3439
ELECTRON SPINRESONANCE SPECTRA OF RADICAL ANIONSOF STYRENE
Electron Spin Resonance Spectra of Radical Anions of Styrene and Related Compounds by A. R. Buick, T. J. Kemp, and T. J. Stone School of Molecular Sciences, University of W a r w k k , Coventry CV4 7 A L , Great Britain
(Received April 14, 19'70)
Well resolved electron spin resonance spectra of the radical anions of the following compounds are reported: styrene, a-methylstyrene, o- and m-methylstyrene, o-, m-, and p-fluorostyrene, o- and p-vinylpyridine, cinnamic acid, and phenylacetylene. All were obtained by means of continuous-flow reduction of the parent compound by solutions of electrons in liquid ammonia. Delay between mixing and observation permitted observation of secondary radicals in certain cases. Assignment of coupling constants was achieved in all cases by comparing experimental spin densities with those calculated by the Huckel and McLachlan procedures.
Introduction Radical anions of a very large number of aromatic molecules have been examined spectroscopically by both electronic and electron spin resonance (esr) techniques and the resulting spectra have been discussed in terms of molecular orbital (MO) theory of increasing degrees of sophistication. While many radical anions are sufficiently stable to permit leisurely investigation a t room temperature, this is not the case for some of those of simple structure which undergo either rapid dimerization or disproportionation, and in these instances spectroscopic identification depends on the application either of fast reaction techniques such as flash photolysis or pulse radiolysis (which afford optical and kinetic data) or of a matrix isolation method,' which depends on preparation of the radical anion in a glass or polycrystalline environment, often at low temperature, by allowing the parent molecule to capture a relatively mobile electron generated radiation- or photo-chemically; the immobile radical anion can then be studied optically or by esr. Electrolysis of nonaqueous systems at low temperatures has also proved effective in allowing a steady-state concentration of the radical anions of pyridine2 and butadiene3 to be built up permitting unequivocal esr characterization. We report here the well resolved solution esr spectra of styrene and a number of related compounds. While Hamil14 has reported optical spectra of the radical anion of styrene (sty.-) following y irradiation of 0.1 M solutions in 2-methyltetrahydrofuran a t 77°K) attempts to produce an unambiguous liquid-phase spectrum by pulse radiolysis of liquid styrene and solutions have been unsuccessfulb6fj Williams and colleagues' have obtained the similar optical spectrum of cr-methyls t y . - using a technique similar to that of Hamill, but esr examination of these glassy solutions revealed a broad peak showing slight but unresolved structure due to the line-broadening effectsassociated with solid-state
electron resonance. Special interest is attached to sty.- and also the radical anion of p-vinylpyridine in view of their intermediacy in anionic polymerization by alkali metals and aromatic hyrocarbon radical ions.s-10
Experimental Section Chemicals. Styrenes and vinylpyridines were obtained from Koch-Light Laboratories Ltd. and were purified by vacuum distillation immediately prior to use. Anhydrous ammonia of a quoted purity of 99.98% was supplied by Imperial Chemical Industries Ltd. Very small pieces of sodium were added until a blue color was obtained. Technique. As previously described," ca. iM solutions in ammonia of the substrate and of sodium were conveyed separately from 2-1. flasks surrounded by chips of solid carbon dioxide by applying a pressure of pure nitrogen. The solutions were then mixed in a 15-jet multicapillary mixer of 2-mm internal diameter placed in the resonant cavity of a Decca Radar Ltd. X-1 esr spectrometer incorporating a 7-in. magnet from Newport Instruments Ltd. At the fastest flow rates employed (8 ml sec-I), mixing is calculated to be com(1) W. H. Hamill in "Radical Ions," E. T. Kaiser and L. Kevan, Ed., Interscience, New York, N. Y., 1968,Chapter 9. (2) C. L. Talcott and R. J. Myers, Mol. Phys., 12, 649 (1967). (3) D.H. Levy and R. J. Myers, J. Chem. Phys., 41, 1062 (1964). (4) M. R. Ronayne, J. P. Guarino, and W. H. Hamill, J. Amer. Chem. SOC.,84, 4230 (1962). (6) J. P. Keene, E. J. Land, and A. J. Bwallow, ibid., 87, 5284 (1965). (6) K.W.Chambers, E. Collinson, F. 5. Dainton, and F. Wilkinson, Trans. Faraday SOC.,63, 1699 (1967). (7) J. Lin, K.Tsuji, and F.Williams, ibid., 64,2896 (1968). (8) K. F. O'Driscoll and A. V. Tobolsky, J. Polym. Sci., 31, 116, 123 (1968). (9) M.Szwarc, P T O Phys. ~ . Org. Chem., 6 , 323 (1968). (10) D.Laurin and G. Parravano, J. Polyn. Sei., A6, 1047 (1968). (11) A. R.Buick, T. J. Kemp, G. T. Neal, and T. J. Stone, J. Chem. SOC.A , 666 (1969).
