GEOMETRICAL ISOMERIZATION AND RESTRICTED ROTATION IN IMINOXYL RADICALS FROM BENZALDOXIMES

The E geometrical isomers of benzaldoxime, p-chlorobenzaldoxime, and p -methylbenzaldoxime have been oxidized to the corresponding iminoxyl r adicals within the cavity of an EPR spectrometer by using photolytical ly produced tert-butoxy radicals. The observed iminoxyls retain the sa me geometry (E) of the starting oximes at low temperature (ca. 190 K), while at room temperature they are almost completely converted to the more stable Z isomers. At an intermediate temperature (233 K), where both isomeric species were simultaneously detected, the rate of geomet rical isomerization could be determined as 18 +/- 6 s(-1) for the radi cal from p-chlorobenzaldoxime. The estimated energy barrier (ca. 12 kc al mol(-1)) is much lower than that calculated in the parent oximino c ompounds (ca. 60 kcal mol(-1)). A rationalization of this effect in te rms of Molecular Orbital theory is given. In the Z isomers of the exam ined iminoxyls, selective broadening of the EPR lines due to restricte d rotation of the aryl ring was observed in the temperature range from 250 to 330 K. An analysis of the line shape variations with temperatu re performed on iminoxyl 5 provided a value of the rotational barrier of 6.18 kcal mol(-1).