摘要:

Recent advances in the computational techniques available to characterize spin-forbidden processes with the Breit-Pauli approximation are reviewed and the use of these techniques is illustrated. These computational advances include:

ISSN:0144-235X    

DOI:10.1080/01442359209353270

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Aspects of the aromaticity concept are reviewed: aromatic characters and measures of aromaticity. Aromatic character is defined by certain physical and chemical properties that characterize cyclic organic molecules, including stability, reactivity, and ring current effects. Measures of aromaticity are numerical indices devised to understand and to predict aromaticity. Emphasis is put on recent developments of theoretical indices, especially the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) or chemical hardness.

ISSN:0144-235X    

DOI:10.1080/01442359209353271

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Chemical reactions and physical processes occurring during matrix devitrification are studied to reveal important factors concerning low-temperature, isotropic, amorphous structures. Reactions in this regime allow active observation of supercooled reagents well below their melting temperatures. It is possible to selectively carry out and observe chain reactions in the vitrifying matrix, whereas in crystal lattices such reactions are hindered due to low translational reagent mobility effects. Very low-temperature reactions allow selectivity of pathways in the vitrifying medium; entropy effects are minimized and exothermic pathways are favoured.

ISSN:0144-235X    

DOI:10.1080/01442359209353272

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Velocity-selective excitation can be achieved by tuning a narrow-band laser within the Doppler-broadened absorption profile of a gas. If the excitation modifies the kinetic properties of the molecules (e.g., the kinetic cross-sections or the molecule-surface interactions), the Maxwell velocity distribution will be distorted. This gives rise to a new class of kinetic effects that do not require external gradients to be imposed on the system; rather, the laser intensity and its gradient can be considered as thermodynamic forces. Examples are light-induced drift, surface light-induced drift and light-induced viscous flow. A phenomenological overview is given of the various light-induced kinetic effects which can arise in pure gases or mixtures. This includes effects resulting from velocity-selective excitation immediately followed by collisional de-excitation (‘velocity-selective heating’). The theoretical description of the various effects is briefly reviewed. Experiments are described which investigate these effects in molecular systems, where the excitation is ro-vibrational, and their results discussed. So far, data have been obtained for CH3F, NH3, CO2, OCS, CH3OH, CH3Br, SF6and C2H4. Applications include determination of the vibrational-state dependence of intermolecular potentials, dependence of molecule-surface interactions upon the rotational state (J, MJ), isotope separation, ortho-para conversion rates and possible astrophysical applications.

ISSN:0144-235X    

DOI:10.1080/01442359209353273

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

This review will concentrate on new theoretical methods for solving the quantum dynamics of molecular systems. The approach will be that of solving the time dependent Schrödinger equation, and extracting from it the measurable quantities of experimental interest. Applications to the modelling of molecular photodissociation processes and to the theory of reactive molecular collisions will be discussed.

ISSN:0144-235X    

DOI:10.1080/01442359209353274

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Oxygen planes of clay minerals are formed by a continuous linkage of [SiO4]4-tetrahedra. This plane determines many of the clay colloidal and surface properties. The dπ-pπbond between O and Si in Si-O-Si and Si-O-Al groups is described here according to the valence bond treatment. Partial π interaction between O and Si causes the O atom to serve as a weak base. Tetrahedral substitution of Si by Al leads to changes in the clay surface properties, resulting from an increase in the basic strength of the siloxanes. Substitution of Mg by Al in octahedral sheets increases the polarization of the OH planes. Interlayer hydrogen bonds between oxygens and hydroxyls were detected in substituted serpentines and chlorites. Wettability of tetrahedral-octahedral-tetrahedral (TOT) clay minerals was determined from advancing contact angles of water. This angle decreased with increasing degree of tetrahedral substitution, indicating an increase in strength of hydrogen bonds between water and the oxygen plane, due to an increase in its basic strength. Hydrogen bonded water may serve as bridges between parallel layers and, consequently, the swelling of TOT minerals with tetrahedral substitution is prevented to some extent. Interactions occurring between the oxygen plane and acid sites in the interlayer space of smectites and vermiculites affect the bulk surface acidity of the interlayer and the type of reactions which occur between the clay and adsorbed organic acids and bases. Clays with tetrahedral substitution form π bonds by donating electrons to aromatic cations and radicals.

ISSN:0144-235X    

DOI:10.1080/01442359209353275

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

ISSN:0144-235X    

DOI:10.1080/01442359209353276

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor