2238
NOTES
Similar views have been expressed by DeGraff and Kistiakowsky4 on the basis of a tentative potential energy diagram of ketene.
Electron Spin Resonance Spectra of Polyphenyl Anion Radicals
By A. L. Allred and Lee W. Bush Department of Chemistry and Materials Research Center, Northwestern University, Evanston, Illinois 60801 (Received August 5, 1967)
The esr spectra of benzenide and biphenylide radicals have been studied extensively and their coupling constants are well known. Three sharply conflicting reports on the esr spectrum of the terphenylide radical anion have been The esr spectrum of quaterphenylide was reported4 to contain only eight lines as a consequence of nonplanarity prohibiting complete delocalization of the electron over the entire molecule. The suggested6relation, at = & p i , between the proton hyperfine splitting constant, ai,and the spin density, pi, at the contiguous carbon atom makes attractive a study of the “distribution” of an odd electron in polyphenyl radicals. The goals of the present work include (a) clarifying the above reports, (b) observing the esr spectra of higher polyphenyls, (c) determining if dianions and trianions of polyphenyls form under certain conditions, and (d) comparing the results of Huckel LCAO-AI0 calculations for polyphenyls with experimental spin densities, reduction potentials (energy of the lowest normally unoccupied molecular orbital), and energies corresponding to the p band in the ultraviolet region.
Experimental Section Sources and observed melting points for the polyphenyls were: 1,l’:4’,1”-terphenyl, Eastman Organic Chemicals, 213”; 1,1’:4’, 1”:4”, 1”’-quaterphenyl, K & K Laboratories, 316-318”; 1,1’”’,1”:4”,1”’:4”‘, 1““-quinquephenyl, K & K Laboratories, 3833900; and 1,11:4t, 10 :4tl, 1 1 ! ! : 4 1 1 ’ , 1 1 1 1 1 : 4 / 1 1 1 , 1 ) 1 1 1 1 _ sexiphenyl, K & K Laboratories, >450”. Quinquephenyl and sexiphenyl were sublimed before use. Tetrahydrofuran (Eastman Organic Chemicals) was distilled from lithium aluminum hydride and 1,2dimethoxyethane (Ansul) was distilled from potassium. The apparatus, designed for the preparation of the radicals of the higher polyphenyls and shown in Figure 1, accommodates large quantities of solvent and thus helps alleviate problems associated with low solubility and with formation of dianions. After the adThe Journal of Physical Chemistry
Figure 1. Apparatus for the preparation of radicals of polyphenyls having low solubilities.
dition of sodium to part B and addition of approximately mol of polyphenyl plus a glass-enclosed stirring bar to part A, the apparatus was evacuuated and flamed. The sodium was heated until a mirror formed. Approximately 80 ml of solvent was transferred from a vacuum system to part A and stirred for 4 hr. The solution and stirring bar were transferred to part B and stirring was continued for 6 hr at room temperature. The solution was then poured into part C and solvent was pumped off until only 3 ml of solution remained. A portion of this solution was decanted and sealed in an esr sample tube. The anion radicals of terphenyl and quaterphenyI were more conveniently prepared in a tube having three side arms for the addition of sample and magnetic stirrer and for the 4 mm 0.d. esr sample tube. Electron spin resonance spectra were run with a Varian Associates, Inc., X-band spectrometer operating with lOO-kc/sec modulation and a 12-in. electromagnet. The Fieldial was calibrated with the spectrum of potassium nitrosodisulfonate. Simulated spectra were obtained with a CDC 3400 computer and CALCOMP plotter. Polarograms (ac and dc) were obtained with solid-state instrumentation constructed by Smith and Brown6J and by procedures discussed elsewhere.8 (1) K. H. Hausser, L. Mongini, and R. van Steenwinkel, 2. Naturforsch., A , 19, 777 (1964). (2) H. Nishiguchi, Y . Nakai, K. Nakamura, Y . Deguchi, and H. Takaki, Rev. Phys. Chem. Jup., 32,57 (1963). (3) C. Y. Chu, K. H. Fu, C. L. Pan, H. Y. Cheng, and Y. C. Hu, K’o Hsueh T’ungPao, 2 , 160 (1964); Chem. Abstr., 61, 9077 (1964). (4) C. Y. Chu, K. H. Fu, C. L. Pan, H. Y. Cheng, and Y. C. Hu, K’o Hsueh T’ung Pao, 8, 710 (1964); Chem. Abstr., 62, 12635 (1965). (5) H. M.McConnell, J . Chem. Phys., 24, 632 (1956); T. R. Tuttle, R. L. Ward, and S. I. Weissman, ibid., 25, 189 (1956). (6) E. R. Brown, Ph.D. Thesis, Northwestern University, Evanston, Ill., 1967. (7) E. R. Brown, T. G. MoCord, D. E. Smith and D. D. DeFord, Anal. Chem., 38, 1119 (1966). (8) A. L. Allred and L. W. Bush, J . Amer. Chem. SOC.,in press
2239
NOTES
Figure 2. Observed (top) and computed (bottom) esr spectra of sodium terphenylide in 1,2-dimethoxyethane a t -50'. The arrow on this and following spectra corresponds in length to 1 G and also indicates the direction of increasing magnetic field.
