Anomalous Termination Kinetics of 4-(N,n-Dimethylamino)phenyl Nitroso Oxide Isomers

The termination kinetics of the adduct of molecular oxygen (3Σg- ) and 4-(N,N-dimethylamino)-phenylnitrene generated by pulse photolysis from the corresponding azide in hexane has been studied using UV spectroscopy in a wide temperature range. The adduct is represented by cis- and trans-isomers of 4-(N,N-dimethylamino)- phenylnitroso oxide (4-Me2NC 6 H4 NOO). It was experimentally established that an abnormal increase in the ter- mination rate of aryl nitroso oxide is observed with decreasing temperature of the reaction mixture. A two-center five-step mechanism of nitroso oxide transformation is proposed. A satisfactory description of the kinetic curves of the change in the optical density of the reaction mixture can be achieved according to the mechanism: along with ortho-cyclization of the cis-isomer of aromatic nitroso oxide, accompanied by the formation of the corre- sponding nitrile oxide, the interaction of trans- and cis-isomers of the nitroso oxide fragments, leading to a prereaction complex, is possible. The structural features of the complex allow its decomposition either (channel I) into nitro- and nitroso compounds or (channel II) trans-isomers of aryl nitroso oxide. With a decrease in the temperature of the reaction mixture the stability of the complex increases. The latter ensures a more probable decomposition through one of the aforementioned channels.