Isotopic quantum effects in the structure of liquid methanol: I. Experiments with high-energy photon diffraction

High-energy electromagnetic radiation scattering techniques have been used to measure the structural differences between four isotopic samples of methanol (CH₃OH, CD₃OD, CH₃OD and CD₃OH). The first series of experiments employed room temperature and ambient pressure. The carbon-oxygen intramolecular bond length was measured and found to depend more strongly on the isotopic substitution at the hydroxyl site than at the methyl sites. The oscillations in the isotopic difference of the x-ray structure factor, ΔS_X(Q), are shown at room temperature to be about 2% as large as the oscillations in the total structure factor. Our uncertainties are an order of magnitude smaller than those of previous gamma ray measurements (Benmore C J and Egelstaff P A 1996 J. Phys.: Condens. Matter 8 9429-32). A second series of experiments was carded out at -80 °C at its vapour pressure in order to study the significant temperature dependence of these effects. The ΔS_X(Q) difference at -80 °C is shown to be up to three times larger than the room temperature difference. These studies showed that isotopic structural differences in methanol may be represented as temperature shifts that vary as a function of thermodynamic state and substitution site.