Electron paramagnetic resonance spectra of the naphthacene

1,4-naphthoquinone (Eastman White Label) .ll. The. 5,12-dihydronaphthacene (Chemical Procurement Lab- ... Sir: In their paper, Seo, Fritsch, and Nelso...
0 downloads 9 Views 244KB Size



Experimental Section



Figure 1. Epr spectrum of the 5,12-naphthacznequinone monoanion in acetonitrile. T h e radical was generated a t - 1.2 V us. sce.

thacene in acetonitrile. However, this wave grew in magnitude when the potential sweep was carried out to a value sufficient for the reduction of the hydrocarbon to its dianion. It appears, therefore, that the quinone is being formed by a chemical reaction which follows the generation of the naphthacene dianion. Both sources are probably contributing to the presence of naphthacenequinone anion radical when one electrolyzes at -2.5 V.

The 5,12-naphthacenequinone was prepared by oxidizing naphthacene (K and K Laboratories) with peracetic acid.1° The quinone was chromatographed on neutral alumina with benzene and then recrystallized from benzene. The quinone was also prepared by condensing a,a,a’,a’-tetrabromo-o-xylene (Aldrich) with 1,4-naphthoquinone (Eastman White Label) . l l The 5,12-dihydronaphthacene (Chemical Procurement Laboratories) and the naphthacene (J. Hinton, vacuum sublimed) were used as received. The electrochemical techniques and instrumentation, as well as the purification procedures for acetonitrile and the supporting electrolyte (tetraethylammonium perchlorate), have been described.12 The supporting electrolyte concentration was 0.1 F in all experiments. The epr spectrometer was a Varian Associates V-4500 with 100-kHz field modulation and Fieldial attachment. Acknozuledgnzents. The authors wish to thank Professor R. N. Adams of The University of Kansas for his assistance. Much of the reported work was performed in his laboratory. We also wish to thank Mr. Terry A. Miller for discussions and assistance. (10) A. A. Lamola, W. G. Herkstroetter, J. C. Dalton, and G. S. Hammond, J . Chem. Phys., 42, 1715 (1965). (11) M. P. Cava, A. A. Deana, and K. Muth, J . Am. Chem. Soc.,

81, 6418 (1959). (12) E. T. Sea, R. F. Nelson, J. M. Fritsch, L. Leedy, and R. N. Adams, ibid., 88, 3498 (1966).

S. Marcoux, D. W.


Comment on “Electron Paramagnetic Resonance Spectra of the Naphthacene Trianion and the 5,12-Naphthacenequinone Anion Radicals”

Sir: In their paper, Seo, Fritsch, and Nelson’ have given a reinterpretation of a spectrum which was originally assigned by us to the trinegative ion of naphthacerx2 Since the observed spectrum neither shows any hfs lines with a characteristic group formation nor allows a definite determination of the total width due to the insufficient signal-to-noise ratio, any assignment is principally some\yhat hypothetical, but lastly, “Hgpothesen sind Xetze: nur der wird fangen, der ausn-irft” (XOVALIS). Nevertheless, we agree with the new interpretation of Seo, et al., as being the most probable one. In addition The Journal of Physical Chemistry

to their reasoning, the g factor of the radical species in question which we have measured to be gaxptl = 2.00417 further confirms their assumption, as g factors around 2.004 are typical for quinone^.^ For the particular anion radical of 5,12-naphthacenequinone,we have furthermore calculated the theoretical g factor by using the linear relationship between the g factor and the sum of the spin densities on the oxygen atoms established by B r o ~ n .His ~ method has been modified for XScLachlan type calculations using the 1\10parameters cited by Seo, et aZ.1 The result of our calculation is gtheoret = 2.00408, which is in good agreement with the (1) E. T. Sea, J. 14. Fritsch, and R. F. Nelson, J . Phys. Chem., 72, 1829 (1968). (2) K. Mobius and .M.Plato, 2. Naturfoorsch., 19a, 1240 (1964).

(3) H. W. Brown in W. Low, “Paramagnetic Resonance,” Val. 11, Academic Press, New York, N. Y., 1963, p 704.

