Temperature dependence of structural quantum effects in liquid methanol

High-energy electromagnetic radiation scattering techniques have been used to measure the structural differences between liquids composed of four diff erent isotopes of methanol (CH3OH, CD3OD, CH3OD and CD3OH) at temperatures of 24.5 degreesC and 80 degreesC. We measured the magnitude of the isotopic effect, which increases with decreasing temperature and comprises of both intramolecular and intermolecular effects. Intramolecularly, the C-O, O-H, and to a lesser extent the C-H bond lengths, shorten upon deuteration. Inte rmolecularly, the winding hydrogen bonded chain structure is affected signi ficantly. The results show that hydrogenated simple alcohols are more disor dered liquids than deuterated alcohols at the same temperature. Substitutio n at the methyl site is compared to substitution at the hydroxyl site and t he effect is shown to increase more rapidly as temperature is decreased.