New Determination of the Intermolecular Hydrogen-Bonded Structure in Liquid Methanol Studied by Neutron Diffraction with H/D Isotopic Substitution and X-ray Diffraction Methods

A new determination has been conducted on the structural details of intermolecular hydrogen bonds in liquid methanol. High precision time-of-flight (TOF) neutron diffraction measurements have been carried out on liquid methanol samples with H/D isotopically substituted for the hydroxyl hydrogen atom in order to obtain further geometrical information on the hydrogen-bonded methanol molecules. The intermolecular distances and coordination numbers concerning the neighboring methanol molecules have been determined from the least-squares fitting analyses of the observed intermolecular H₀-H₀, X-H₀, and X-X partial structure factors (H₀: hydroxyl hydrogen, X: other atoms except for H₀) <i>a</i>_H0H0^inter(<i>Q</i>)-1, <i>a</i>_XH0^inter(<i>Q</i>)-1, and <i>a</i>_XX^inter(<i>Q</i>)-1. The nearest neighbor hydrogen-bonded H₀···H₀ and H₀···O distances have been determined to be 2.34 ± 0.04 and 1.91 ± 0.02 Å, respectively. The bond angle determined from <i>a</i>_XH0^inter(<i>Q</i>)-1, ∠H₀···O-C = 114 ± 6°, agrees well with the value obtained from <i>a</i>_XX^inter(<i>Q</i>)-1, ∠O···O-C = 109 ± 8°, indicating the formation of a linear hydrogen bond between the neighboring methanol molecules. X-ray diffraction measurements have additionally been carried out for liquid methanol with natural abundance to reveal the structural parameters concerning orientational correlation of the neighboring methanol molecules in the liquid state. The determined bond angle, ∠O···O-C = 103.0 ± 0.5°, agrees well with that derived from the present neutron data. It has been confirmed that a strong orientational correlation with a nearly antiparallel configuration of molecules is present between the nearest neighbor hydrogen-bonded methanol molecules.