EPR spectra and structure of the radical cations of fluorinated ethylenes and propenes

EPR spectra for the radical cations of a series of fluorinated ethylenes an d propenes, generated by irradiation with gamma-rays in halocarbon solid ma trices, have been observed at low temperature. The spectra consist of a hyp erfine structure with a nearly axial symmetry mainly due to fluorine nuclei . For the trifluoro olefin cations, CF2=CFX+ (X = H, CH3, or CS), the exper imental spectra were successfully analyzed in terms of the three F-19 nucle i with coaxial parallel components. This may strongly suggest a planar stru cture for the radical cations, similarly to the case of the CF2=CF2+ cation already reported. On the other hand, for mono- and di-fluoro olefin radica l cations, it is difficult to conclude the planar structure unequivocally f rom the experimental F-19 and H-1 hf splittings. Thus, the optimized geomet ry of the radical cations was calculated by an ab initio MO method. In cont rast with the ethylene cation and the propene cation, having nonplanar twis ted structures, the fluorinated ethylene and propene cations are concluded to have planar structures. Calculations at the INDO levels including the ev aluation of interatomic dipole-dipole interactions were done for the geomet ries of the radical cations optimized by the ab initio method. The calculat ed results strongly support the EPR spectra observed and assigned to the ra dical cations.