Alexandre Larin

Theoretical analysis of the synergism in the dielectric strength for SF6/CF4 mixtures

Available cross section data of electron–molecule processes are scaled to simulate the behavior of the dielectric strength (DS) in SF6 and CF4 gases at the level of the two-term approximation of the Boltzmann equation solution corresponding to the homogeneous electric field model. Then, the DS of mixtures is evaluated and compared to experimental data. The reasons of the synergism and its “asymmetry” relative to both components in SF6/CF4 mixtures are analyzed in terms of rate constant variations with respect to their values for the pure gases as well as in terms of weighted rate constants for all channels of electron energy losses relevant under breakdown conditions.

Lower order atomic multipole moments of the oxygen atoms of “small size” H-form aluminosilicate frameworks

The geometries of all possible Si‐O(H)‐Al Bronsted centres within five hydrogen form (H-form) aluminosilicate frameworks, ABW, CAN, CHA, EDI, and NAT, were optimised using a full periodic ab initio Hartree‐Fock LCAO scheme at the STO-3G level. A distributed multipole analysis (DMA) was then applied at the ps-21G(Al, Si)/6-21G(O, H) level in order to obtain the multipole moments. Simple analytical approximations of the dependence of the Mulliken charges and atomic dipole moments with respect to the average bond distance, the anisotropy between the Al‐O and Si‐O distances, and the Si‐O‐Al angle for all crystallographically independent types of oxygen atom were derived on the basis of all optimised models. The nuclear quadrupole coupling constants for 17 O are calculated for all oxygen positions and compared to the experimental values

Approximations of the Mulliken Charges for the Oxygen and Silicon Atoms of Zeolite Frameworks Calculated with a Periodic Hartree)Fock Scheme

Distributed multipole analysis DMA on the basis of periodic . Hartree)Fock PHF calculations, using the CRYSTAL code, is applied to five different all-siliceous zeolite models: chabazite, gmelinite, merlinoite, montesommaite, and RHO. Mulliken charges of the framework atoms were calculated with a pseudopotential ps-21G* basis set for silicon and a 6-21G* basis for oxygen. The charge values of the silicon atoms were approximated by a simple one-dimensional function with respect to the average Si—O distance within the respective SiO tetrahedra, whereas a two- 4 dimensional function with respect to the average Si—O distance and the Si—O—Si angle was used for the oxygen atoms. Both dependences were then utilized to evaluate the Mulliken atomic charges of 10 other frameworks with a larger number of atoms per unit cell. The validity of such application is confirmed by comparison with results obtained through direct PHF calculation for all-siliceous mordenite. Q 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 993)1001, 1998

Approximation of the Mulliken charges and dipole moments of the oxygen atoms of aluminophosphate sieves

Distributed multipole analysis on the basis of periodic Hartree‐Fock (PHF)-type calculations, using the CRYSTAL95 code is applied to 12 aluminophosphate molecular sieves. Several approximations of the dependence of the Mulliken atomic charges and dipole moments of the crystallographically independent oxygen atoms calculated with the STO-3G, ps-21G, and 6-21G basis sets are obtained with respect to three oxygen parameters (i.e. average value and difference between the Al‐O and P‐O bond distances, and Al‐O‐P angle). Some deflections from these approximate forms for the different oxygen types are discussed.

Theoretical estimation of the vibrational perturbation of the molecular properties of hydrogen adsorbed within a zeolite A framework

Abstract An iterative numerical procedure is proposed to evaluate the variation of the dependence versus the internuclear distance of several molecular properties (polarizabilities, multipole moments) of hydrogen adsorbed within zeolite A. Dealing with a method which includes only the vibrational perturbation, it is shown that the dependence on internuclear distance of the properties of H2 does not change upon adsorption in NaA as compared to the gas.

Quick scheme for evaluation of atomic charges in arbitrary aluminophosphate sieves on the basis of electron densities calculated with DFT methods

It is demonstrated that unique and simple analytical functions are justified for the atomic charge dependences q of the T (T = Al, P) and O atoms of aluminophosphates (AlPOs) using DFT calculations with several basis sets, starting from STO‐3G to 3‐21G and 6‐21G**. Three internal (bonds, angles, …) coordinates for the charge dependences of the T atoms and four coordinates for the O are sufficient to reach a precision of 1.8% for the fitted q(Al), 1.0% for q(P), and 2.5% for q(O) relatively to the values calculated at any basis set level. The proposed strategy consists in an iterative scheme starting from charge dependences based on the neighbors positions only. Electrostatic potential values are computed to illustrate the differences between the calculated and fitted charges in the considered AlPO models.

Electric field convergence versus atomic basis sets in all-siliceous zeolites

The electrostatic potential (EP) and electric field (EF) are calculated in the TON and CHA zeolites using periodic hybrid B3LYP, PBE, and PW91 functionals considering eight basis sets. Relative root mean square differences between the EP or EF values estimated between the different basis sets are evaluated in several domains available for adsorbate molecules in both zeolites. The EP is interpreted in terms of ionicity of the framework.

Theoretical and experimental analyses of the synergism in the dielectric strength for C3F8∕C2HF5 mixtures

Two theoretical approaches are applied in order to describe the behavior of the experimental breakdown voltage in C3F8∕C2HF5 mixtures. First, available cross section data of the electron-molecule processes are scaled to simulate the dielectric strength (DS) behavior in both C3F8 and C2HF5 gases at the level of the Boltzmann equation (BE) solution corresponding to a homogeneous electric field model. Then, the DS of C3F8∕C2HF5 mixtures is evaluated and compared to the experimental breakdown voltage measured using different electrode geometries as sphere/plane type via experimental techniques, i.e., power frequency and negative or positive polarity lightning impulse. Due to the limitation of the BE approach to describe the observed experimental data, the Hunter-Christophorou [J. Appl. Phys. 57, 4377 (1985)] model regarding the positive synergism phenomenon is finally applied. Both models allow us to compare relative values of the “chemical” and “physical” DS components in C3F8∕C2HF5 mixtures.

The Cumulative Coordinate Method for Describing the Electrostatic Potential and Field in Silica Zeolite Polymorphs

Hartree-Fock calculations with periodic boundary conditions were performed for the X-ray structure models of all-silica zeolites to determine the dependences of atomic multipole moments up to the fourth order inclusive with respect to the so-called cumulative coordinate. These dependences were used to describe the electrostatic potential and electric field in cross sections of several zeolites. The description was shown to be satisfactory provided the atomic charges were known. The deviations of atomic multipole moments in strongly distorted structures from those determined using the approximation suggested are discussed. The applicability scope of the cumulative coordinate method to all-silica zeolites is considered.

Similarities between Atomic Properties in Dense and Open Aluminophosphate Sieves

Abstract Experimental studies on the AlPO 4 -15 sieve are combined with theoretical analyses of the charge partitioning between the sieves and different (water, ammonium, i -propylamine) template molecules (TM) to ascertain the ionic character of the interactions occurring between the different partners. Positions of the TM protons are optimized using ab initio periodic Hartree-Fock calculations under fixed positions of the non-hydrogen atoms obtained by XRD. The critical points of the X temp -H…O sieve (X=N, O) hydrogen bonds as well as of the Al-O bonds of the experimental electron density are compared to the ones computed at different theory levels.