The structure of liquid methanol by H/D substitution technique of neutron diffraction

Abstract

Neutron diffraction (ND) measurements on liquid methanol (CD3OD,CD3O(H/D),CD3OH) under ambient conditions have been performed to obtain the total (intra-+intermolecular), Gdist(r) and intermolecular, Ginter(r) radial distribution functions (rdfs) for the three samples. The extent to which intermolecular structure is affected by using two different intramolecular models is discussed. The H/D substitution on hydroxyl–hydrogen (Ho) has been used to extract the partial distribution functions, GXHodist/inter(r) (X=C, O, and H—a methyl hydrogen) and GXXdist/inter(r) from the difference techniques of ND at both the distinct and intermolecular levels. The O–Ho bond length, which has been the subject of controversy in the past, is found purely from the partial distribution function, GXHodist(r) to be 0.98±0.01 Å. The C–H distance obtained from the GXXdist(r) partial is 1.08±0.01 Å. These distances determined by fitting an intramolecular model to the total distinct structure functions are 0.961±0.001 Å and 1.096±0.001 Å, respectively. The GXXinter(r) function, dominated by contributions from the methyl groups, apart from showing broad oscillations extending up to ∼14 Å is featureless, mainly because of cancellation effects from six contributing pairs. The Ho⋯Ho partial pair distribution function (pdf), gHoHo(r), also determined from the second-order difference, shows that only one other Ho atom can be found within a mean Ho⋯Ho separation of 2.36 Å. The average position of the O⋯Ho hydrogen bond determined purely from experimental GXHointer(r) partial distribution function, at 1.75±0.03 Å is found to lie in the range (1.75–1.95 Å) of values reported from computer simulation results.