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.

Interplay between intramolecular and intermolecular structures of 1,1,2,2-tetrachloro-1,2-difluoroethane

“…47 Because the fit is intended to extract the intramolecular structure, in reciprocal space we will focus only on the intramolecular contribution of the aforementioned q-dependent term as performed previously, [8][9][10]48,49 1…”

“…Both the decoupled 5 and the coupled 6 approaches have been applied in structural modeling applications. In more complex systems, such as alcohols, 7,8 the intramolecular structure has been proven to have an effect on the intermolecular structure at short and even intermediate distances. 9 The determination of the intramolecular structure in the liquid or the solid phases is not an easy task.…”

Liquid methanol Monte Carlo simulations with a refined potential which includes polarizability, nonadditivity, and intramolecular relaxation

“…16,17 It has therefore been thought that something similar could be occurring in the liquid phase and be responsible for its peculiar behavior. Several numerical simulations [18][19][20][21][22][23] and neutron diffraction experiments [24][25][26] have produced data that support this view. On the other hand, experimental results of neutron diffraction, 27,28 x-ray scattering, 29,30 and x-ray emission spectroscopy 31 have been taken to support the existence of cyclic clusters of methanol in the liquid phase.…”

“…5. The first two curves were constructed from the partial radial distribution functions using the weighting scheme suggested by Adya et al 26 In Fig. 5 the comparison shows that the disagreement between the theoretical and the experimental lines is similar to that shown between both experimental curves.…”

“…The total radial distribution function predicted by the model was estimated and compared with those of Yamaguchi et al 25 and Adya et al 26 in Fig. 5.…”