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 radicals within the cavity of an EPR spectrometer by using photolytically produced tert-butoxy radicals. The observed iminoxyls retain the same 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 geometrical isomerization could be determined as 18 f 6 s-1 for the radical from p-chlorobenzaldoxime. The estimated energy barrier (ca. 12 kcal mol-') is much lower than that calculated in the parent oximino compounds (ca. 60 kcal mol-'). A rationalization of this effect in terms of Molecular Orbital theory is given. In the Z isomers of the examined iminoxyls, selective broadening of the EPR lines due to restricted rotation of the aryl ring was observed in the temperature range from 250 to 330 K. An analysis of the line shape variations with temperature performed on iminoxyl5 provided a value of the rotational barrier of 6.18 kcal mol-l.