The Journal of Physical Chemistry, Vol, 74, No. 19, 1970
3440
A. R. BUICK,T.J. KEMP,AND T. J. STONE
Table 1: Coupling Constants and Experimental and Theoretical Spin Densities
Substrate
7
Position
8
CH=CH,
&i” I
8
8
I
S
8
4F
--------Theor Huckel
spin densities-MoLachlan
3.82 0.87 5.51 0.59 2.00 1.51 7.35
0.142 0.032 0.205 0.022 0.074 0.056 0.367
0.134 0,008 0.182 0.042 0.082 0.085 0.307
0.142 -0.024 0.204 0.020 0,074 0.044 0.368
2 3 4 6 7 8
3.44 0.29 5.07 0.65 2.15 1.31 6.65
0.128 0.011 0.188 0.024 0.080 0.047 0.333
0.138 0.008 0.188 0.045 0,081 0.080 0.278
0.146 -0.024 0.213 0.024 0.074 0.045 0.334
2 3 4 5 6 7 8
3.77 0.37 4.92 0.89 1.48 2.60 7.68
0.140 0.014 0.183 0.033 0.053 0.097 0 384
0.157 0.002 0.181 0.065 0.062 0.084 0.292
0.171 -0.032 0.203 0.049 0.049 0.046 0.350
2
3.98 0.64 5.49 0.86 2.17 1.52 7.35
0.148 0.024 0.204 0.031 0,081 0.056 0 368
0.145 0.006 0.176 0,045 0.075 0.085 0.307
0.156 -0.024 0.197 0.026 0.065 0.044 0.368
5.89 0.38 4.57 0.94 1.84 0.94 7.51
0.118 0.014 0.170 0.035 0.068 0.035 0.376
0.116 0.018 0.188 0.029 0.099 0.087 0.301
0.119 -0.011 0.213 0.004 0.097 0.048 0.360
4 5 6 7 8
3.98 1.22 5.23 1.00 2.43 1.53 7.66
0.148 0.024 0.194 0.037 0,090 0.057 0.383
0.142 0.006 0.178 0.044 0.077 0.085 0.307
0.152 -0.034 0.200 0.024 0.068 0.044 0.368
2 3 4 5 6 7 8
5.29 0.49 11.17 1.26 2.53 1.41 8.53
0.197 0,018 0.223 0.047 0.094 0.052 0.426
0.142 0.004 0.176 0.035 0.082 0.089 0.304
0.153 -0.029 0.196 0.014 0.076 0.050 0.365
4.50
0.167 0 0.267 0.041 0.067 0.061 0.317
0.145 0.004 0.181 0.039 0.081 0.084 0.275
0.158 -0,029 0.204 0.017 0.075 0.050 0.330
3 4 5 6 7 8 2 3 4
5 6 7 8 2
T
Exptl spin densities (@XiH 26.9)
2 3 4 5 6 7 8
5
7
Exptl coupling constants, Oe
3
2 3 4 5 6 7 8 The Journal of Physical Chemistry, Vol. 74, No. 18, 1870
d line width 13.34 1.09 1.80 1.70 6.34
I
I
3441
ELECTRON SPINRESONANCE SPECTRA OF RADICAL ANIONSOF STYRENE
Table I
(Continued)
Substrate
7
Position
Exptl coupling constants, Oe
2 3 4 5 6 7
8
CH=CHp
0.097 0.042 0.153 0.082 0.061 0.302
0.086 0.028 0.159 0.076 0.051 0.015 0.370
3.44
