215
SPECTROSCOPY OF 1-METHYLPHENALENYL AND 1-PHENYLPHENALENYL RADICALS
Electron Spin Resonance Spectroscopy of the 1-Methylphenalenyl and the 1-Phenylphenalenyl Radicals by I. C. Lewis and L. S . Singer Union Carbide Corporation., Carbon Products Division, Parma Technical Center, Parma, Ohw 44130 (Received August 6,1968)
Electron spin resonance measurements have been made of the stable free radicals 1-methylphenalenyl and 1-phenylphenalenyl in solution. Analyses of the proton hyperfine structure indicate that the unpaired spin densities at the unsubstituted positions are only slightly affected by the presence of a methyl or phenyl group. The observed large methyl proton splittings prove the existence of a significant hyperconjugation effect. The small phenyl-ring splittings in 1-phenylphenalenylare consistent with a nonplanar conformation of the radical.
Introduction The phenalenyl (perinaphthenyl) radical (I) which was first observed in 1957 by Sogo, Nakazaki, and Calvin1 is one of the best known examples of a stable neutral hydrocarbon radical. Phenalenyl can be formed at room temperature by t.he dissociation of hydrogen from the hydrocarbon perinaphthene and has been ob-
firm this conformational effect by determining the unpaired spin distribution in the radical. I n this paper, the isotropic esr hyperfine spectra for the 1-methylphenalenyl and 1-phenylphenalenyl radicals are presented. The proton hyperfine coupling constants for t.hese radicals have been determined and compared with those for phenalenyl. The chemical implications of the coupling constants are discussed.
Experimental Section 7
6
I
served in petroleum d i s b i l l a t e ~and ~ ~ in ~ the pyrolysates of aromatic hydrocarbon^.^ There have been a number of recent esr studies of phenalenyl in both ordinary510 and liquid crystal solvent^.^^^ The magnitudes and signs of the spin densities for I have been of general theoretical interest. O J O No substituted phenalenyl radicals have been prepared by dissociation reactions. Weiss, et aZ.,ll have identified a hydroxylphenalenyl radical formed during the photolysis of perinaphthenones. We have now prepared the 1-methyl and 1-phenyl derivatives of I by simple dissociation reactions and have measured their esr spectra in solution. The methyl and phenyl groups are of particular interes t as substituents in phenalenyl. The magnitude of the hyperfine interaction for the protons of methyl groups in aromatic ions has been taken as direct evidence for hyperconjugation.12l 3 Large hyperfine splittings have also been observed for the methyl protons in substituted neutral triphenylmethyl radi~a1s.l~The magnitude of the hyperfine interaction of the methyl pro tons in the methylphenalenyl radical would provide a significant test of this hyperconjugation effect. I n the case of 1-phenylphenalenyl, molecular models show that the phenyl group cannot be coplanar with the phenalenyl ring system. It should be possible to con-
Preparation of Radicals. ( I ) Phenalenyl. The phenalenyl radical was obtained directly from perinaphthene in solution a t room ternperature.l The perinaphthene was prepared by the reduction of perinaphthenone tosylhydrazone with NaBH4.l5 This method proved to be more effective than the direct reduction of perinaphthenone with LiA1H4.l 6 (a) 1-Methylphenalenyl. The 1-methylphenalenyl radical was obtained from 1-methylperinaphthene in solution at room temperature, The l-methylperi(1) P. B. Sogo, M. Nakazaki, and M. Calvin, J . Chem. Phys., 26, 1343 (1957). (2) J. E. Bennett, Proc. Chem. SOC.,144 (1961). (3) F. C. Stehling and K. W. Bartz, J. Chem. Phys., 34, 1076 (1961). (4) L. S. Singer and I. C. Lewis, Carbon, 2, 115 (1964). (5) F. Gerson, Helv. Chim. Acta, 49, 1463 (1966). (6) B. G. Segal, M. Kaplan, and G. K. Fraenkel, J . Chem. Phys., 43, 4191 (1965). (7) 8. H. Glarum and J. H. Marshall, ibid., 44, 2884 (1966). (8) H. R. Falle and G. R. Luckhurst, Mol. Phys., 11, 299 (1966). (9) R. Lefebvre, H. H. Dearman, and H. M. McConnell, J . Chem. Phys., 32, 176 (1960). (10) L. C. Snyder and T. Amos, J. Chem. Phys., 42, 3670 (1965). (11) G. P. Rabold, K. H. Bar-Eli, E. Reid, and K. Weiss, ibid., 42, 2438 (1965). (12) J. R. Bolton, A. Carrington, and A. D. MoLachlan, Mol. Phys., 5, 31 (1962). (13) E. deBoer and 5. P. Colpa, J. Phys. Chem., 71, 21 (1967). (14) H. Judeikis and D. Kivelson, J. Amer. Chem. SOC.,84, 1132 (1962). (15) L. Caglioti and P. Grasselli, Chem. Ind., (London), 153 (1964j. (16) V. Boekelheide and C. E. Larrabee, J. Amer. Chem. Soc., 72, 1245 (1950). Volume 73,Number 1 January 1069
