NOTES
3423
-
meet at the known critical point of 1733'K. Independently, the law off rectilinear diameter leads one to practically the same value. We thus consider the critical viscosity of mercury equal to 0.41 cp. a t 1733OK. to be a good estimate. To illustrate this further, water has the viscosity equal to 1.00 cp. at 20'; thus mercury, at the critical point, is 2.5 times as fluid as water at 20'. Since the critical density2 is equal to 5.0 g . / ~ n i . the ~, critical kinematic viscosity of mercury equals 0.082 centistoke. The same method may be used to estimate the critical viscosity of a number of other metals whose qliq
and qsatd vsp are known over a substantial temperature range. Such is the case of the alkali metals; the critical teniperatures and densities of sodium, potassium, rubidium, and cesium have been estimated (see ref. 3). Self-diffusion is another transport property which is closely related to viscosity. In a number of papers of the authorz6it has been stressed that the values of the diffusion constant, D,of metals can be obtained from 7 thanks to the Stokes-Einstein relation.
(26) The latest one is A. V. Grosse, Science, 145, 50 (1964).
NOTES
current constant of 4.1 pa. mg.-'/' set."' mlM-l is the same as that for the known one-electron reduction of Anion Radical of Benzocyclobutadienoquinone nitrobenzene in a~etonitrile.~The shape of the polarographic wave is close to that for a reversible one-electron reaction; E,,, - E,,, is -65 mv. compared with the by David H. Geske and Alan L. Balch value of -56 mv. for a reversible process. Department of Chemistry, Cornel2 Uniueraity, Ithaca, New York The anion radical of I, which is conveniently secured (Receiued June 4, 1964) by intra muros electroreduction' of I, is pale yellow. The e.s.r. spectrum obtained on reduction of a 1 mM The considerable investment of experimental effort solution of I in acetonitrile is shown in Fig. 1. The line width is 0.1 gauss and the g value is 2.0040. by organic chemists in attempts to synthesize cycloThe seven-line spectrum is readily assigned to isobutadiene and its derivatives has borne fruit in the tropic proton coupling constants of 1.87 gauss and 3.74 case of a number of quinones related to cyclobutadiene. The preparation of phenylcyclobutenedione by Sinutny gauss for two sets of two equivalent protons. The assignment of these coupling constants to molecular and Roberts1s2 was followed by the synthesis of diphenylcy~lobutenedione,~ dihydroxycy~lobutenedione,~ positions is shown in Table I. This assignment is and dimethylcyclobu tenedione.6 Recently Cava and supported by comparison with the accepted assignments for several other ortho-disubstituted anion co-workers6 have secured benzocyclobutadienoquinone (I). This paper reports on an electron spin resonance (1) E. J. Smutny and J. D. Roberts, J . Am. Chem. Soc.. 7 7 , 3420 (e.s.r.) study of the anion radical of I. An Electron Spin Fksonance Study of the
T % :I (I)
A polarographic study of millimolar solutions of I in acetonitrile (0.1 M in tetraethylammonium perchlorate as supporting electrolyte) shows a reduction wave at -1.20 v. vsi. aqueous 6.c.e. The diffusion
(1955).
(2) E. J. Smutny, M. C. Caserio, and J. D. Roberts, ibid.,8 2 , 1793
(1960). (3) A. T. Blomquist and E. A. LaLancette, ibid., 83, 1387 (1961); 84, 220 (1962). (4) 8. Cohen, J. R. Laoher, and J. D. Park, ibid., 81, 3480 (1959). ( 5 ) A. T. Blomquist and R. A. Vierling, Tetrahedron Letters, 655 (1961). (6) M. P. Cava and D. R. Napier, J . Am. Chem. Soc., 79, 3606 (1957); M. P. Cava, D. R. Napier, and R. J. Pohl, i b i d . , 85, 2076 (1963). (7) D. H. Geske and A. H. Maki, ibid., 82, 2671 (1960).
Volume 68, Number I 1
Xoeember, 1964
NOTES
3424
Table I : Ring Coupling Constants in Anion Radicals 2
I aring j ,
gauss
Ref.
r
6
C - 2 G . 4
5 . 6c
d
H-+
Figure 1. Derivative electron spin resonance spectrum of anion radical of benzocyclobutadienoquinone as obtained by electrolysis a of 1miW solution of benzocyclobutadienoquinone in acetonitrilelrolution.
4.1 M. Adams, $1. S.Blois, Jr., and R. H. Sands, J . Chem. Phys., 28, 774 (1958). * P. H. Rieger, I. Bernal, W. H. Reinmuth, and G. K. Fraenkel, J . Am. C'hem. Soc., 85,683 (1963). The phthalonitrile anion radical was also observed by A. Carrington and P. F. Todd, Mol. Phys., 6, 161 (1963), who found ring proton coupling constants of 4.24 and 0.33 gauss for the radical in tetrahydrofuran. The phthaldehyde anion radical exists in the rneso form as shown in Table I. Consequently there are not two sets of exactly equivalent proton coupling constants; the actual values are 2.91, 2.19, 0.49, and
