摘要:

Dedicated to Dr Gerhard Herzberg on the occasion of his 90th birthday Recent large-scaleab initiocalculations for small negative molecular ions are reviewed. Accurate equilibrium geometries are established for several species like NH2−, HCC−, NO2−, CH2N−C5−and C6−. Predictions are made for various spectroscopic properties like vibrational frequencies, rotational constants and infrared intensities. The effects of a shallow energy minimum in the T-shaped configuration on the rovibrational term energies of HCC−are investigated. The calculated vibrational structures of the photoelectron spectra of SiH3−and CH2N−are in very good agreement with the experiment. The present calculations support the assignment of an absorption observed at 608 nm in a neon matrix to the [Ctilde]2Πg← [Xtilde]2Πutransition of C6−. Electron affinities are obtained with an accuracy of 0-05 eV or better. Spectroscopic properties of intermediates in simple SN2reactions are calculated.

ISSN:0144-235X    

DOI:10.1080/01442359509353308

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Because the connection factors between theoretical and experimental results of thermodynamic quantities are given through the molecular interaction parameters, for chemical engineering applications it is necessary to use exact values of these parameters for a determined molecular interaction model. Because the results of computer simulation may be considered ‘exact’ for a determined intermolecular potential, it makes an excellent tool for investigating this connection. The purpose of the present work is to propose a procedure for determining interaction parameters for fluids by forcing agreement between the values of pressure obtained from empirical Equations of state in phase space regions where we are sure they are most exact and those obtained from computer simulation.

ISSN:0144-235X    

DOI:10.1080/01442359509353309

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We discuss the control of the kinetics and dynamics of chemical reactions by the solvent, from a molecular point of view. The kinetics are discussed using a transition state theory (TST) approach, applied to the reactants and their surrounding solvent as one supramolecule. The topics discussed include a molecular interpretation for the changes that take place when one solvent is being replaced by another; the use of local against normal vibrational modes and/or joint description, i.e., local modes for part of the system and normal modes for the other part; and the effect of pressure on the rate in solution. The notion of free volume and volume of activation is extended to a more general phase space in which geometrical volumes may overlap, the approximations that are inherent to cell theory are examined and a molecular interpretation for internal and chemical pressures is suggested. The link to the dynamics is provided by an analysis of the breakdown of TST due to diffusion/cage control of the rate of the reaction. A unified description which interpolates from activation control to diffusion control is presented with a special emphasis on the motion within the solvation cage. Results of molecular dynamics simulations for both activated and activationless reactions are presented. The very detailed computer experiment is interpreted using a reduced mechanical description and the separation of time-scales is discussed using an adiabatic separation of variables. Spectroscopic methods for probing the different time epochs are suggested. The rather short duration typical of the motion within the solvent cage is emphasized, and the possibilities that this affords for studying the short-time dynamical role of the solvent via experiments in clusters or in glasses are noted.

ISSN:0144-235X    

DOI:10.1080/01442359509353310

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Dipolar systems, both liquids and solids, constitute a class of naturally abundant systems that are important in all branches of natural science. The study of orientational relaxation provides a powerful method to understand the microscopic properties of these systems and, fortunately, there are many experimental tools to study orientational relaxation in the condensed phases. However, even after many years of intense research, our understanding of orientational relaxation in dipolar systems has remained largely imperfect. A major hurdle towards achieving a comprehensive understanding is the long range and complex nature of dipolar interactions which also made reliable theoretical study extremely difficult. These difficulties have led to the development of continuum model based theories, which although they provide simple, elegant expressions for quantities of interest, are mostly unsatisfactory as they totally neglect the molecularity of inter-molecular interactions. The situation has improved in recent years because of renewed studies, led by computer simulations. In this review, we shall address some of the recent advances, with emphasis on the work done in our laboratory at Bangalore. The reasons for the failure of the continuum model, as revealed by the recent Brownian dynamics simulations of the dipolar lattice, are discussed. The main reason is that the continuum model predicts too fast a decay of the torque-torque correlation function. On the other hand, a perturbative calculation, based on Zwanzig's projection operator technique, provides a fairly satisfactory description of the single particle orientational dynamics for not too strongly polar dipolar systems. A recently developed molecular hydrodynamic theory that properly includes the effects of intermolecular orientational pair correlations provides an even better description of the single-particle orientational dynamics. We also discuss the rank dependence of the dielectric friction. The other topics reviewed here includes dielectric relaxation and solvation dynamics, as they are intimately connected with orientational relaxation. Recent molecular dynamics simulations of the dipolar lattice are also discussed. The main theme of the present review is to understand the effects of intermolecular interactions on orientational relaxation. The presence of strong orientational pair correlation leads to a strong coupling between the single particle and the collective dynamics. This coupling can lead to rich dynamical properties, some of which are detailed here, while a major part remains yet unexplored.

