ИСТИНА

Several representatives in a series of radical cations originating from three- and four-membered heterocycles containing oxygen, sulfur and nitrogen were studied regarding their photochemical behavior in low-temperature freonic matrices by the means of EPR and UV/Vis spectroscopy and with references to DFT calculations for geometries and magnetic resonance parameters. These radical cations can be viewed as convenient model compounds in the studies of reactivity for heterocyclic intermediates: on the one hand, reliable quantum chemical techniques are readily available for them; on the other hand, their transformations are not as varied as those for larger heterocycles. Several types of pathways have been discovered for the mentioned species, some of them quite unusual: they include reversible photocyclization for methyloxirane, methylthiirane and 2,3-dimethyloxirane radical cations; C–N bond cleavage with the formation of two stabilized rotamers for azetidine radical cations; formation of one ring-open form from two ring-closed isomers of aziridine radical cation of comparable statistical significance; and lack of any photochemical activity for tetramethyloxirane radical cations. Implications are drawn to connect several crucial parameters defining radical cations (charge and spin distribution, molecular symmetry and conformation) to the availability of different pathways and the choices between the possible photoproducts.