Deuterium isotope effects on rotation of methyl hydrogens. A study of the dimethyl ether radical cation by ESR spectroscopy and ab initio and densityfunctional theory

Deuterium isotope effects on the methyl group conformation and ESR spectra of selectively deuterated dimethyl ether radical cations are demonstrated b y employing experimental matrix-isolation ESR techniques in combination wit h accurate ab initio and density functional theory (DFT) quantum chemical m ethods. The experimental study shows strong deuterium isotope effects on th e proton hyperfine coupling constants (HFCC) for CD3OCH3+, CD3OCH2D+, and C D3OCHD2+ as well as a temperature dependence in the HFCC for CD3OCH2D+ and CD3OCHD2+. The deuterium isotope effects and temperature dependencies can b e understood by incorporating the mass difference of the two hydrogen isoto pes in addition to their magnetic properties, and utilizing new, improved q uantum chemical structure calculations. Good agreement between experiment a nd theory is obtained when correlated ab initio (MP2) geometries are used i n conjunction with DFT hyperfine coupling constants, whereas an erroneous t emperature behavior is obtained for the HFCC when older methods or DFT calc ulations are used for the geometry determinations.