Maurice Leslie

Bridging hydrodyl groups in zeolitic catalysts: a computer simulation of their structure, vibrational properties and acidity in protonated faujasites (HY zeolites)

Abstract The structure and properties of the four possible bridging hydroxyl groups in silicon-rich H-faujasite (zeolite Y) are studied by lattice-energy minimization carried out within the classical shell model. The most likely proton-accepting sites are the O1 and O3 oxygens. Their hydroxyl stretching frequencies are shown to be responsible for the characteristic high-frequency (HF) and low-frequency (LF) bands, respectively, in the infrared spectra of H-faujasites. The variation of the OH stretching frequencies for the four isolated sites is correlated with changes of local geometry such as the Oue5f8H bond length and the Siue5f8O(H)ue5f8Al and the Siue5f8Oue5f8H angles. However, there appears to be no correlation with differences in the acid strength as evidenced by the values of the calculated deprotonation energy of the O(1)H and O(3)H hydroxyl groups for which only a minor difference is predicted.

THE PARAMETRIZATION OF A THOLE-TYPE ALL-ATOM POLARIZABLE WATER MODEL FROM FIRST PRINCIPLES AND ITS APPLICATION TO THE STUDY OF WATER CLUSTERS (N=2-21) AND THE PHONON SPECTRUM OF ICE IH

We present the parametrization of a new polarizable model for water based on Thole’s method [Chem. Phys. 59, 341 (1981)] for predicting molecular polarizabilities using smeared charges and dipoles. The potential is parametrized using first principles ab initio data for the water dimer. Initial benchmarks of the new model include the investigation of the properties of water clusters (n=2–21) and (hexagonal) ice Ih using molecular dynamics simulations. The potential produces energies and nearest-neighbor (H-bonded) oxygen–oxygen distances that agree well with the ab initio results for the small water clusters. The properties of larger clusters with 9–21 water molecules using predicted structures from Wales et al. [Chem. Phys. Lett. 286, 65 (1998)] were also studied in order to identify trends and convergence of structural and electric properties with cluster size. The simulation of ice Ih produces a lattice energy of −65.19 kJ/mol (expt. −58.9 kJ/mol) with an average dipole moment of 2.849 D. The calculated...

The MD simulation of the equation of state of MgO: Application as a pressure calibration standard at high temperature and high pressure

Abstract Molecular dynamics (MD) simulation is used to calculate the elastic constants and their temperature and pressure derivatives, and the T-P-V equation of state of MgO. The interionic potential is taken to be the sum of pairwise additive Coulomb, van der Waals attraction, and repulsive interactions. In addition, to account for the observed large Cauchy violation of the elastic constants of MgO, the breathing shell model (BSM) is introduced in MD simulation, in which the repulsive radii of O ions are allowed to deform isotropically under the effects of other ions in the crystal. Quantum correction to the MD pressure is made using the Wigner-Kirkwood expansion of the free energy. Required energy parameters, including oxygen breathing parameters, were derived empirically to reproduce the observed molar volume and elastic constants of MgO, and their measured temperature and pressure derivatives as accurately as possible. The MD simulation with BSM is found to be very successful in reproducing accurately the measured molar volumes and individual elastic constants of MgO over a wide temperature and pressure range. The errors in the simulated molar volumes are within 0.3% over the temperature range between 300 and 3000 K at 0 GPa, and within 0.1% over the pressure range from 0 up to 50 GPa at 300 K. The simulated bulk modulus is found to be correct to within 0.7% between 300 and 1800 K at 0 GPa. Here we present the MD simulated T-P-V equation of state of MgO as an accurate internal pressure calibration standard at high temperatures and high pressures

THE RELAXATION OF MOLECULAR CRYSTAL STRUCTURES USING A DISTRIBUTED MULTIPOLE ELECTROSTATIC MODEL

We describe a method for minimizing the lattice energy of molecular crystal structures, using a realistic anisotropic atom–atom model for the intermolecular forces. Molecules are assumed to be rigid, and the structure is described by the center of mass positions and orientational parameters for each molecule in the unit cell, as well as external strain parameters used to optimize the cell geometry. The resulting program uses a distributed multipole description of the electrostatic forces, which consists of sets of atomic multipoles (charge, dipole, quadrupole, etc.) to represent the lone pair, π electron density, and other nonspherical features in the atomic charge distribution. Such ab initio based, electrostatic models are essential for describing the orientation dependence of the intermolecular forces, including hydrogen bonding, between polar molecules. Studies on a range of organic crystals containing hydrogen bonds are used to illustrate the use of this new crystal structure relaxation program, DMAREL, and show that it provides a promising new approach to studying the crystal packing of polar molecules.

