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.