216
I. C. LEWISAND L. S. SINGER
Table I : Experimental Proton Coupling Constants for Phenalenyl Radical (I) Measuring
Solvent
temp,
CClr CCl,
'C
25 25 25 20 25 25 102 145 150
DME4 DMEa CClr Petroleum dkillate p-Asoxyankole p-Azoxyanisole mQuinquepheny1
ai, G
6.32 f 0.01 6.306 f 0.002 6.333f0.002 6.336 6.29 i 0.04 6 . 3 =k 0.1. 6.280 6.270 6.27
azl
R of
G
This work
l . 8 1 f 0.01 1.821 & 0.003 1.823 =!= 0.002 1.826 1.805 & 0.015 1.82 f 0.05 1.822 1.833 1.81
6 6 7
5 2
8 7 4
'DME, dimethoxyethane. naphthene was produced from the Grignard reaction of perinaphthenone with CH3MgBr.17J* (3) 1-Phenylphenalenyl and 1-Phenyl-dj-phenalenyl. The 1-phenylphenalenyl radical was formed from 1phenylperinaphthene in solution a t room temperature. The 1-phenylperinaphthene was prepared from the Grignard reaction of perinaphthenone with C6H6MgBr. The deuterated radical, 1-phenyl-db-phenalenyl, was prepared from perinaphthenone with CsDjMgBr (Merck Sharp and Dohme). Esr Measurements. The esr spectrum for phenalenyl was obtained for a degassed solution of radical in CCL a t room temperature. The spectrum for l-rnethylM soluphenalenyl was measured for a degassed, tion of the radical in dirnethoxyethane a t -50'. These conditions have been used by Segal, Kaplan, and Fraenkel in obtaining excellent resolution of the phenalenyl radical spectrum.6 The esr spectra for 1-phenylphenalenyl and l-phenyl-d6-phenalenyI were measured on degassed M solutions of the radical in COHO a t 25". The spectra were measured with an X-band esr spectrometer employing superheterodyne detection a t a microwave power of 0.1 mW. Low-temperature measurements were performed with the Varian variabletemperature cavity.
Results Phenalenyl Radical ( I ) . The esr spectrum of phenalenyl shows a large hyperfine interaction from the six
'
4
GAUSS
'
Figure 1. Comparison of the experimental curve (a) and the computed ourve (b) for the I-methylphenalenyl free radical. Computed values: 1 H, 6.48 G; 3 H, 6.27 G; 4 H, 6.05 G; 3 H, 1.77 G. The Journal of Physical Chemistry
equivalent protons at the active positions and a small splitting by the three equivalent protons at the inactive positions.lg The coupling constants which we have determined for phenalenyl (I) in CC1, at 25" are: 6 H, 6.32 =k 0.01 G; and 3 H, 1.81 f 0.01 G. These values are in good agreement with those obtained in other recent investigations. Table I lists the recent proton coupling constant data that have been reported for I. The magnitudes of the hyperfine splittings show a slight dependence on solvent and temperature. I-Methylphenalen yl Radical (11) e
H3 4 5
11
The 1-methylphenalenyl radical (11) is observed immediately after the dissolution of the parent hydrocarbon 1-mebhylperinaphthene a t room temperature. The extreme ease with which the hydrogen dissociation from methylperinaphthene to form the radical I1 takes place is indicative of the high stability of radicals incorporating the phenalenyl ring system. Since the methyl group in I1 is substituted at one of the active starred positions, one would expect an appreciable hyperfine interaction by the three methyl protons. I n addition, large hyperfine splittings are expected for the five protons a t the remaining active ring sites. Figure l a shows the esr spectrum obtained for the 1-methylphenalenyl radical formed in a dirnethoxyM in methylperiethane solution approximately naphthene. The spectrum contains 3.16 resolvable lines with an individual line width of 0.1 G. This spectrum has been reduced to the following coupling constants: 1 H , 6.48 f 0.02 G; 3 H, 6.27 f 0.01 G; (17) L. F. Fieser and L. W. Newton, J. Amer. Chem. Xoc., 64, 917 (1943). (18) L. C. Craig, W. A. Jacobs, and G. I. Lavin, J . B i d . Chem., 139, 277 (1941). (19) A. Streitwieser, "Molecular Orbital Theory for Organic Chemists,'' John Wiley & Sons, Inc., New York, N. Y., 1961,p 46.
