NOTES
to be relieved predominantly by rotation of the nitro groups out of the ring plane. Comparison of the reduction potential of 2,4,6trinitroterphenyl with that of s-trinitrobenzene suggests that the 3- and 6-nitro groups in this molecule are also twisted to some extent. Thus, both mechanisms for the relief of steric strain are operative in 2-nitro- and 2,4,6-trinitroterphenyl.
1657
It is not possible from these rather limited data to estimate angular degrees of noncoplanarity, but the intensity of the nitroterphenyl absorption band near 330 mp seems to be decreased more dramatically by nitro group decoupling than by rotation about the C1C1!bond. This band, regarded as an intramolecular charge-transfer band, is much more intense in 2,4dinitroterphenyl than in the 2,bdinitro isomer.
NOTES
Nitro-p-terphenyls. 111. Electron Paramagnetic Resonance Spectra of the Radical Anions
by Richard L. Hansen, R. H. Young, and P. E. Toren Contribution No. 343 jrom the Central Research Laboratories, Minnesota Mining and Manufacturing Company, St. Paul, Minnesota 66119 (Received November 12, 1966)
The dual charge-transfer abilities and polarographic properties of several nitro-p-terphenyls can be explained reasonably well in terms of Huckel MO theory.'S2 A consequence of this model is that the lowest vacant MO's of these molecules should be essentially nitrobenzene levels. For example, the LVO of 2,6-dinitroterphenyl should be very similar to the LVO of m-dinitrobenzene with low electron density in the biphenyl portion of the molecule. The lowest vacant molecular orbitals of aromatic nitro compounds have been extensively mapped by epr spectroscopy using the electrochemical anionradical generation technique pioneered by Geske and Makia3 We have used this technique to test the Huckel model. 'We also hoped to learn more about steric strain in these molecules.
Experimental Section The materials which were used have been described.lJ The anion radicals were generated by electrochemical reduction at potentials corresponding to the first polarographic waves of the nitroterphenyls.2 The
solutions in DMF were 1-5 mM in the nitro compound and contained 0.1 M tetrabutylammonium perchlorate. The reductions were conducted in a vessel similar to that described by Rieger and Fraenkel' placed in the dual cavity of a Varian Model 4205 spectrometer. The reference contained peroxylamine disulfonate.
Results and Discussion The epr spectra of seven nitro-p-terphenyls were analyzed in terms of NI4 and proton hyperfine coupling constants. The results are summarized in Table I. The assignments have been made largely on the basis of analogies reported in the literature. The hypefine coupling constants of the mononitro compounds are similar to those reported for nitrobenzene16although the nitrogen coupling constants are smaller. The effect on the N14 coupling constant of rotation about the C-N bond is well documented both experimentally and noncoplanarity causes an increase in a N . The nitrogen coupling constants of the three mononitro terphenyls do not follow the behavior expected on this basis, but vary in a manner (1) Part I: R. L. Hansen, J . Phys. Chem., 70, 1646 (1966). (2) Part 11: R.L. Hansen, P. E. Toren, and R. H. Young, ibid., 70, 1653 (1966). (3) D. H. Geske and A. H. Maki, J. Am. Chem. SOC., 8 2 , 2671 (1960). (4) P. H.Rieger and G . K. Fraenkel, J . Chem.Phys., 39, 609 (1963). (5) P.Ludwig, T.Layloff, and R. H. Adams, J . Am. Chem. SOC.,86, 4568 (1964). ( 6 ) D. H.Geske, J. L. Rrtgle, M. A. Bambenek, and A. L. Balch, ibid., 86, 987 (1964). ( 7 ) D.H.Geske and J. L. Ragle, ibid., 83, 3632 (1961). (8) P. H.Rieger and G . K. Fraenkel, J. Chem. Phys., 39, 609 (1963).
