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)
COMMUNICATIONS TO THE EDITOR
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~~
Table I : Charge-Transfer Energies of Pyromellitic Dianhydride and Mellitic Trianhydride with Aromatic Hydrocarbon Donors“ -E~, Donor
PMDA
Xaphthalene Anthracene Phenant,hrene Acenaphthene Pyrene Fluorene 2-1Llethylnaphthalene l,%Dimet,hylnaphthalene 1-Chloronaphthalene 1-Bromonaphthalene 2-Methylanthracene
3.01 2.40 3.07 2.66 2.46 2.91 2.91 2,70 3.07 3.05 2.33
evMTA
2.74 2.10 2.74 2.35 2.10 2.56 2.63 2.46 2.71 2.70 2.08
AE~? ev
0.27 0.30 0.33 0.31 0.36 0.35 0.28 0.24 0.36 0.35 0.25
Av 0 . 3 1 f 0.04 a
I n ch1,oroform solution.
* ErrPMDA - ErrMTA.
eter, equipped with a four-place digital voltmeter. The wavelength maxima were determined to a precision of +2 mp using 5-cm matched silica cells with solvent as reference. All complexes gave typical broad structureless charge-transfer absorption bands.
We report in Table I the charge-transfer energy at wavelength of maximum absorption for a series of mellitic trianhydride-aromatic hydrocarbon complexes. These are compared with pyromellitic dianhydride complexes having the same donors and the differences in charge-transfer energy for both series shown in the table. From eq 1, assuming W to be constant for complexes having the same donor, we get
ErrPMDA - E ~ M T A = E A MTA - E A PMDA
(2)
The electron affinity of mellitic trianhydride is shown to be 0.31 f 0.04 ev higher than that, of pyromellitic dianhydride. Hence, its electron affinity is 1.17 based on the reported value of 0.86 ev for pyromellitic dianhydride.6 Since the electron affinity of phthalic anhydride is 0.1,6 it is seen that consecutive addition of cyclic anhydride substituents to phthalic anhydride does not yield a uniform increase in electron affinity, the first addition being considerably more effective than the second addition. (6) G. Briegleb, Angew. Chem., 7 6 , 326 (1964).
C O M M U N I C A T I O N S TO THE E D I T O R
The Ultraviolet Spectrum of Trimethylborane and the Ethylene Problem
Sir: The spectra of small molecules are of considerable interest as they provide a testing ground for theoretical and empirical calculations of spectroscopic properties. Attempts’ have been made to obtain the ultraviolet spectrum of trimethylborane, and the observation of rising end absorption in the 225-mp region has been interpreted as indicating an absorption maximum in the region of 220-230 mp. We have previously suggested2 that the 220-230-mr transition observedlb in the spectra of tributylboranes was a charge-transfer tranOrbital to boron, whose from a transition energy should be proportional to the ionization potential of the hydrocarbon. We would therefore expect this charge-transfer transition of alkyl-
boranes to move to higher energies with increasing hydroca,rbonionization potential. We have recently measured the ultraviolet spectrum of trimethylborane between 4000 and 1770 A (Figure band systems, l). There are clearly two at energies substantially lower than the Rydberg transitions observed for methane, which cannot therefore be assigned to analogous transitions in the methyl group. At 1770 A we see the beginning of the transition reported in earlier qualitative unpublished results by Goodman and Love.a This transition, A,, = 1750 A, 7.1 ev, log E = 2.9, we have previously assigned to (1) (a) J. Rosenbaum and M. Symons, Proc. Chen. Soc., 92 (1959); (b) A. G. Davies, D. Hare, and L. Larkworthy, Chem. I d . (London), 1519 (1959); (0) H.Jaffe and M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy,” John Wiley and Sons, Ino., New York, N. y , ,1962, 455,
(2) B.G. Ramsey, J.Phys. Chem., 70,611 (1966).
Volume 70, Number 12 December 1968
