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Benzene in MTHF glass produces a very broad absorption through the visible apparently benzene does not produce a well defined anion under these conditions. The spectrum is quite different from that of trapped electrons in MTHF but does not show the narrower band at 435 nm produced by potassium reduction of benzene in dimethoxyethane at - 80°K.8 (In the latter case, the ion may be stabilized by the Acknowledgment. We thank the Australian Research solvent or the counterion.) Stabilization of benzene Grants Committee and the Australian Institute of anions in a nonpolar solvent therefore appears unNuclear Science and Engineering for the support of likely, but Ekstrom assigns bands a t 525 and 930 nm in this research. 3-methylpentane to CeH6- and (CBH6)2- on the basis of DEPARTMENT OF PHYSICAL CHEMISTRY K. P. DAVIS their removal by carbon tetrachloride. * However, OF NEWSOUTH WALES J. L. GARNETT these wavelengths agree well with ours (550 and 930 THEUNIVERSITY KENSINQTON, N.S.W. 2033, AUSTRALIA nm) obtained in the halide glass and assigned to monoRECEIVED JANUARY18, 1971 mer and dimer cations while Gallivan and Hamill state that the 930-nm band in 3-methylpentane is enhanced by the electron scavengers, carbon tetrachloride and isopropyl chloride, but suppressed by the positive Comment on 6‘Ionic Species Formed from charge scavengers, 2-methylpentene-1, triethylamine, and MTHF.g Benzene during Radiolysis of Its Solutions I n solutions containing carbon tetrachloride, Ekstrom in 3-Methylpentane at 77°K” by A. Ekstrom observed bands at 320 and 1030 nm which he ascribed Publication costs borne completely by The Journal of to monomer and dimer cati0ns.l The 320-nm band Physical Chemistry cannot be the same transition as our band at 550 nm; (which is consistent with photoelectron spectral data;1° Sir: The object of this note is to question Ekstrom’s this band is masked in CC14 solutions). It could be a assignment of an optical absorption band to the species, higher transition8 but it may be due to the radical, (CeHa)z -. CaH,..” The identity of the species absorbing at 1030 The term “dimer cation” is used for species, M2+, nm is not clear. where M is a stable neutral molecule such as an aromatic I conclude that there is as yet no definite evidence hydrocarbon: as in the similar case of excimers, M2*, for a dimer anion. Their apparent instability remains the interaction between the two partners is believed to puzzling. An explanation given recently for the case be weak, i.e., no rearrangement of bonds occurs. of the naphthalene dimer anion4 is not likely to be Dimer cations have been clearly identified by their generally applicable. esr spectra; their optical spectra have been studied and they have been detected in mass spectrometers (see ref 1-3 and references therein). Simple explanations (1) A. Ekstrom, J . Phys. Chem., 74,1705 (1970). (2) B . Badger, B. Brocklehurst, and R . D. Russell, Chem. Phys. of the stability of dimer cation^^-^ suggest that dimer Lett., 1,122 (1967). anions, Mz-, should be equally stable. It is surprising, (3) B . Badger and B. Brocklehurst, Trans. Faraday SOC.,65, 2576, therefore, that the numerous esr studies of aromatic 2582,2588 (1969). hydrocarbon anions in solution have not detected such (4) B. Badger and B. Brocklehurst, ibid.,66,2939 (1970). species. (Anion dimers, M22-, do exist, e.g., where h!I is (5) K. Kimura, H. Yamada, and H. Tsubomura, J . Chem. Phys., 48, 440 (1968). benzoquinone;6 these species have two bonding elec(6) W. H. Hamill in “Radical Ions,” E. T . Kaiser and L. Kevan, Ed., trons instead of one). Interscience, New York, N. Y., 1968, p 321. The radiolysis of solutions in organic glasses forms a (7) T . Shida and W. H. Hamill, J . Chem. Phys., 44,4372 (1966). convenient method of preparing ions of the solutes. (8) C. L. Gardner, ibid., 45,572 (1966). Anions are produced in ethers (such as methyltetrar (9) J. B . Gallivan and W. H. Hamill, ibid., 44,2378 (1966). (10) D. W. Turner, Advan. Phys. Org. Chem., 4 , 3 1 (1966). hydrofuran, MTHF), cations in halides, and both in (11) T . Shida and W. H . Hamill, J . Amer. Chem. Soc., 88, 3689 hydrocarbons.6 Having studied the formation of (1966); T . Shida and I. Hanazaki, Bull. Chem. SOC.Jap., 43, 646 dimer cations in butyl chloride-isopentane mixtures,a (1970). we made a search for dimer anions in MTHF. Only monomer anions could be detected in naphthalene B. BROCKLEHURST solutions up to 1 M and pyrene to 0.5 M concentration CHEMISTRY DEPARTMENT THEUNIVERSITY even on annealing after radiolysis. Comparison with 53 7HF, ENQLAND SHEFFIELD, the cation results suggests that the dimer anions are not RECEIVED SEPTEMBER 8, 1970 stable.
conditions using both forward and back exchange methods, This remarkable correlation in isotope orientation and multiplicity of exchange during initial isotope incorporation suggests that analogous n-complex mechanisms operate under homogeneous and heterogeneous conditions. The data thus unequivocally support the general n-complex theory of metal catalysis. lv2
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The Journal of Physical Chemistry, Vol. 76, No. 8, 1971