Volume 70, Number 6 May 1968
1658
NOTES
Table I : Coupling Constants of Nitro-pterphenyl Anion Radicals in DMF Compd
Position
bNlr gausa
Iaal, gauss 1
ZNitroterphenyl
2 3 476 5
8.82 f 0.05
3 2,4 5 6
9.27 =k 0.05
4 2,6 3,5 2’,6’
8.3 10.1
2,4Dinitroterphenyl
2 4
3.3 f 0 . 2 6.6 1 0 . 4
2,6Dinitroterphenyl
28 395 4
3.94 f 0.05
4,4”-Dinitroterphenyl
4,411 3,5,3”,5”
3.6 f 0 . 2
2,4,6Trinitroterphenyl
2,6 4 375 2’,6’ 2/’,6”
Zero 5.3 A0.l
3-Nitro terphenyl
CNitroterphenyl’
a
and
3.0 f 0 . 1 1.1 *O.l 4.2 10.1 3.18 1 0.05 1.07 f 0.05 4.05 f 0.07 0.94 A 0.06 3.1 1 0 . 1 1.07 i 0.05
t-
108.
I
Figure 1. The epr spectrum of 2,4,6-trinitroterphenyl anion radical. 3.94 f 0.05 0.96 f 0.05
1.8 f O . l
3.1 f0.07 0.64 f 0.08 0.32 f 0 . 0 4
Huckel calculations give electron densities of 0.007 for CZ c 6 and 0.018 for c2tand Cef in a planar model.’
more suggestive of the ability of the nitro group to interact with the biphenyl ring system. It has been reported that electron-withdrawing substituents para to the nitro group decrease a ~ The . ~ proton coupling indicates that the LVO of 4-nitroterphenyl does extend into the biphenyl ring system. The small nitrogen coupling in the 2- and 3-nitro isomers compared to nitrobenzene suggests a certain amount of delocalization in these cases as well, although splittings attributable to biphenyl protons were not detected. Of the three dinitroterphenyls only 2,6-dinitroterphenyl gave a well resolved epr spectrum. In the other cases, the coupling constants which could be determined were assigned on the basis of the expected steric effect, and by analogy to 4,4’-dinitrobiphenyl.* I n the case of 2,6-dinitroterphenyl the coupling constants are just slightly smaller than those reported for 2,6-dinitrotoluene,* and UN is identical with that of m-dinitrobenzene.8 The oxidation and reduction potentials of this material suggested that steric strain was relieved predominantly by rotation of the nitro The Journal of Physical Chemistry
9 = 2.0016
groups out of the ring plane.* The NI4 coupling constant does not reveal decoupling of the nitro groups from the n-electron system. It is possible that the conformation of the anion is different than that of the neutral molecule. There is no evidence that the lowest vacant MO extends into the biphenyl ring system. The epr spectrum of the 2,4,6-trinitroterphenyl anion radical is shown in Figure 1. The reconstruction is based on the coupling constants in Table I. It is significant that the 2- and 6-nitro groups are not revealed in the spectrum. Bernal and Fraenkel have also observed less than the expected number of nitro groups in the epr spectra of electrolytically generated nitromesitylene and nitrodurene anion radicals in DMF.l0 However, their electrolyses were conducted at potentials corresponding to higher polarographic waves and evidently produced amino derivatives.“ Glarum and Marshall have studied a series of polynitroaromatic anion radicals generated electrolytically and report coupling constants for all of the N14 nuclei expected,12 although the alternating line width phenomenon13was observed in some cases. The alternating line width phenomenon cannot be ruled out in the present case although spectra obtained at several modulation amplitudes did not yield any new ~
(9) A. H. Maki and D. H. Geske, J. Am. Chem. Soc., 83, 1852 (1961). (10) I. Bernal and G. K. Fraenkel, ibid., 86, 1671 (1964). (11)This was pointed out by the referee. See R. D. Allendoerfer and P. H. Rieger, Abstracts, 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965. (12) S. € I . Glarum and J. H. Marshall, J. Chem. Phya., 41, 2182 (1964). (13) J. H. Freed and G. K. Fraenkel, ibid., 41, 699 (1964).
NOTES
lines. It may be significant that even though the polarographic reduction of 2,4,6-trinitroterphenyl was a one-electron reversible electrochemical process, cyclic voltammetry indicated chemical irreversibility.2 If the spectrum we have obtained is that of the 2,4,6-trinitroterphenyl anion radical, it is evident that the lowest vacant MO is delocalized over the entire molecule, since proton splittings from all three benzene rings were observed. We had also hoped to be able to report the nitroterphenyl cation-radical spectra. However, we have been unable to obtain spectra of these species in sulfuric acid or by electrochemical oxidation in acetonitrile or DMF.