ISSN:0144-235X    

DOI:10.1080/01442359509353311

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

New microcanonical theories for the angular distributions of reactive scattering arising from long-lived collision complexes and their application to a wide range of transition state geometries are reviewed. Transition states formed at the top of centrifugal barriers on potential energy surfaces without a potential energy barrier are well described by symmetric top transition states, provided that the loosening of bending mode vibrations into product rotation is included. A phase space description appropriate for collisions at small impact parameters can be incorporated into a consistent mathematical description. Tight transition states formed at the top of a potential energy barrier and involving H atom displacement are described by an asymmetrical top transition state theory which includes all the symmetric top configurations as limiting cases. However the effects of transition state flexibility rapidly obscure the observable consequences of preferred geometry for directions of dissociation away from the principal axes. These theories are used to explore the effects of structural isomerism of collision complexes in studies of a wide range of F and O atom abstraction and displacement reactions, where the structure and interconversion of highly vibrationally excited complexes is related to the nature of the chemical bonding involved.

ISSN:0144-235X    

DOI:10.1080/01442359509353312

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

… these atoms move in the infinite void, separate one from another and differing in shapes, sizes, position and arrangement; overtaking each other they collided, and some are shaken away in any chance direction, while others, becoming intertwined one with another according to the congruity of their shapes, sizes, positions, and arrangements, stay together and so effect the coming into being of compound bodies. (Simplicius)

ISSN:0144-235X    

DOI:10.1080/01442359509353313

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Reactions of solids exposed to non-isothermal heating reveal diverse isokinetic relationships (IKR). Some of these effects are exclusively inherent to non-isothermal reactions of solids. The present paper briefly surveys the most commonly encountered IKRs, classifying them into two main types: ‘false’ and ‘true’. The origin of ‘false’ IKRs, their meaning and their possible practical application, based on the related isokinetic temperatures are elucidated. For ‘true’ IKRs emphasis is put on the correlation between the experimentally found isokinetic temperature, characteristic bands in infrared spectra, and the mechanism of process. Theoretical considerations are illustrated by experimental examples for reactions of inorganic substances and polymers.

ISSN:0144-235X    

DOI:10.1080/01442359509353314

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A great deal of experimental effort has been put toward measurements of integral and differential, state-to-state cross-sections for rotationally inelastic energy transfer. Throughout the years measurements in thermal gas cells, and in crossed molecular beams, have been performed at increasingly impressive levels of quantum state detail. Because the term ‘rotational energy transfer’ can include collisional interchange among nuclear rotational states, magnetic sublevels, electron spin and orbital quantum levels, and vibrational angular momentum states, and can also include rotation-translation/vibration energy transfer, the field is an expansive one. In this review an array of experimental studies is encapsulated, including discussion of quantum-state propensities, their known or speculative physical origins, and the success or failure of simple energy transfer models. Discussion of progress toward the development of accurate, intermolecular potential energy surfaces, and the results of classical or quantum scattering calculations, accompanies the overview of experimental work.

ISSN:0144-235X    

DOI:10.1080/01442359509353315

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

In order to interpret spectral observations of interstellar clouds in terms of the physical and chemical conditions which prevail within them, a large body of atomic and molecular data is required. We consider both diffuse clouds, which are permeated by the background ultraviolet radiation field, and dense clouds, whose interiors are shielded from the external ultraviolet radiation by dust scattering and absorption in the outer layers. The categories of chemical reaction which are known or believed to be important are reviewed. The results of chemical models that attempt to simulate conditions in the clouds are summarized. The phenomenon of bistability of the steady-state solutions of the chemical rate equations is considered in detail. Interstellar clouds are perturbed from static equilibrium by shock waves, produced by a variety of mechanisms. The structure of shocks and their influence on the physical and chemical state of the medium are described. Processes intervening in the thermal balance of the medium and the exchange of energy with the environment are discussed.

ISSN:0144-235X    

DOI:10.1080/01442359509353316

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

ISSN:0144-235X    

DOI:10.1080/01442359509353317

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor