ESRSPECTRA OF ZWITTERIONRADICALS
2479
are typical of aqueous binaries.6 It also differs from the acetone-triacetin binary (Table VI) in which a steady exothermic trend is found towards triacetin, the superior solvating medium. This superiority of 2.27 kcal/mol (16.18 - 13.91) is worth comparing with that of ethyl acetate (14.57 - 13.91 = 0.66 kcal/mol) over acetone. If triacetin were behaving as three ethyl acetate residues bound together, we might expect it to be superior to acetone by about 2.0 kcal/mol. The fact that triacetin exceeds this by about 0.3 kcal/mol might be due to the fact that the acetate residues are vicinal to each other on the three-carbon backbone of the molecule, thus permitting a chelation effect such as is found in the binding of alkali metal ions by other polybasic ligands.22 The results in Table VI1 show that on a mole fraction basis glucose pentaacetate (a possible model for the fundamental molecular unit of completely acetylated cellulose) is comparable to triacetin in its ability to solvate sodium ions. Table VI11 shows that in dilute acetone solution cellulose acetate is superior to diacetin, triacetin, and glucose pentaacetate in its interaction with sodium tetraphenylboron. Since cellulose acetate
is partially hydroxylated and also carries ether linkages between its glucose residues, these groups may be contributing exothermally to the solvation of sodium ions. Magnesium perchlorate is used as an additive in casting cellulose acetate reverse osmosis membranes. K e ~ t i n has g ~ ~proposed that cations of high charge such as Mg2+form complexes with the hydroxyl and acetate groups of cellulose acetate and contribute to the swelling of the membrane by subsequent hydration after immersion in water. The results in Tables VI1 and VI11 show clearly that Mg(C10& interacts strongly with cellulose acetate in acetone and that cellulose acetate is far superior to glucose pentaacetate on this score. It remains to be seen how much this difference is due to conformational factors and how much is due to the specific superiority of hydroxyl and ether functions in solvating cations. In any event, Kesting's postulation of complexing between this salt and cellulose acetate is clearly supported. (22) J. J. Christensen, J. 0. Hill, and R.M. Izatt, Science, 174, 459 (1971). (23) R.E. Kesting, J . A p p l . Polym. Sci., 9, 663 (1965).
Electron Spin Resonance Spectra of Zwitterion Radicals and Isoelectronic Anion Radicals1 by Ronald P. Mason" Department of Chemistry, Cornell University, Ithaca, Neur York 14860
and John E. Harriman Department of Chemistry, University of Wisconsin, Madison, Wisconsin 65706 (Received January 12, 1972) Publication costs borne completely b y The Journal of Physical Chemistry
Electron spin resonance spectra have been obtained for the p-nitrobenzyltrimethylammonium zwitterion radical and the 2,2-dimethyl-l-p-nitrophenylpropaneanion radical in acetonitrile at -40" and for the p nitrophenyltrimethylammonium switterion radical and the p-nitro-tert-butylbenzene anion radical in dimethyl sulfoxide at room .temperature. All radicals were generated electrolytically. Hyperfine coupling constants are reported and discussed in terms of probable radical structures. I n a recent note, Knolle and Bolton2 have discussed differences in the fluorine hyperfine splitting in two isoelectronic and essentially isostructural radicals, one a neutral nitroxide and the other a ketyl. We report here an investigation of the Droton hvDerfine CouDIing constants of two pairs of isoelectronic para-substituted nitrobenzene radicals. One member of each pair is an
-
" I
I
V
anion radical with an alkyl substituent while the other is a zwitterion radical containing a trimethylammonium substituent. The radical pairs are -0&CsH&H&(1) This research was supported by the National Science Foundation under Grants No. GP-7512 and GP-9539. (2) W. R. Knolle and J. R. Bolton, J. Amer. Chem. Soc., 91, 5411 (1969).
The Journal of Physical Chemistry, Vol. 76, No. 17, 1972
2480 (CH3)B (I) and -0&CeH&H2IS+(CH3)3 (11), - 0 2 T;jC6H4C(CH3)3(111) and -02fiC6H4N+(CH3)3(IV). Experimental Section The esr spectra of the p-nitrobenzyltrimethylammonium zwitterion radical and the 2,2-dimethyl-l-pnitrophenylpropane anion radical have been obtained at -40" with acetonitrile (ACN) as the solvent. The zwitterion radical was generated by intra UTO OS^ electrolysis at -40" of a 1 mM solution of p-nitrobenzyltrimethylammonium chloride. This radical was unstable at higher temperatures. The anion radical was generated by essentially complete electrolytic bulk reduction, in an external cell, of a 1 m M solution of the parent compound. The esr spectra of the p-nitrophenyltrimethylammonium zwitterion radical and the p-nitro-teTt-butylbenzene anion radical were obtained at room temperature in dimethyl sulfoxide (DMSO). These radicals were prepared by external, electrolytic bulk reduction of 0.5 mM solutions of p-nitrophenyltrimethylammonium perchlorate and p-nitro-tert-butylbenzene, respectively. The spectrum of p-nitro-tert-butylbenzene anion has been reported p r e v i ~ u s l y . ~ I n all cases 0.1 M tetra-n-propylammonium perchlorate served as supporting electrolyte. Solvents and supporting electrolyte were prepared by standard techniques. All spectra were obtained using a Varian Model E-15 X-band spectrometer equipped with a 40cm magnet and a E 80-A recorder. fieparation of Compounds. p-Nitrobenzyltrimethylammonium chloride was obtained from Chemicals Procurement Laboratories and used without further purification. p-Nitrophenyltrimethylammonium perchlorate was prepared by the method of Zaki and Fahim.6 The precursor, N,N-dimethyl-p-nitroaniline, was obtained from Eastman Organic Chemicals. p Xtro-tert-butylbenzene and 2,2-dimethyl-l-p-nitrophenylpropane were obtained by nitration of the alkylbenzeness which were obtained from Chemical Samples Co. The highest boiling isomer, the para isomer, was isolated and purified by gas chromatography. A 10-ft, 25% Carbowax column at 200" cleanly separated the three isomers.
Results and Discussion The spectra were analyzed to obtain hyperfine coupling constants which are summarized in Table I. The splitting due to the nine methyl protons, which is only barely resolved in the spectrum of p-nitro-tert-butylbenzene, is clearly resolved in the isoelectronic zwitterion. Computer simulation of the first few lines confirmed that the splitting was due to nine equivalent protons. This splitting was not resolved if acetonitrile was used as solvent. The members of each of these zwitterion-anion radical pairs differ only in the replacement of PI'+ by C. The Journal of Physical Chemistry, Vol. 76, No. 17,1971
RONALD P. MASONAND JOHN E. HARRIMAN It is not surprising that the charge difference changes the hyperfine coupling constants, since it will certainly alter the electronic charge distribution, and thus the spin distribution, in one relative to the other. The changes in the ring proton and nitro nitrogen constants are relatively small and appear to fit the pattern for para-substituted nitrobenzene first investigated by Maki and Geske7 and subsequently extended by many workers. The other coupling constants will be affected by geometrical or configurational considerations, as well as by changes of spin density distribution within the ring. It is convenient to introduce a coordinate system in terms of which to discuss these effects. The nitro nitrogen, atoms 1 and 4 of the ring, and the first carbon or nitrogen of the substituent are taken (in their equilibrium position) to define the z axis, and the plane of the ring is taken to be the yx plane. It is well known that, for protons like the methylene protons of the first pair of radicals, the proportionality constant relating the proton hyperfine coupling constant to the ring carbon 7r electron spin density is angle dependent.8 To a good approximation upH =
pc"(B0
+B
COS'
6)
xhere 0 is the angle between the H-C-C,,,, plane and the xz plane. The spin density distributions, as reflected by the hyperfine splittings of the nitrogen and ring protons, are nearly identical for all para alkyl nitrobenzene anion radicals in It has also been shown that for these systems the constant BOis essentially zero while pczBis about 8 G.g The observed value of 2.02 G for the methylene proton coupling constant in I1 is thus not consistent with essentially free rotation of the substituent', leading to (cos26) = I/*. It is consistent with a dominant configuration with the central carbon of the substituent in the zz plane, having cos2 0 = 0.25. Small torsional motions are not excluded, so long as their amplitude is not sufficient to appreciably alter (cos2 6). Of course transitions between two equivalent configurations, with the protons above or below the plane, are allowed and no conclusions regarding rate of conversion are possible so long as negligible time is spent in the intermediate region. Similar results have been obtained by Carrington and Todd for alkyl-substituted cyclooctatetraene anion radicals.1° (3) D. H. Geske and A. H. Maki, J . Amer. Chem. SOC.,82, 2671 (1960) (4) P. B. Ayscough, F. P. Sargent, and R. Wilson, J . Chem. Soc., 5418 (1963). (5) A. Zaki and H. Fahim, ibid., 270 (1942). (6) K. L. Nelson and H. C. Brown, J . Amer. Chem. Sac., 73, 5605 (1951). (7) A. H. Maki and D. H. Geske, ibid., 83, 1852 (1961). (8) D. H. Geske, Progr. Phys. Org. Chem., 4, 125 (1967). (9) E. G. Janeen and J. L. Gerlock, J . Org. Chem., 32, 820 (1967). (10) A. Carrington and P. F. Todd, Mol. Phys., 8 , 299 (1964).
