1829
XOTES Electron Paramagnetic Resonance Spectra of the Naphthacene Trianion and the
Table I : Proton Hyperfine Coupling Constants for the 5,12-Naphthacenequinone Monoanion“
5,12-NaphthacenequinoneAnion Radicals
Positions
21 3 6, 11 81 9 11 4 7 , 10
by Eddie T. Sleo, Physical Research Center, TRW Systems Group, TRW Inc., Redondo Beach, California 90278
John 11, Fritsoh, Central Research Department, Monsanto Company, St. Louis, Missouri 63241
and Robert F. Nelson1 Department of Chemistry, Sacramento State College, Sacramento, California 96819 (Received September 14, 1967)
Mobius and Plato have reported the electron paramagnetic resonance spectrum of a species obtained by electrochemical reduction of naphthacene in acetonitrile a t - 2 . 5 V us. me.2 They have assigned this spectrum to the trianion radical of naphthacene. We, and other workers, 3 have observed that this spectrum is obtained with unusual ease, especially in solutions that have not been carefully deoxygenated. This observation and previous experience with the electrochemical reduction of aromatic hydrocarbons led us to suspect that the naphthacene trianion radical was actually the anion radical of 5,1%naphthacenequinone. We have electrochemically generated both the hydrocarbon trianion and quinone anion radicals and recorded their respective spectra. The epr spec’trum obtained from naphthacene under the conditions described by Mobius and Plato and that obtained from 5,12-naphthacenequinone in acetonitrile appear to be identical (Figure 1 and ref 2). We repeated the experiment of Mobius and Plato involving the in situ electrolysis of naphthacene in not too carefully deaerated acetonitrile at -2.5 V us. see. The spectrum obtained was somewhat broadened, but obviously identical with that of Mobius and Plato and with that of the quinone. The quinone spectrum shown in Figure 1 was generated a t -1.2 V and the same spectrum ]persists at -2.5 V because the electrochemical formation of the dianion is also reversible (an appreciable radical concentration is maintained by the dianion-parent radical equilibrium). I n Figure 1, the total width of the naphthacenequinone anion spectrum is about 5.2 G with line widths of about 0.17 G. The corresponding values found by Mobius and Plato are 5.3 and 0.195 G. The spectrum can be interpreted, although not essential to the present argument, in terms of the naphthacenequinone anion. The coupling constants are listed in Table I; splittings for the various positions are tentatively assigned with the aid of McLachPan calculation^.^ The lack of any additional outside hyperfine lines and the approximate
--------Coupling Exptl
constants, G--
0.71 rt 0.03 0.71 i 0.03 0 . 3 7 =t0.02 0 . 3 7 0.02 0 . 3 7 rt 0.02
Calcd
1.06
0.80 0.30 0.23 0.04
Calculated constants were obtained through the RIcLachlan ~ ~ Karplus ~ and G. K. Fraenkel, procedure using a H = 2 3 . 7 (RI. J. Chem. Phys., 35, 1312 (1960)), h = 1.2, a0 = a c 1.26Pc0, and pc-0 = 1.55poc (J. Gendell, J. H. Freed, and G. K. Fraenkel, ibid., 37, 2832 (1962)). a
+
equality of the two large calculated couplings make any other interpretation unlikely. Protonation of naphthacene dianion in solution to form S112-dihydronaphthacene has been proposed recently.5-7 Hoijtink has suggested that the radical obtained by Mobius and Plato may be the monoanion radical of dihydronaphthacene.8 In fact, cyclic voltammetry experiments show that naphthacene first undergoes a reversible one-electron reduction step, followed by a second one-electron reduction step which exhibits no reverse anodic peak, even at a scan rate of 1000 V/min.9 The electrochemical reduction of 5,12dihydronaphthacene mas, therefore, investigated. The dihydronaphthacene was reducible at -2.2 V us. sce, but in situ electrolysis in the epr spectrometer cavity at -2.5 V us. sce produced no radical species. Therefore, the possibility that the spectrum obtained by Mobius and Plato was due to the 5,12-dihydronaphthacene anion radical seems remote. The origin of the quinone could be from either or both of two sources. Commercial samples of naphthacene were found to contain small quantities of naphthacenequinone which was probably formed by air oxidation of the hydrocarbon as noted previously.1° In fact, one can obtain an appreciable signal due to the quinone anion radical by electrolyzing a sample of the hydrocarbon at about -0.9 V. This potential is sufficient to reduce the quinone but not the hydrocarbon. If one then steps the potential to - 1.2 V, the relatively weak quinone signal is swamped out by that of the naphthacene anion radical. The quinone reduction could barely be discerned on a cyclic polarogram of naph(1) To whom correspondence should be directed. (2) K. Mobius and M. Plato, 2.Araturforsch., 19a, 1240 (1964). (3) W. C. Landgraf, T’arian Associates, private communication. (4) A. D. McLachlan, Mol. Phys., 3, 233 (1960). (5) N. H. Velthorst and G. J. Hoijtink, J . Am. Chem. SOC.,87, 4529 (1965). (6) N. H. Velthorst and G. J. Hoijtink, ibid., 89, 209 (1967). (7) I. Bergman, “Polarography 1964,” Vol. 11, G . J. Hills, Ed., Interscience Publishers, Inc., New York, N . Y . , 1966, p 925. (8) G. J. Hoijtink, 2. Physik. Chem. (Frankfurt), 45, 248 (1965). (9) R. F. Nelson, unpublished data.
Volume 72, Number 6
M a y 1968
1830
COM~~IUNICATIOPJS TO THE EDITOR 4
Experimental Section
1
I
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 . A m . 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.
C O M M U N I C A T I O N S TO T H E EDITOR
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.
