1H, 13C, and 17O nuclear magnetic shielding of methanol and its deuterated isotopomers from gas phase measurements

Abstract

Small amounts of methanol isotopic mixtures (CH 3OH + CH 3OD) in fluoromethane and trifluoromethane as gaseous buffers were studied using 1H, 13C, and 17O NMR spectra. Strictly linear density dependence of nuclear magnetic shielding was observed when the pressure of each buffer was changed. After extrapolation of results to the zero-density limit it was possible to determine the appropriate shielding constants free from intermolecular interactions, σ 0(H), σ 0(C), and σ 0(O) at 300 K. They were recalculated to the absolute scales of nuclear magnetic shielding. Additionally, high-resolution 1H, 13C, and 17O NMR spectra of pure liquid methanol isotopomers were also recorded and their NMR chemical shifts were measured. Suitable gas-to-liquid shifts and the magnitudes of isotope effects on nuclear shielding constants caused by H/D substitution are reported. It is shown that the experimental NMR parameters obtained from the gas phase and therefore free from lattice effects, better correspond to the results of quantum-chemical calculations.