Results M sodium terphenylide The esr spectrum of 5 X in 1,Zdimethoxyethane at -50" is shown in Figure 2. An identical spectrum was observed with tetrahydrofuran (THF) as the solvent. The anion was yellow to yellow-green depending upon the concentration. Exposure of the anion to the mirror for periods of time longer than 2 hr resulted in a blue color, presumably due to the dianion, since the latter solution fails to give an esr signal. A solution containing T H F and the monoanion changes from yellow to pink during 1 week at room temperature, but a 1,Bdimethoxyethane solution remains yellow for several months. A computed spectrum based on the hyperfine splitting constants a4 = 3.34, a3 = 0.56, a2 = 2.07, and az' = 0.98 G and a line width of 0.04 G matches closely the experimental spectrum as shown by Figure 2. The esr spectrum of sodium quaterphenylide in T H F a t -50" is shown in Figure 3. Because of the low solubility of quaterphenyl in l12-dimethoxyethane, only T H F was used as a solvent. The anion was orange and again excessive exposure to the mirror produced the blue color, presumably due to the dianion. The orange color persists indefinitely at room temperature in a sealed tube. A computed spectrum based on the experimental hyperfine splitting constants given in Table I and on a line width of 0.03 G also is presented in Figure 3. The assignment of the splitting constants to the 2' and 3 positions is not certain, since both positions are predicted to have low spin density. With the apparatus described in the Experimental Section, sufficient concentrations for esr studies of the monoanions of quinquephenyl and sexiphenyl formed during a short exposure of the solution to sodium metal and before dianion formation became noticeable. Electron spin resonance spectra of the red-violet solution of sodium quinquephenylide and the purple solution of sodium sexiphenylide were observed at several different
Figure 3. Observed (top) and computed (bottom) esr spectra of sodium quaterphenylide in T H F a t -50'.
Table I : Hyperfine Splitting Constants and Spin Densities of Polyphenyl Anion Radicals
.-Anion
Biphenylide
Atoma
4
3 2 1 Terphenylide
4 3 2 1
1' 2' Quaterphenylide
4
3 2 1 1' 2' 3' 4'
Pi-
Exptl
Huokel
McLachlan
5. 4ObgG 0.193 0.44 0.016 2.70 0.097
.*.
...
0.158 0.020 0.090 0.123
0.213 -0.027 0.106 0.128
3.34 0.56 2.07
0.119 0.020 0.074
0.088 0.008 0.060 0.054 0.124 0.049
0.122 -0.023 0.078 0.046 0.148 0.037
0,054 0.004 0.040 0.029 0.095 0.028 0.048 0.081
0.076 -0.016 0.054 0.020 0.119 0.011 0.049 0.090
ni
... ...
... ...
0.98
0.035
2.10 0.42 1.47
0.075 0.015 0.052
...
...
... ...
0.105 1.47
0.004 0.052
...
*..
. equivalent, only the lowest number is listed.
When positions are
' Reference
10.
concentrations, as indicated by changes in color intensity. There was considerable hyperfine splitting but no well-resolved spectra were obtained. For example, see the quinquephenylide spectrum in Figure 4. As expected, both anions appear to be stable indefinitely in the esr tube when stored at room temperature. The formation of a trianion of sexiphenyl was attempted. Part B, containing the reactants, solvent, and stirring bar, was sealed off to permit stirring for long periods of time. With sodium as the reducing agent and after the solution was stirred a t room temperature for 2 weeks, only the blue color, assumed to Volume 78, Number 6 June 1968
2240
NOTES
Table I1 : Polarographic Data and Energies from Reduction Potentials and from Huckel Molecular-Orbital Calculations 1st wave-Epeak,aca-
Ag-AgNOa
Biphenyl Terphenyl Quaterphenyl
3.16 2.90 2.77
Hg Poolc
2.06 1.80 1.67
2nd wave Potentialsa (Ag-AgNOa ref)--------. -Epeak,ac Half-widthd
_-I____
~---em+~b------
7-
Exptl
HMO
0.691
0.705 0.593 0.636
0.583 0.529
-fil/z,da
e 3.23 3.01
...
e 3.26 3.02
140 105
a Volts. Units of 8. The potentials were observed with respect to a Ag-AgNOa electrode a t a precision of rtO.01 V and then reFor biphenyl, the difference in observed potentials for the two reference elecferred to a mercury-pool electrode by adding 1.10 Width in millivolts of ac wave a t half-height. trodes is 1.10 V. Not observed.
v.
