COMMUNICATIONS TO THE EDITOR745 I For the ethyl radical, B has a value of 58.5 gauss.' Solution of the equations, aaxia1.H = Bp COS2 eaxial, Uequat-H = Bp COS2 eequat, and Oequat-H = Baxial-H f 120' (tetrahedral a-carbon atom) gives the results listed in Table I. 6 It is interesting that the value of Oaxial for the a-hydrogens of the 4-t-butylcyclohexane-1,2-dione radical-anion is the same as found for cyclohexene by n . m .~-.~ Our results demonstrate the application of e.s.r. to conformational analysis of nonrigid systems and suggest the use of e.s.r. for structural determinations of rigid ketonic systems, such as the steroidal ketones. Work in this area is in progress. (5) R. W. Fessenden and R . H . Schuler, J . Chem. P h y s . , 89, 2147 (1963). These authors determined Qp (CHa). Since c 0 s~4 5~ (free rotation) is * / n , B is twice their value of Qp (CHd (6) Two values of Bp and two values of 0 (except when cos2 Oequat.H = 0) are obtained (7) G. V . Smith and H . Kriloff, J . A m . Chem Soc., 86, 2016 (1963) These authors use a dihedral angle for axial hydrogens equivalent to 90' (8) (a) Alfred P . Sloan Foundation Fellow, 1959-1963; (b) National Unrealistic solutions ( B p > 58.5/2) have not been listed e. .
Photoisomerization of Azomethane Sir :Although cis-trans isomerization is well-established for aromatic azo compounds,1,2 it has not been observed in the aliphatic series. We wish to report experiments in which the photochemical cis-trans isomerization of azomethane and the isolation of pure cis-azomethane have been achieved.Azomethane (I) ~ which exists in the trans configurat i~n ,~-~ photolyzes readily in the gas phase, yielding nitrogen and two methyl radicals. In agreement with previous work,6 the quantum yield ( 4~~) for the photolytic decomposition of gaseous azomethane (I) by 365mp light was found to be 1.0 f 0.1. In marked contrast, the quantum yield decreases dramatically when azomethane is irradiated in solution. Thus, for carefully degassed 5 X M solutions in isooctane, ethanol, N,N-dimethylformamide, and water a t 25", the quantum yields, determined by nitrogen formation, (1) G. Zimmerman, I,. Chow, and U. Paik, J. A m Chem. Soc.. 80, 3528 (1958); E. Fischer, ibid., 81, 3249 (1960).(2) c i s and trans isomers of difluorodiazine (NzFd are known: C. B.were, respectively, 0.17 f 0.01, 0.07 f 0.01, 0.05 f 0.01, and 0.01 f 0.01. In addition, fluorescence was not detected in any of these solutions. However, chemical transformation does attend irradiation of azomethane in degassed solution and can be observed by ultraviolet absorption and by proton magnetic resonance spectroscopy. A solution of azomethane (I, to lo-' M ) in degassed DzO shows a single sharp resonance lying a t 6 $0.984 ~. p . m . ,~ and is indefinitely stable in the dark a t room temperature. Upon irradiation of the solution a t room temperature, a new sharp singlet (11) appears 0.10 p.p.m. to high field from the resonance of I . By following the ratio of the height of the new resonance (11) to that of I as a function of irradi...
