Multinuclear magnetic resonance studies of gaseous and liquid dimethyl ethers

Abstract

Dimethyl ethers (DME): (CH 3) 2O, CH 3OCD 3, (CD 3) 2O have been studied using advanced NMR spectroscopy in liquid and gaseous states at 300 K. Extrapolation of the gas-phase results to the zero-density limit permitted the measurements of 1H, 13C and 17O nuclear magnetic shieldings of isolated DME molecules. The significance of intermolecular effects in the gaseous state as well as by coming from gas to liquid are also reported; measured gas-to-liquid shifts for 13C and 17O nuclei are negative (deshielding effects) but positive for 1H nuclei. The very weak linear dependence on density has also been observed for the carbon-proton and carbon-deuteron scalar coupling constants over one bond in the appropriate methyl group of DME. It was possible to obtain the coupling constants free from intermolecular interactions: 1 J 0(CH)=138.84 Hz in (CH 3) 2O, 1 J 0(CH)=138.81 Hz and 1 J 0(CD)=21.17 Hz in CH 3OCD 3 and 1 J 0(CD)=21.16 Hz in (CD 3) 2O. The magnitude of isotope effects on nuclear shieldings and indirect coupling constants caused by H/D substitution is also discussed. These new experimental results are suitable for a better verification of ab initio calculations of indirect coupling constants and nuclear magnetic shieldings of all nuclei in molecules under study.