Table I11: Experimental and Calculated Transition Energies
-
----
1.57 1.32 1.20 1.15 1.10
1.41 1.19 1.07 1.01 0.96
(em+t -em)“
p-bandb
Figure 4. Electron spin resonance spectrum of sodium quinquephenylide in T H F a t - 50”.
be that of the dianion, was observed. Potassium was also tried as the reducing agent but without success: several days of stirring at -23” (decomposition in the blue solution occurred at room temperature) produced no color change after the formation of the blue color. The concentration as indicated by-the intensity of the blue color was never great enough for an esr signal to be expected. The polarograms of biphenyl, terphenyl, and quaterphenyl each contained a reversible, one-electron reduction wave and, except for biphenyl, a second wave for which the total diffusion current was twice that of the first wave. Reversibility of the second wave increased with increasing chain length as judged by the criteriae 90 mV = line width a t half-height of the ac wave and El/,,do = Epeak,ao. The initial waves satisfied these requirements within experimental error. The polarographic data are recorded in Table 11. Data for quinquephenyl and sexiphenyl are not available due to low solubilities. Eigenfunctions and eigenvalues were obtained by the Huckel procedure and with the assumptions given previously.10 Spin densities were calculated from the Hiickel coefficients and also by AfcLachlan’s procedure (A 1.2),10711 which takes into account configuration interaction. The polyphenyls were assumed to be planar and a value of 28 G was used for &. The calculated values of spin densities, energies of the lowest normally unoccupied molecular orbital em+1, and the energies for excitation from the highest occupied to the lowest unoccupied molecular orbitals (Em+l - em) are presented in Tables 1-111, respectively.
Discussion Polyphenyls are readily reduced to stable monoThe Journal of Physical Chemistry
Biphenyl Terphenyl Quaterphenyl Quinquephenyl Sexiphenyl
HMO
a Units of p. ’Based on frequencies reported in E. M. Layton, Jr., J . MoE. Spect., 5 , 181 (1960), and the equation of A. Streitwieser, Jr., “Molecular Orbital Theory for Organic Chemists,” John Wiley and Sons, Inc., New York, N. Y., 1961, p 217.
anion radicals and to stable dianions. Although the energy, emf2, of the second orbital above the highest normally occupied molecular orbital in sexiphenyl is only -0.66@, no evidence for trianion formation could be obtained. Observed spin densities agree closely with those calculated by the McLachlan method, as shown in Table I. For polyphenyl systems, the McLachlan spin densities are clearly more accurate than the simply obtained Huckel spin densities. The splitting constants found for terphenyl in this investigation agree with those of Hausser, et d . , l and differ from the results of ref 2 and 3. The previously reported4 spectrum of quaterphenylide apparently was unresolved, and the coupling constants for quaterphenylide in Table I are consistent with an odd electron being delocalized along the entire chain. For a large number of neutral, aromatic compounds, the calculated (HMO) energy, Em+l, of the lowest normally unoccupied molecular orbital is linearly related12 to the half-wave reduction potential, E,,,, and the calculated difference in energy, em+1 - Em, is linearly relsted13 to the frequency of the p-band (‘La transition). (9) D. E. Smith, “Electroanalytical Chemistry,” Val. I, A. J. Bard, Ed., M. Dekker, Inc., New York, N. Y., 1966, Chapter 1. (10) M. D. Curtis and A. L. Allred, J . Amer. Chem. Soc., 87, 2643 (1965). (11) A. D. McLachlan, Mol. Phys., 3, 233 (1960). (12) A. Streitwieser, Jr., and I. Schwager, J. Phys. Chem., 66, 2316 (1962). The equation is applicable if DMF is the solvent and a mercury pool is the reference.