217
SPECTROSCOPY OF 1-METHYLPHENALENYL AND 1-PHENYLPHENALENYL RADICALS
A
Figure 2. Comparison of the experimental curve (a) and the computed curve (b) for the 1-phenylphenalenyl free radical. Computed values: 5 H, 6.12 G; 3 H, 1.78 G; 3 H, 0.48 G; 2 H, 0.39 G.
4 H, 6.05 f 0.01 G; and 3 H, 1.77 =k 0.01 G. A spectrum simulated from this assignment is shown in Figure lb. The assignment of these coupling constants to the specific ring positions of I1 can be made from a comparison with the coupling constants for the unsubstituted phenalenyl radical. The splitting of 6.27 G must be assigned to the three equivalent protons of the methyl group. The small splitting of 1.77 G is attributed to the three protons at the inactive 2, 5, and 8 positions. Of the five remaining protons in the molecule, the 4,6,7, and 9 protons are the most similar chemically. The coupling constant of 6.05 G can, therefore, be assigned to these protons, leaving the single splitting of 6.48 G for the proton at the 3 position. 1-Phenylphenalenyl (111). An intense esr spectrum for the phenylphenalenyl radical P m 4
6
1
I11
was observed for solutions of phenylperinaphthene at room temperature. Figure 2a shows the esr spectrum observed for a M solution of I11 in benzene at 25'. Since this spectrum is incompletely resolved and since there is considerable ambiguity in its analysis, we have also obtained an esr spectrum for the l-phenyl-&phenalenyl radical prepared from l-phenyl-&-perinaphthene. The splittings by the protons in the phenyl ring are expected to be small in comparison with those for the protons of the phenalenyl ring. Deuteration of the phenyl ring should, therefore, result in a simplified spectrum. Figure 3a shows the esr spectrum obtained for a solution of 1-phenyl-de-phenalenyl radical in benzene a t 25". A simple pattern of 24 hyperfine lines 0.36 G wide is observed. This spectrum can readily be reduced to the following coupling constant assignment: 5 H, 6.12 f 0.01 G and 3 H, 1.781 f 0.003 G. A stick plot from these values is shown in Figure 3b.
Figure 3. Comparison of the experimental curve (a) and the computed stick plot (b) for the 1-phenyl-ds-phenalenyl free radical. Computed values: 5 H, 6.12 G; 3 H, 1.781 G.
The hyperfine splittings by the deuteriums of the phenyl group are considerably less than 0.36 G and are not resolved. With the aid of the results for the deuterated radical, we have been able to analyze the spectrum for I11 in terms of the following coupling constants: 5H16.12G; 3H,1,78G; 3 H l 0 . 4 8 G ; and2H,0.39G. An esr curve computed from these values is shown in Figure 2b. The slight additional resolution apparent in the experimental curve indicates that the assumed proton equivalences are not exact.
