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of o-hydroxybenzophenone which is due to an electronic transition involving the carbonyl group. The high solubility of the compound in orgaiic E& vents resembles similar soluhility characteristics of ~ also have high some mesoionic c o m p n ~ i n d swhich dipole moinrnts and involve a separation of charge within the five-membered ring. It should be pointed out, however, that compound V should not be classified as a mesoionic compound even though it cannot be represented by a wholly covalent structure. Although a sextet of electrons1° is available, they are n o t associated with all of the atoms comprising the ring---ri otably the phosphorus. Thus, structure V is a novel system involving intracyclic charge separation. The remarkable stability of the compound is also cxplicable on the basis of structure V. A t least twenty-two resonance hybrids can be written, of which five examples (Va\'e) are shown
The electrolyses were performed in a cell with a platinum anode and a mercury-pool cathode. The r - h v ~ n t 5 ~ 7 : 5 N,N-dimethylformainide (DMF), acetonitrile, or mixtures of the two, and the supporting electrolyte was 0.1 M tetra-72-propylamrtioniurn perchlorate. The solvents were degassed carefully and most of the studies were made with solutions 5 mM. in reducible material. A Ag-AgCIOa reference electrode, 0.1 M in AgC104, was employed which, with DMF as solvent, has a potential about -0.5 v . with respect to the standard calomel electrode. The details of the electrolytic cell3 and of the electron spin resonance spectrometer' are described elsewhere. Polarographic studies, both d.c. and a s . , also were made.s The oscillopolarograms were performed a t a number of frequencies in the range from 0.1 to 100 cps. Tetracyanoethylene (TCNE) is reduced to the anion radical merely by dissolving it in DICZF. The resulting solution is pale yellow. A potential of -0.2 v., corresponding to the half-wave potential, is required to reduce T C N E in acetonitrile, and the radical produced gives an e.s.r. spectrum identical with that observed from the D M F solution. OscilIopolarograrns in acetonitrile show that the reduction is reversible. TCNE in solvent mixtures of acetonitrile and D M F gives this same e.s.r. spectrum for mole fractions of acetonitrile less than 0.5 except that the intensity of the spectrum decreases and the line width increases as the amount of acetonitrile is increased. The increased line width undoubtedly arises from the exchange reaction between TCNE- and the unreduced TCNE. PassVd Ve age of current in the D M F solution of TCNE inA niolecular orbital picture would involve a delocali- creases the line width of the e.s.r. spectrum. This zation of the positive charge from the trigonal car- phenomenon has not been explained. The half-wave potential of 1,1,2,2-tetracyanobon into the two benzene rings and the two oxygens. Further studies designed to elucidate the cyclopropanes (TCNP) in D M F is - 1.4 v. Reducrequirements for the unusual charge separation are tion a t this potential gives a bright red-orange solution and bubbling is observed a t the mercury-pool in progress. cathode. The e s r . spectrum obtained is identical Acknowledgments.-The authors express appreciation to the Public Health Service, National with that found for TCNE- except that the TCNP Institutes of Health, for support of this work under solution yields a spectrum with narrower lines. A Research Grant No. CY-3753 and acknowledge comparison of the optical absorption spectrum of the help of Mr. C. G. Moscley in the synthesis of TCNE- in D M F and the solution obtained by reduction of T C N P in D M F shows that the charN-benzylsydnone and purification of acetonitrile. acteristic band (A,,, = 4320 A.) of TCNE- re(9) W. Baker and W. D. Ollis, Quarf. Revs., 11, 15 (1957). ported by Phillips, et is present in both solu(10) Two from each oxygen and two from the benu, ring. tions, and that the T C N P solution also contains DEPARTMENT OF CHEMISTRY A. G. PINKUS additional bands, Oscillopolarograms of T C N P BAYLORUNIVERSITY S. Y . MA \VACO, TEXAS H. C. CUSTARD, JR. in acetonitrile indicate that the reduction is irreversible, and they exhibit a strong oxidation wave a t RECEIVED JULY 19, 1961 approximately -0.2 v. This is the half-wave potential of T C N E in acetonitrile. Small waves ELECTRON SPIN RESONANCE STUDIES OF corresponding to other reduction products also are ELECTROLYTICALLY REDUCED observed. Finally, when TCNP in acetonitrile TETRACYANOETHYLENE DERIVATIVES' is reduced completely by electrolysis, the resulting Sir: We wish to report the investigation by electron solution gives a strong oxidation wave a t -0.2 V. (2) W. D. Phillips, J. C. Rowell and S. I. Weissman. J . Chcm. Phrs., spin resonance of the electrolytic reduction products of tetracyanoethylene, 1,1,2,2-tetracyanocy- 88,(3)626P.(IQGO) H. Rieyer. I. Bernal and 0.K.Pracnkel. t o be published. clopropane, and 1,1,2,2-tetracyanoethane. The (4) J. hl. Hirshon and G. K. Fraenkel, Reo. Sci. Insfr.. 16, 34 anion radical of tetracyanoethylene dissolved in (1955); H. L. Straws and G. IC. Fraenkel, J . Chcm. P h y s , io press. (5) D. E. Smith and W. H. Reinmuth. A n d . Chem., submitted for tetrahydrofuran was first studied by Phillips, Ropublication. and D. E. Smith, Thesis, Columbia University, New well and Weissman.2 York, 1961. @
(1) Supported in part by the U. S. Air Force through the Office of Scientific Research.
( 6 ) hlade according to the procedure given by R. M . Schrriber, G . h'. Sancen and W. W , Pritchard, J . Ore, Chetn., 16, 1440 (1960).
Sept. 20, 1961
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= 1.574 =t0.005 gauss ala = 9.541 0.01 gauss c'c