COMMUNICATIONS TO THE EDITOR
2481
Table I : Observed Hyperfine Coupling Constants’ Radical (solvent, temp)
p-Nitrobenz yltrimethylammonium zwitterion (I) (ACN, -40’)
9.13
3.26
0.97
2.73
1.52
C
2,2-Dimethyl-l-p-nitrophenylpropane (11)(ACN, -40’) p-Nitrophenyltrimethylammonium zwitterion (111)(DMSO, room
10.72
3.36
1.09
...
2.02
c
8.33
3.30
0.85
0.78
...
0.126d
10.13
3.28
1.06
...
...
0. 060d
temp )
p-Nitro-tert-butylbenzene anion (IV) (DMSO, room temp)
a Values are in gauss, with a n uncertainty of about =!=0.01 unless otherwise noted. Ortho and meta t o the nitro group; Hb refers to t h e methylene protons. Assignment of these sets of 2 equivalent protons each is b y analogy t o other substituted nitrobenzenes and Uncertainty -0.005 G. b y comparison with calculated results. c N o t resolved.
I t is most reasonable to assume that I has the same geometry as 11. The smaller value of 1.54 G for the methylene proton coupling constant indicates a smaller value of p,“B, but present data do not allow us to distribute the change between par and B. The greatest relative difference observed for corresponding splittings between members of an isoelectronic pair occur for the nine methyl protons in I1 and IV. The value of 0.78 for / a N ~ + ~implies e a l p,“ is about 0.19 in 111 if Q N C N + H ~ equals QNCN*Mea, since laNN+Hal is4.1 in .CH-JJ+Ha.11*12 Calculationsla and experimental coupling constants14 suggest that para is 0.16 in nitrobenzene. This spin density is probably very similar in IV, and is thus
smaller than in 111. This would suggest that, if the methyl proton coupling constants are proportional t o pa“, the constant of proportionality is rather different for the C and K+species. These comparisons of coupling constants in isoelectronic radicals will provide an interesting test of the semiempirical calculation of spin density distributions.
(11) R. P. Kohin and P. G. Nadeau, J . Chem. Phys., 44, 691 (1966). (12) P. E. Ayscough, “Electron Spin Resonance in Chemistry,” Methuen, London, 1967. (13) J. A. Pople, D. L. Beveridge, and P. A. Dobosh, J . Amer. Chem. Soc., 90, 4201 (1968). (14) J. E. Harriman and A. H. Maki, J . Chem. Phys., 39,778 (1963).
C O M M U N I C A T I O N S T O THE E D I T O R Raman Scattering in Sodium-Liquid Ammonia Solutions1 Publication costs assisted by Texas Christian University
Sir: The nature of the species present in a dilute solution of an alkali metal in liquid ammonia continues to be a matter of speculation. Numerous models have been proposed,2 but as yet no single theory is clearly superior. According to Jortner, et C Z ~ . , very ~ dilute solutions contain unassociated, solvated cations and electrons. The electron is described as existing in a “cavity” created by preferentially oriented solvent molecules. Calculations on the ground-state energy
lead to a prediction of a totally symmetric vibration between 25 and 60 cm-l. According to R u s c ~this ,~ vibration may lie between 400 and 700 cm-l. Rusch5 has reported that the vibrational spectrum of ammonia is perturbed by the addition of alkali metal. Specifi(1) This work was supported by The Robert A. Welch Foundation. Gratitude is expressed to the University of Dallas for the loan of the monochromator. (2) “Metal-Ammonia Solutions,” J. J. Lagowski and M. J. Sienko, Ed., Butterworths, London, 1970. (3) D. A. Copeland, N. R. Kestner, and J. Jortner, J. Chem. Phys., 53, 1189 (1970). (4) P. F. Rusch, Universite Catholique de Lille, France, private
communication. (5) P. F. Rusch, Ph.D. Dissertation, The University of Texas a t Austin, 1971. The Journal of Physical Chemistry, Vol. 76, No. 17, 1972
