It is very important to understand the relation between the structure of a zeolite and the adsorption and fate of specific molecules. In this review, we will restrict our interest to molecular dynamics (MD) simulation of zeolites. Our goal is to illustrate this powerful technique and outline results and problems in its application to these complex systems. We shall not follow a chronological order, because the studies in this field are devoted to different phenomena occurring in zeolites, which will be treated separately. The basic features of the MD simulation technique are illustrated first. The application to zeolites of closely related simulation techniques like the Monte Carlo (MC) method (refs 7−11 are recent examples) will not be treated, because the review would become cumbersome and too complex, and we do not feel sufficiently involved in this field.

Structure and Dynamics of Zeolites Investigated by Molecular Dynamics.

Rotational Tunneling and Neutron Spectroscopy: A Compilation.

We report the results of molecular dynamics simulations on the ice clathrates of methane, tetrafluoromethane, cyclopropane, and ethylene oxide. Although the translational phonon density of states of the host lattice are found to be broadly similar, there are subtle differences which can be correlated to the properties of the guests. The simulations also predict low frequency vibrations that are associated with the rattling and liberations of the guest molecules in the hydrate cages. The occurrence of these low frequency modes may be responsible for the lower thermal conductivities of the clathrate hydrates relative to ordinary ices.

Computer simulation studies of the structure I clathrate hydrates of methane, tetrafluoromethane, cyclopropane, and ethylene oxide

Organic free radicals and most transition metal ions are characterized by the presence in their structure of one or more unpaired electron(s), that is, they are paramagnetic, and may produce electron spin resonance (ESR) signals in both the free state and any organic and inorganic association in which they maintain unpaired electron(s), in either solid or solution state.

Application of Electron Spin Resonance (ESR) Spectroscopy in Soil Chemistry

A molecular dynamics study of methane in zeolite NaY

An intermolecular potential for methane

The rotational energy levels of NH + 4 in dilute solution in KBr have been calculated for three possible orientations of a tetrahedral ion in an octahedral field. Both tunneling and librational levels have been obtained. It is concluded that the inelastic neutron scattering spectra, which were reported in part I [J. Phys. Soc. Jpn. 52 (1983) 2401], are best accounted for on the basis of C 3v orientation of the NH + 4 ions on the cation lattice sites.

Rotational States of NH+4 in Dilute Solution in Alkali Halide Lattices. III. Calculated Energy Levels of NH+4 in KBr

Energy levels within the librational ground state manifold of CH3D in a trigonal field are calculated for a monolayer system physisorbed on the surface of graphite. They are classified into two groups: The upper two levels correspond to the stable tripod orientation with the CD bond pointing upward from the surface, and the lower five levels correspond to those with the CD bond pointing toward the surface. The inelastic neutron scattering cross sections are calculated. The transitions between the two groups are found to be negligible. The spectrum for a powdered sample of graphite (Vulcan III) is well reproduced, and that for a partially oriented sample (Papyex) is predicted with momentum transfer both parallel and perpendicular to the surface. Heat capacity anomalies are calculated and found to be sensitive to the spacing between the two groups of levels.

Splittings of the librational ground state of CH3D physisorbed on the surface of graphite

We have investigated certain aspects of the dynamics of methane monolayers physisorbed on graphite. In particular, phonon frequencies have been calculated by applying the self‐consistent harmonic approximation to a two‐dimensional array of methane molecules arranged as stable tripods on a triangular lattice. The methane–graphite interaction was not considered explicitly, and only three degrees of freedom per molecule were taken into account: two translations parallel to the surface and rotation about the axis normal to the surface. A new intermolecular potential proposed by Righini and present authors was employed. It was found that the translation–rotation coupling plays an important role in determining the anharmonic frequency shift. The results are in good agreement with a recent neutron incoherent inelastic scattering experiment in which the momentum transfer is parallel to the graphite surface. The two main peaks observed around 9 and 5 meV are assigned to librational and translational modes, respect...

Phonon spectra of methane physisorbed on graphite

A high resolution neutron scattering study has been made of a solid solution of CH3D (15 mol %) in CH4 at temperatures down to T=1.5 K in order to obtain information about the energy states of CH3D molecules in the phase II structure, i.e., on sites of O and D2d symmetries. Measurements were made with spectrometers IN5 and IN10 at ILL, Grenoble, in the energy range 0<e<1.3 meV. ‘‘Free’’ rotation and tunneling transitions were observed for both CH3D and CH4 molecules. The results obtained for the tunneling transitions are used to refine the parameters of a model used previously to correlate neutron scattering and thermodynamic data for partially deuterated methanes in the more complicated phase III structure.

Kazuo Maki

Papers

An intermolecular potential for methane

Rotational States of NH+4 in Dilute Solution in Alkali Halide Lattices. III. Calculated Energy Levels of NH+4 in KBr

Splittings of the librational ground state of CH3D physisorbed on the surface of graphite

Phonon spectra of methane physisorbed on graphite

Energy states of CH3D molecules in the phase II structure of solid methane