The structure of molecular liquids: neutron diffraction and molecular dynamics simulations
Neutron diffraction (ND) measurements on liquid methanol (CD3OD, CD3O(H/D), CD3OH) under ambient conditions were performed to obtain the distinct (intra- + inter-molecular), Gdist (r) and inter-molecular, Ginter(r) radial distribution functions (rdfs) for the three samples. The H/D substitution on hydroxyl-hydrogen (Ho) has been used to extract the partial distribution functions, GXHo(r) (X=C, O, and H - a methyl hydrogen) and GXX (r) at both the distinct and inter-molecular levels from the difference techniques of ND. The O-Ho bond length, which has been the subject of controversy in the past, is found purely from the distinct partial distribution function, GXHo(r) to be 0.98 ± 0.01 Å. The C-H distance obtained from the distinct GXX(r) partial is 1.08 ± 0.01 Å. These distances determined by fitting an intra-molecular model to the total distinct structure functions are 0.961 ± 0.001 Å and 1.096 ± 0.001 Å, respectively. The inter-molecular GXX(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), 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 for the first time purely from experimental inter-molecular GXHo(r) partial distribution function is found to be at 1.75 ± 0.03 Å. The experimental structural results at the partial distribution level are compared with those obtained from molecular dynamics (MD) simulations performed in NVE ensemble by using both 3- and 6-site force field models for the first time in this study. The MD simulations with both the models reproduce the ND rdfs rather well. However, discrepancies begin to appear between the simulated and the experimental partial distribution functions, showing that the agreement at the rdf level does not provide a critical evaluation of appropriateness of a chosen potential model to reproduce the observed liquid structure. Both the simulations reproduce equally well the X-X partial comprising of six correlations. The ability of the 3-site model simulations to satisfactorily reproduce this function dominated by contributions from the methyl group, demonstrates that the methyl group does not participate in any bonding in the liquid. However, the main peaks of the simulated Ho-Ho pdf are found to be slightly higher and shifted to larger distances as compared to the ND results. A comparison of the simulated and ND X-Ho inter-molecular functions dominated by H-Ho correlations shows that although the 3-site model reproduces at least qualitatively the experimental features, the six-site model fails badly. The structure of liquid benzene at 298 K is investigated by performing molecular dynamics (MD) simulations in NVE ensemble using three different force field models differing both in their functional form and in the way they were devised. Surprisingly however, they lead to similar results for the pdfs. The structural results from MD simulations are compared with the neutron diffraction (ND) results where the newly C-C, C-H and H-H inter-molecular pdfs are obtained in this study by the H/D substitution on hydrogen atoms of benzene. A good agreement is found between the simulated and experimental total inter-molecular rdfs for C6D6 and C6(H/D)6 experimental, but not for C6H6. Most of the structural properties of benzene discussed in the past have been based on the models, which showed a reasonable agreement between the simulated and neutron inter-molecular rdf or X-ray C -C pdf. The C-Cp d f extracted from the present ND studies however differs from the one obtained earlier from the X-ray measurements. Apart from that, the simulated C-C p d f reproduces the corresponding ND function better than that obtained from the X-rays. Nevertheless, comparisons between the MD and ND results for the C-H and H-H pdfs show significant discrepancies, which highlight the need to further refine the existing force field models. Neutron diffraction (ND) measurements were also performed on benzene-methanol liquid mixture (molar ratio 1:2) under ambient conditions. The H/D isotopic substitution technique on the hydrogens of both the hydroxyl group of methanol (Ho) and benzene (HB) was used to extract the solvent-solvent, solute-solute and solutesolvent correlations. The ND structural results of the mixture are interpreted with the help of the experimental results of its pure components. The results reveal that the self-association of methanol due to hydrogen bonding is hardly disrupted by the addition of benzene. Investigations of the solute-solvent and solute-solute correlations show that although a weak association exists between benzene and methanol molecules, there is no evidence to suggest the formation of a π-hydrogen bond between them in the liquid state. The benzene molecules thus, play the role of an inert solute in the mixture. The experimental structural results for the benzene-methanol liquid mixture are compared with those obtained from molecular dynamics (MD) simulations performed with an inter- molecular potential model built from the two force field models used in simulating the behaviour of the two pure components. The simulated structural results of the mixture are interpreted with the help of the simulated results of the pure components. Although an overall agreement is obtained between the simulated and experimental inter-molecular rdfs, a comparison of the partial distribution functions reveals that model potentials for the mixture need to be refined.
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