NOTES
2241
Values of ern+] obtained from reduction potentials and the application of the former relation are very close to the ern+l values from Huckel calculations as shown in Table 11. The em+1 - ern values calculated from the frequencies of the p bands are each higher by approximately 0.14p than the corresponding HMO values, as indicated by Table 111. The average deviation of SO.14P is reasonable, since the equation relating em+l e, to frequency is based on data for 40 fused ring compounds and no polyphenyls. The anion radicals probably do not deviate greatly from planarity, in view of the results of calculations based on the usual HMO approximations plus the assumption that the resonance integral of the inter-ring bond is proportional to overlap and, therefore, is given by (cos 8)p,14 where e is the dihedral angle. With this model and e equal to 25”, agreement between calculated and experimental spin densities and energies is not as close as in Tables 1-111.
limit to the evaporation coefficient for magnesium nitride sublimation. Their calculated equilibrium pressures are lower by more than a factor of 10 than pressures calculated from thermochemical data,3 and Hildenbrand and Theard concluded that either the heat of formation for iVIgaNz(s) is in error or the rate of vaporization is limited by a secondary process which leads to false apparent-equilibrium pressures. This communication reports a torsion-eff usion and torsion-Langmuir study of magnesium nitride. The torsion-Langmuir study was made both in vacuo and in controlled pressures of nitrogen and argon gases. The torsion-effusion study provides a direct check of the experimental findings of Hildenbrand and Theard, and the torsion-Langmuir study provides a more direct means for calculating the evaporation coefficient and temperature dependence and provides evidence on the effects of adsorbed gases on the rate of a sublimation reaction.
Acknowledgments. This research was supported in part by the Advanced Research Projects Agency of the Department of Defense, through the Northwestern University Materials Research Center. L. W. B. expresses appreciation for a predoctoral fellowship from the Division of General Medical Sciences, United States Public Health Service. We thank Drs. E. R. Brown, D. E. Smith, and D. F. Shriver for valuable assistance.
Experimental Section
(13) A. Streitwieser, Jr., “Molecular Orbital Theory for Organic Chemists,” John Wiley and Sons, Inc., New York, N. Y., 1961, p 217. (14) M. J. S. Dewar, J. Amer. Chem. Xoc., 7 4 , 3345 (1952).
The Rate of Sublimation of Magnesium Nitride from Effusion Cells and from Free Surfaces
in Yucu.0 and i n Argon and Nitrogen Gases
by Bette A. H. Blank and Alan W. Searcy Inorganic Materials Research Division, Lawrence Radiation Laboratory, and Department of Mineral Technology, College of Engineering, University of California at Berkeley, Berkeley, California (Received October 3,1967)
+
For the reaction ?cfg,Nz(s) = 3Mg(g) Nz(g), Soulen, Sthapitononda, and Margravel have reported Hertz-Knudsen effusion pressures which lie well below pressures that they calculated from thermochemical data. This fact plus an increase in measured pressures when the effusion orifice was decreased suggested that magnesium nitride has a kinetic barrier to decomposition. Hildenbrand and Theard,2 from a study of the dependence of torsion-eff usion pressures on orifice area, calculated apparent equilibrium pressures and an upper
The torsion-eff usion and torsion-Langmuir methods as used in this laboratory have been previously des~ribed,~ and J only special features of the present study need be described here. That leakage from the effusion cells of National Carbon ZTlOl grade graphite caused no deflection was demonstrated by heating magnesium nitride in the cell before effusion holes mere drilled. The orifices used in effusion studies were approximately 1-2 mm in diameter. For the torsion-Langmuir measurements, 1 mm in diameter knife-edged orifices defined the area of sublimation. The vapor pressure of tin was measured as a test of our experimental methods. A value of 72.0 kcal/mol for the heat of sublimation at 298” K was obtained by the third-law method, which agrees well with the value of 71.8 kcal/mol measured by Schulz6 by the torsioneffusion method and with 72.2 f 0.5 kcal/mol calculated by Hultgren, et al.,’ from results of earlier studies. For experiments 011 the effect of gases on the rate of sublimation, nitrogen or argon was passed through a liquid nit,rogen cold trap and an oxygen getter of copper (1) J. R. Soulen, P. Sthapitononda, and J. L. Margrave, J. Phys. Chem., 59, 132 (1955). (2) D. L. Hildenbrand and L. P. Theard, ASTIA Unclassified Report 258410, Aeronutronic Report U-1274, Newport Beach, Calif., 1961. (3) D. R. Stull, et al., “JANAF Thermochemical Tables,” DON
Chemical Co., Midland, Mich. (4) A. W. Searcy and R. D. Freeman, J . Amer. Chem. floc., 76,5229 (1964). (5) Z. A. Munir and A. W. Searcy, J. Chem. Phys., 42, 4223 (1965). (6) D. A. Schulz, Ph.D. Thesis, University of California at Berkeley. Berkeley, Calif., 1961. (7) R. Hultgren, R. L. Orr, P. D. Anderson, and K. K. Kelley, “Selected Values of Thermodynamic Properties of Metals and Alloys,” John Wiley and Sons, Inc., New York, N. Y., 1963.
Volume 78,Number 6
June 1968
