CNDO/2 Calculations of α‐Quartz Electronic Properties of the Perfect Cluster and EPR Parameters of the O^−; Vacancy

Abstract

The CNDO/2 method is applied to a cluster cut out from a perfect a-quartz crystal. The result is compared with that of a n extended Hiickel theory (EHT) calculation of the same cluster. The dominant character of valence band states in terms of atomic orbitals and the width of the energy gap are discussed. The formal charges of central cluster atoms are given. Furthermore E P R parameters of a relaxed 0vacancy in a-quartz are calculated on the basis of CNDO-RHF and -U H F approximations. The results are compared with those of an EHT calculation of the same model and with the experimental result of the E; centre in SiO?. Die CNDO/P-Methode wird auf einen ungestorten a-Quarz-Cluster angewandt und das Ergebnis mit dem einer EHT-Rechnung des gleichen Clusters verglichen. Der Charakter der Zustande im Valenzband nach ihrem Atomorbital-Anteil und die Breite der verbotenen Zone werden erortert. Die Formalladungen der zentralen Clusteratome werden angegeben. Ferner werden die auf der Basis von CNDO-RHF und -UHF-Rechnungen ermittelten EPR-Parameter fur eine relaxierte O--Vakanz in a-Quarz angegeben und diskutiert. Das Ergebnis wird mit dem einer EHT-Rechnung a m gleichen Nodell und dem experimentellen Ergebnis fur das E;-Zentrum in SiO, verglichen.

Mobility and Clustering of Rh and Ru in Zeolite. Experiment and Theory

“…For insulators and semiconductors dangling bonds at the truncating surface are usually saturated with pseudoatoms. For silica supports the dangling bonds of oxygen and silicon at the surface often are very successfully simply saturated with hydrogen atoms. − The specific model structures for the supported Rh mono- and dicarbonyl adsorbed systems are shown in Figure . The use of three hydrogen atoms to truncate the silica cluster is regarded as a minimal first approximation to a silica structure.…”

Quantum chemical investigation of support-metal interactions and their influence on chemisorption. 1. The system atomic hydrogen...nickel-silicon dioxide

“…Siy orbitals were not taken into consideration since the essential electronic valence band properties of Si02 can be explained quite well within the CNDO/2 framework using SÍ& and Si3p orbitals only.32 Such cluster calculations gave realistic charge distribution compared with ab initio and experimental results, realistic gap and valence band widths, and the correct sequence of peaks in the valence band density of states. 32 The open-shell systems were exclusively calculated by using the CNDO-UHF method. 30 To overcome convergence difficulties we adapted a special mixing procedure proposed by Bacon and Zerner33 which proved more successful with our models than the level shifting procedure.34 The The interaction energy of two subsystems X and Y was calculated from the total energies of these systems, E(X) and E{Y), and of the whole system, E(XY): AE(X,Y) = E(XY) -(E(X) + E(Y))…”

Determination of Titanium CNDO Parameters and Application in Cluster Calculations of Different Titanium Compounds

“…The gap width of 19.7 eV, calculated as the energy difference A& between LUMO (lowest unoccupied MO) and HOMO, is much too large compared to the experimental value but, as pointed out in [39], the gap should rather be identified with the singletsinglet transition energy AEG of the closed-shell system within the framework of the Hartree-Fock approximation. The corresponding value of 12.7 eV is smaller but still [50].…”

“…The nuclear charge of the pseudo hydrogen atoms H is +4/3. The O H distance is chosen to be 97 pm like the OH distance in the case of SiO, cluster calculations [39]; straight Ti-0-H bonds are used.…”