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Ideas of Quantum Chemistry P74

Ideas of Quantum Chemistry P74 shows how quantum mechanics is applied to chemistry to give it a theoretical foundation. The structure of the book (a TREE-form) emphasizes the logical relationships between various topics, facts and methods. It shows the reader which parts of the text are needed for understanding specific aspects of the subject matter. Interspersed throughout the text are short biographies of key scientists and their contributions to the development of the field. | 696 13. Intermolecular Interactions Fig. 13.5. A perturbation of the wave function is a small correction. Fig. a shows in a schematic way how a wave function spherically symmetric with respect to the nucleus can be transformed into a function that is shifted off the nucleus. The function representing the correction is shown schematically in Fig. b . Please note the function has symmetry of a p orbital. starting 0 function. This tiny deformation is the target of the expansion in the basis set 1 . In other words the perturbation theory involves just a cosmetic adjustment of the q0 add a small hump here Fig. 13.5 subtract a small function there etc. Therefore the presence of the excited wave functions in the formulae is not an argument for observing some physical excitations. We may say that the system took what we have prepared for it and we have prepared excited states. This does not mean that the energy eigenvalues of the molecule have no influence on its induction or dispersion interactions with other molecules.11 However this is a different story. It has to do with whether the small deformation we have been talking about does or does not depend on the energy eigenvalues spectrum of the individual molecules. The denominators in the expressions for the induction and dispersion energies suggest that the lower excitation energies of the molecules the larger their deformation induction and dispersion energies. intermolecular distance 13.6.3 INTERMOLECULAR INTERACTIONS PHYSICAL INTERPRETATION Now the author would like to recommend the reader to study the multipole expansion concept Appendix X on p. 1038 also cf. Chapter 12 p. 624 . The very essence of the multipole expansion is a replacement of the Coulombic interaction of two particles one from molecule A the other from the molecule B by an infinite sum of interactions of what are called multipoles where each interaction term has in the denominator an integer power of the distance called the intermolecular distance R