THE LATTICE-DYNAMICS AND THERMODYNAMICS OF THE MG2SIO4 POLYMORPHS

We use an approach based upon the Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the phonon dispersion of forsterite and the lattice dynamical behaviour of the beta-phase and spinel polymorphs of Mg2SiO4. The potential used (THB1) was derived largely empirically using data from simple binary oxides, and has previously been successfully used to model the infrared and Raman behaviour of forsterite. It includes ‘bond bending’ terms, that model the directionality of the Si-O bond, in addition to the pair-wise additive Coulombic and short range terms. The phonon dispersion relationships of the Mg2SiO4 polymorphs predicted by THB1 were used to calculate the heat capacities, entropies, thermal expansion coefficients and Gruneisen parameters of these phases. The predicted heat capacities and entropies are in outstandingly good agreement with those determined experimentally. The predicted thermodynamic data of these phases were used to construct a phase diagram for this system, which has Clausius-Clapeyron slopes in very close agreement with those found by experiment, but which has predicted transformation pressures that show less close agreement with those inferred from experiment. The overall success, however, that we have in predicting the lattice dynamical and thermodynamic properties of the Mg2SiO4 polymorphs shows that our potential THB1 represents a significant step towards finding the elusive quantitative link between the microscopic or atomistic behaviour of minerals and their macroscopic properties.

Siting of AI and bridging hydroxyl groups in ZSM-5: A computer simulation study

Abstract The siting of AI and of bridging OH groups in zeolite HZSM-5 is studied by defect energy minimization adopting the classical shell model. The small energy differences between the various lattice sites suggest that there would be little deviation from a random distribution of bridging hydroxyl groups and AI over the possible framework sites in contrast to previous suggestions.

Molecular dynamics studies of hydrocarbon diffusion in zeolites

Molecular dynamics simulation techniques have been used to investigate behaviour of sorbed CH4 and C2H4 in zeolite ZSM-5. Our calculations allowed for framework in addition to molecular motions. Diffusion trajectories were obtained for the sorbed molecules. Simulated diffusion coefficients were in acceptable agreement with experiment.

Computer modelling studies of defects and valence states in La2CuO4

Inter-atomic potential models have been derived for La2CuO4. The models correctly reproduce the structure of the orthorhombic phase, and yield calculated phonons dispersion curves showing a soft mode. Defect simulations show a large energy barrier to the Cu2+ disproportionation reaction, but find that doping of the material and oxidation of the doped material are energetically favoured. The calculations show that the latter takes place with formation of Cu3+ ions. Bipolaron formation is also investigated.

The thermal stability of lattice-energy minima of 5-fluorouracil: Metadynamics as an aid to polymorph prediction

This paper reports a novel methodology for the free-energy minimization of crystal structures exhibiting strong, anisotropic interactions due to hydrogen bonding. The geometry of the thermally expanded cell was calculated by exploiting the dependence of the free-energy derivatives with respect to cell lengths and angles on the average pressure tensor computed in short molecular dynamics simulations. All dynamic simulations were performed with an elaborate anisotropic potential based on a distributed multipole analysis of the isolated molecule charge density. Changes in structure were monitored via simulated X-ray diffraction patterns. The methodology was used to minimize the free energy at ambient conditions of a set of experimental and hypothetical 5-fluorouracil crystal structures, generated in a search for lattice-energy minima with the same model potential. Our results demonstrate that the majority ( approximately 75%) of lattice-energy minima are thermally stable at ambient conditions, and hence, the free-energy (like the lattice-energy) surface is complex and highly undulating. Metadynamics trajectories (Laio, A.; Parrinello, M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12562) started from the free-energy minima only produced transitions that preserved the hydrogen-bonding motif, and thus, further developments are needed for this method to efficiently explore such free-energy surfaces. The existence of so many free-energy minima, with large barriers for the alteration of the hydrogen-bonding motif, is consistent with the range of motifs observed in crystal structures of 5-fluorouracil and other 5-substituted uracils.

DL_MULTI—A molecular dynamics program to use distributed multipole electrostatic models to simulate the dynamics of organic crystals

DL_MULTI has been developed to extend the Molecular Dynamics simulation program DL_POLY to model rigid molecules whose intermolecular interactions include a distributed multipole model for the electrostatic interactions. The adaptations use anisotropic atom–atom potentials, corresponding to atomic multipoles up to hexadecapole. The lattice sums of these multipoles are evaluated using the Ewald method, using a technique utilizing Stones S functions which describe the multipoles in a molecule fixed reference frame. An algorithm for determining suitable cutoffs is described and errors in the direct space part of the Ewald summation discussed. Thus DL_MULTI provides a general purpose MD program for studying polar rigid organic molecules in their liquid and crystalline states with a realistic intermolecular potential suitable for studying polymorphism. Example applications to uracil and 5-azauracil show that, with the new summation method, a realistic electrostatic model can be used without excessive computer time.