Deuterium isotope effects on the methyl group conformation and ESR spectra of selectively deuterated dimethyl ether radical cations are demonstrated by employing experimental matrix-isolation ESR techniques in combination with accurate ab initio and density functional theory (DFT) quantum chemical methods. The experimental study shows strong deuterium isotope effects on the proton hyperfine coupling constants (HFCC) for CD 3 OCH 3 + , CD 3 OCH 2 D + , and CD 3 OCHD 2 + as well as a temperature dependence in the HFCC for CD 3 -OCH 2 D + and CD 3 OCHD 2 + . The deuterium isotope effects and temperature dependencies can be understood by incorporating the mass difference of the two hydrogen isotopes in addition to their magnetic properties, and utilizing new, improved quantum chemical structure calculations. Good agreement between experiment and theory is obtained when correlated ab initio (MP2) geometries are used in conjunction with DFT hyperfine coupling constants, whereas an erroneous temperature behavior is obtained for the HFCC when older methods or DFT calculations are used for the geometry determinations.

The EPR spectra of the radical cations of CH 3 OCH 2 F and CH 3 OCH 2 Cl generated in CF 3 CCl 3 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 1 H nuclei in the CH 3 and CH 2 X (X ¼ 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 CH 3 OCH 2 Cl + in CF 3 CCl 3 observed between 50 K and 100 K were analyzed in terms of a three-site chemical exchange model among the three protons, caused by CH 3 group rotation. The observed activation energy for the rotation, 3.3 AE 0.5 kJ mol À1 , is in reasonable agreement with the value calculated for the potential barrier of the rotation.