COMI~IUXICATIONS TO THE EDITOR measured value. This high a g factor seems to be another argument against Hoijtinlds suggestion4 that the spectrum in question might be due to the 5,12dihydronaphthacene mononegative ion. We would like to present a slightly different hfs analysis of the spectrum of the 5,12-quinone anion radical which appears to be in better agreement both with the observed intensity distribution and with the splitting constants predicted by JIcLachlan’s MO method. Our proposed set of hfs constants is: a2,3 = 1.11; a6,11 = 0.74; a8,9 = al,4= 0.37; a7,lO < 0.1 Oe. These values give the following intensity ratios of the 15 lines: 1:4:8:14:22:28:33:36:33, etc. The analysis of Seo, et al., gives 1:6:19:44:81:122:155:168:155, etc., requiring a much higher increase of intensity toward the center of the spectrum than is being observed. Speciiically, if we regard the second, third, and fourth outmost lines (the intensity of the first line cannot be determined exactly because of noise) we have the experimental ratios of 1:2 :3, whereas the analysis of Seo, et al., would require about 1:3 : 7. Apart from this discrepancy the achieved signal-to-noise ratio would prohibit the observation of the outmost line in their analysis. Our proposed analysis, however, fits the observed spectrum in all details. In the course of the electrochemical reduction of naphthacene and at an improved signal-to-noise ratio, one can observe an additional epr spectrum beside that of the 5,12-quinone monoanion. This spectrum has a noticably higher g factor (2.00464) and consists of a quintet and a subsequent septet splitting, which can be assigned to four equivalent protons and two different pairs of protons with the following respective constants: a1 = 2.98; u2 = 0.75; a3 = 0.35 Oe. The higher g factor indicates that this radical is possibly a further oxidation product of the 5,12-quinone. This assumption seems to be supported by the observation that a considerable increase of the spectrum intensity is obtained on the expense of the 6,12-quinone spectrum by purging oxygen into the solution during the electrochemical reduction process. The fact that the total width (14.10 Oe) of the new spectrum is considerably larger than that of the 5,12-quinone is not necessarily a contradiction 1o our assumption, since a similar effect has been observed in the case of an oxidation product of anthraq~inone.~The specific form of the new radical cannot easily be determined on account of the many possible structures from which even protonated forms cannot be excluded. If, for instance, protonation has occurred a t the 3,14 positions of the 5,12-quinone, the observed hfs splittings and g factor could also be attributed t o the keto form of this radical anion, as in this case, neglecting hyperconjugation, the spin density is restricted to a T system identical with that of 1,4-naphthosemiquinone the epr spectrum of which is well known.6 The observed large splitting would have to be assigned to the

1831 four methylene protons on account of hyperconjugative coupling. As p4 = 0.1,6 this assumption leads to the correct order of magnitude for Qc-H,. (4) G. J. Hoijtink, 2. Phys. Chem. (Frankfurt), 45, 248 (1965). (5) U. Deffner and E. Brunner, ibid., 51, 290 (1966). (6) E. W. Stone and A. H. Maki, J . Chem. Phys., 36, 1944 (1962).



The Complete Macroscopic Characteristic of Isothermal Diffusion in Binary Systems

of Neutral Components

Sir: Fick’s law gives only one mutual diffusion coefficient D”,in the volume-fixed frame of reference, if diffusion in a binary system is investigated in the traditional way. Two intrinsic diffusion coefficients, D1 and Dz, can be introduced if the flow of the convectionfixed frame of reference is known.’J Further, two self-diffusion coefficients, D1* and Dz*, can be determined. Recently13r4since many experimental results in binary - ~ ~ relation systems are available for d i s c u ~ s i o n , ~the among these five coefficients has been widely discussed. I n many cases a clear discrepancy between the experimental values and Darken’s equation has been ob~ e r v e d . ~To , ~explain this discrepancy, the formation of association polymers was recently assumed.4 We discuss diffusion in binary neutral systems by linear nonequilibrium thermodynamic^.^^ 3 The flows can be represented in a convection-fixed frame of refer(1) J. Crank, “Mathematics of Diffusion,” Oxford University Press, Oxford, 1956. (2) L.S. Darken, Trans. A I M E , 175, 184 (1948). (3) L. E. Trimble, D. Finn, and A. Gosgara, Jr., Acta Met., 13, 501 (1965). (4) P. C. Carman, J. Phvs. Chem., 71, 2565 (1967). (5) D. K. Anderson, J. R. Hall, and A. L. Babb, ibid., 62,404 (1958). (6) R. R. Irani and A. W. Adamson, ibid., 62, 1517 (1958). (7) L. Miller and P. C. Carman, Trans. Faraday Soc., 55, 1831 (1959). (8) P. C. Carman and L. Miller, ibid., 55, 1838 (1959). (9) A. P. Hardt, D. K. Anderson, R. Rathbun, B. W. Mar, and A. L. Babb, J . Phys. Chem., 63, 2069 (1959). (10) A. L. Van Geet and A. W. Adamson, ibid., 68, 238 (1964). (11) D. W. McCall and D. C. Douglass, ibid., 71, 987 (1967). (12) S. R. de Groot and P. Mazur, “Non-equilibrium Thermodynamics,” Amsterdam, N. Y., 1962. (13) R. Haase, “Thermodynamik der Irreversiblen Processe,” Darmstadt, 1963.

Volume 72, Number 6 May 1968