Imre Bakó

Synchrotron X-ray diffraction measurements have been conducted on aqueous mixtures of propan-2-ol (a.k.a. isopropanol, or 2-propanol), for alcohol contents between 10 and 90 molar %, from room temperature down to 230 K. Molecular dynamics simulations, by using an all-atom parametrization of the propan-2-ol molecule and the well-known TIP4P/2005 water model, were able to provide semi-quantitative descriptions of the measured total structure factors. Various quantities related to hydrogen bonding, like hydrogen bond numbers, size distribution of cyclic entities and cluster size distributions, have been determined from the particle co-ordinates obtained from the simulations. The percolation threshold at room temperature could be estimated to be between isopropanol concentrations of 62 and 74 molar %, whereas at very low temperature, calculations yielded a value above 90 molar %.

It is demonstrated that the localized orbitals calculated for a water cluster have small delocalization tails along the hydrogen bonds, that are crucial in determining the resulting dipole moments of the system. (By cutting them, one gets much smaller dipole moments for the individual monomers—close to the values one obtains by using a Bader-type analysis.) This means that the individual water monomers can be delimited only in a quite fuzzy manner, and the electronic charge density in a given point cannot be assigned completely to that or another molecule. Thus, one arrives to the brink of breaking the concept of a water cluster consisting of individual molecules. The analysis of the tails of the localized orbitals can also be used to identify the pairs of water molecules actually forming hydrogen bonds.

The results of dipole moment as well as of intra- and intermolecular bond order calculations indicate the big importance of collective electrostatic effects caused by the nonimmediate environment in liquid water models. It is also discussed how these collective effects are built up as consequences of the electrostatic and quantum chemical interactions in water clusters.

Neutron diffraction plays an important role in structural chemistry. In order to reveal the solution structure, the partial radial distribution functions have to be determined by using isotope substitution technique yielding different diffraction pattern while the structural parameters remain unchanged. The extraction of parameters from the series of measurements thus reduces to solving a system of linear equations that is affected by experimental errors. In this article, we give an estimation of the size of this error and also directions on how to minimize this effect by properly selecting the systems to be studied.

X-Ray and neutron diffraction measurements are presented for liquid dioxane. The analysis of intramolecular terms by the MCGR method yields bond length parameters in good agreement with previous studies. The liquid structure is similar to that of cyclohexane and is characterized by a weak orientational correlation with a regular long range order arising from the distribution of molecular centres, typical of a pseudo-spherical van der Waals molecule.

The structure of liquid pyridine is investigated by molecular dynamics simulation and the RISM integral equation methods. The calculated total radial distribution functions (RDF) are compared with the experimental RDF’s, obtained by X-ray diffraction measurement. It is shown that the local structure of liquid pyridine is mainly determined by the dipole-dipole interaction between the molecules. The local order is characterized by the pair distribution functions, angular correlation functions and orientational distributions. Dynamical properties (self-diffusion coefficients, rotational correlation times and reorientational correlation times) calculated on the basis of MD results are in good agreement with experimental data.