Static and dynamic structures of halogenated dimethyl ether radical cations: An EPR and MO study

Abstract

The EPR spectra of the radical cations of CH₃OCH₂F and CH₃OCH₂Cl generated in CF₃CCl₃ matrices by ionizing radiation at 77 K were fully analyzed with the help of their selectively deuterated compounds. The spectra were analyzed in terms of hyperfine coupling constants (hfcc) to the ¹H nuclei in the CH₃ and CH₂X (X = F, Cl) groups. Based on the EPR results combined with ab initio and DFT MO calculations, the conformation of the cations was determined; the halogen atom preferentially occupies a position in the molecular C–O–C plane which is perpendicular to the unpaired electron orbital of the central oxygen and the C–O–C–Cl framework has a geometry corresponding to a cis conformation. The temperature-dependent EPR spectra for CH₃OCH₂Cl⁺ in CF₃CCl₃ observed between 50 K and 100 K were analyzed in terms of a three-site chemical exchange model among the three protons, caused by CH₃ group rotation. The observed activation energy for the rotation, 3.3 ± 0.5 kJ mol⁻¹, is in reasonable agreement with the value calculated for the potential barrier of the rotation.