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

Neutron diffraction (ND) measurements on liquid methanol (CD3OD,CD3O(H/D),CD3OH) under ambient conditions have been performed to obtain the total (intra-+intermolecular), G(dist)(r) and intermolecular, G(inter)(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, G(XHo)(dist/inter)(r) (X=C, O, and H-a methyl hydrogen) and G(XX)(dist/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, G(XHo)(dist)(r) to be 0.98 +/- 0.01 Angstrom. The C-H distance obtained from the G(XX)(dist)(r) partial is 1.08 +/- 0.01 Angstrom. These distances determined by fitting an intramolecular model to the total distinct structure functions are 0.961 +/- 0.001 Angstrom and 1.096 +/- 0.001 Angstrom, respectively. The G(XX)(inter)(r) function, dominated by contributions from the methyl groups, apart from showing broad oscillations extending up to similar to 14 Angstrom is featureless, mainly because of cancellation effects from six contributing pairs. The Ho ... Ho partial pair distribution function (pdf), g(HoHo)(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 Angstrom. The average position of the O ... Ho hydrogen bond determined purely from experimental G(XHo)(inter)(r) partial distribution function, at 1.75 +/- 0.03 Angstrom is found to lie in the range (1.75-1.95 Angstrom) of values reported from computer simulation results. (C) 2000 American Institute of Physics. [S0021-9606(00)51309-9].