys., 28, 1255 (1958).
The Journal of Physical Chemistry
=
ID - EA - W
(1)
the energy of the charge-transfer (CT) transition of T complexes as a function of the ionization potential and electron affinity of the acceptor of the donor (ID) (EA). W is a collective term which includes all other energy interactions arising principally from solvation and coulombic attraction and is essentially constant for a similar series of ?r complexes. Electron affinities are directly proportional to the Hammett u p of the substituents of p-benzoquinone. Increasing the number of electron-withdrawing substituents enhances the electron affinity, although this effect does not appear to be simply additive but is dependent on the positions of sub~titution.~ It has been noted that substituents with u p > 0.60 (CN, 0.66; NO2, 0.78) are particularly effective in CT acceptor^.^ Since u constants for disubstituents, in general, have not been evaluated, the properties of functional groups such as cyclic anhydrides must be determined experimentally. A comparison of the electron affinities of pyromellitic dianhydride and 1,2,4,5-tetracyanobenzene reveals that the cyclic anhydride group is more effective than two adjacent cyano group^.^ It is therefore not surprising that pyromellitic dianhydride complexes have been actively in~estigated,~ although the complexes of mellitic trianhydride have been neglected since their initial observation by Mustafin.6 All complexes were prepared by adding appropriate amounts of donor to saturated solutions of mellitic trianhydride in chloroform. Spectroscopic data were obtained on a Cary Model 11 recording spectrophotom(1) H. McConnell, J. S. Ham, and J. R. Platt, J. Chem. Phys., 21, 66 (1953). (2) P. R. Hammond, J. Chem. SOC.,471 (1964). (3) A. R.Lepley and J. R. Thelman, Tetrahedron, 22, 101 (1966). (4) L. I. Ferstandig, W. G. Toland, and C. D. Heaton, J. Am. Chem. SOC.,83, 1151 (1961); Y.Nakayama, Y.Ichikaw, and T. Matsuo, Bull. Chem. Soc. Japan, 38, 1674 (1965); T. Matsuo, ibid., 38, 2110 (1 965). (5) I. S.Mustafin, J. Gen. Chem. USSR, 17, 560 (1947)