Discussion The ease of formation of the substituted phenalenyl radicals from both 1-methylperinaphthene and 1phenylperinaphthene is further confirmation of the extreme stability of radicals incorporating the phenalenyl ring system. The coupling constants obtained for the phenalenyl, methylphenalenyl, and phenylphenalenyl radicals are summarized in Table 11. The close similarity in the values for the different radicals makes the assignment to ring positions relatively unambiguous. The most interesting aspects of the coupling constant
Table I1 : Comparison of Coupling Constants in Gauss for Phenalenyl, 1-Methylphenalenyl, and LPhenylphenalenyl Radicals Poaition
Phenalenyl (I)
1-Methylphenalenyl (11)
1
6.32
6.27 (CH3)
2 3 4 5 6
1.81 6.32 6.32 1.81 6.32 6.32 1.81 6.32
1.77 6.48 6.05 1.77 6.05 6.05 1.77 6.05
7 8 9
1-Phenylphenalenyl (111)
0.48 ( P I 0.48 (0) 0.39 ( m ) 1.78 6.12 6.12 1.78 6.12 6.12 1.78 6.12
Volume 78,Number 1 January 1060
218
RICHARD E. LINDSTROM AND HENRY E. WIRTH
data for the methylphenalenyl radical are the large tions, which are positions of negative spin hyperfine splittings for the methyl protons. The are virtually unchanged by the methyl substituent. magnitude of the methyl splitting is approximately the The hyperfine interactions a t the remaining active same as that for the ring protons in unsubstituted positions are altered only slightly by the 1-methyl subphenalenyl. Large methyl proton splittings have been stituen t. observed in methyl-substituted aromatic i o n ~ . ' ~ J ~ The phenyl substituent in I11 effects a slight reducThis effect has been described in terms of a combined tion in the magnitude of the hyperfine interaction at all inductive and hyperconjugation mechanism. The the phenalenyl ring positions. The splittings by the large methyl splitting in I1 demonstrates the importance phenyl protons are in turn quite small, a result which is of these mechanisms in aromatic neutral radicals. consistent with the sterically enforced nonplanar conAn examination of the coupling constant data for I1 formation of the phenyl group. shows only a small perturbation by the methyl group on the unpaired electron distribution in the remainder Acknowledgments. We wish to thank Mrs. S. B. of the molecule. The splittings a t the inactive posiWallon for synthesizing the compounds.
Estimation of the Bisulfate Ion Dissociation in Solutions of Sulfuric Acid and Sodium Bisulfate by Richard E. Lindstrom and Henry E. Wirth Department of Chemistry, Syracuse University, Syracuse, New Y O T ~13210 (Received August 7, 1068)
Young's rule was applied to the observed mean apparent molal volumes of sulfuric acid and sodium bisulfate to obtain estimates of the dissociation quotient (Qv) in the volume ionic strength (g") range 0-4. The results in sulfuric acid solution are given by the equation log Qy log 0.0102 -p 2.036fi,'/2 1.376~" 0.8862~.,s/~0.2171fiv2and in sodium bisulfate solution by the equation Iog Qy = log 0.0102 2.036pV'/$ 1 . 5 4 3 ~ "f 0.8297p,?/2 0.1703gv2. The volume change at infinite dilution ( W )for the process H+ 502- -+ HSOawas found to be 21.6 ml.
+
-
+
+
-
Cz and C3 are the molar concentrations of the dissociated and undissociated species, respectively. I n terms of the degree of dissociation ( a ) , eq 1 becomes
Some years ago Klotz and Eckertl investigated the dissociation of the bisulfate ion in sulfuric acid solutions utilizing apparent molar volumes. Their approach was to assume that a bisulfate solution is a mixture of two electrolytes : the completely dissociated species, H +, H+,SOd2-, and the undissociated species, H+,HSOe-. The observed or mean apparent molar volume, a, could then be expressed as a function of the mole fraction and apparent molar volumes of the two solute specieg. This relationship is essentially that defined more explicitly by Young and Smith2 and tested by Wirth and coworkers.a It is
Simultaneous solution of eq 2 and 3 yields an CY which may be used to calculate the dissociation quotient, Qv, at the given ionic strength where
where CP is the mean or observed apparent molar volume of the solute at ionic strength pv, C#J2 is the apparent molar volume of H+,H+,S042- in a pure solution of the same ionic strength p,., and +3 is the apparent molar volume of H+,HS04- also in a pure solution a t ionic strength p".
(1) I. M. Klotz and C. F. Eckert, J . Amer. Chem. Soc., 64, 1878 (1942). (2) T.F. Young and M. B. Smith, J. Phys. Chem., 58, 716 (1954). (3) H.E.Wirth, R. E. Lindutrom, and J. N. Johnson, ibid., 67, 2339 (1963).
The Journal of Physical Chemistry
ch = a42
+ (1 - a)+3
(2)
The ionic strength a t a total solute molarity (C) is PV =
(1
+ 2a)C
(3)