Acknowledgment. We wish to thank Mr. George Filipovich for the use of the epr equipment.
Radiation Chemistry of Aqueous Solution of Silver Ion'
by Gideon Czapskiz and A. 0. Allen Chemistry Department, Brookhaven National Laboratory, Upton, (Received October 16, 1966)
New York 11979
Little work has appeared on the radiation chemistry of solutions of silver salts. Shchegoleva, et u L . , ~ have reported a few experiments on the effect of alcohols on the yields of reduction. The reduction of silver ion by hydrogen atoms was demonstrated in 1928 by Bonhoeffer and Harteck4and more recently by Littman, Carr, and Bradyj and by Czapski and Steins6 The rate constant for the reaction of solvated electron with silver ion has recently been determined.',* The present results show that the chemistry is more complicated than might be anticipated from looking at a table of rate constants. Our attention was recently , ~ ircalled to a paper by Ryabchikova, et ~ l . who radiated relatively high concentrations of Ag+ (>0.1 AI), tvith results in qualitative agreement with ours.
Experimental Section The irradiations were made in two Co60y-ray sources having dose rates of 5800 and 250 radslmin. The Pyrex irradiation cells were steam-cleaned and preirradiated. The water was triply distilled. The silver salts were of analytical grade and were used without further purification. Irradiated solutions contained a brown precipitate which was removed on a fine sintered glass filter and
1659
washed several times with distilled water. When the precipitate was washed with NH40H or 1 N HClO4 solution, no silver was present in the washing, showing that there was no AgzOor hyponitrite in the precipitate, which was therefore assumed to consist entirely of metallic silver. The precipitate was dissolved in 30% H N 0 3 containing a small amount of NaNOZ to catalyze the dissolution of metallic silver, and the resulting solution was titrated for silver using 0.05 N KCNS. The limit of detection was 0.5 pmole. Nitrite ion was determined by the method of Shin.'" Hydrogen was determined by the method used by Schwarz, Losee, and Allen." Attempts were made to determine peroxide both by the iodide method and by measuring the amount of oxygen produced on the addition of ceric sulfate to the solution. The results were not well reproducible and merely served to indicate roughly the amount of peroxide present. It is possible that the metallic silver and peroxide interact in a somewhat irreproducible manner.
Results Results of the irradiation of silver perchlorate solution are shown in Table I. The yields of metallic silver were small at low concentration but increased with increasing concentration. The presence of air resulted in a reduction of the yield as compared to airfree solutions, as did also the addition of hydrogen peroxide to the air-free solution; G(Ago) in 0.01 M Ag+ fell from 0.22 to 0.05, then to zero, as (Hz02)was increased from zero to 1 mM, then to 6 mM. The reduction in yield of hydrogen with increasing concentration of silver ion is expected12from the high rate of reaction of silver ions with solvated electrons. (1) Research performed under the auspices of the C. S. Atomic Energy Commission. (2) Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel. (3) I. S. Shchegoleva, -4.V. Egunov, T . S. Glikman, and 1'. Ya Dain, Dokl. Akad. ,Vauk S S S R , 148, 633 (1963). (4) K.F.Bonhoeffer and P. 2. Harteck, 2.Physik. Chem., A139, 64 (1928). (5) F. E. Littman, E. M. Cam, and A. P. Bradq-, Radiation Res., 7, 107 (1957). (6) G. Czapski and G. Stein, Israel J . Chem., 2, 15 (1964). (7) S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, Discussions Faraday SOC.,36, 193 (1963). (8) J. H.Baxendale, et al., S a t u r e , 201, 468 (1964). (9) G. G. Ryabchikova, V. I. Duzhenkov, and 1'. Ya. Glazunov, T r . Tashkentsk. Konf. PO Mirnomu Ispol'z. A t . Energii, Akad. S a u k U z . SSR, 1, 361 (1961); Chem. Abstr., 57, 1787f (1962). (10) M.B. Shin, I d . Eng. Chem. Anal. Ed., 13, 33 (1941). (11) H.A. Schwarz, J. P. Losee, and A. 0. Allen, J . Am. Chem. SOC., 76, 4693 (1954). (12) H.A. Schwarz, Radiation Res. suppi., NO.4,io3 (1964).
Volume '70,-+-umber 5 M a y 1966
