TAILIEUCHUNG - Ideas of Quantum Chemistry P106

Ideas of Quantum Chemistry P106 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. | 1016 S. POPULATION ANALYSIS N EEE EPrsSrs-A reA B seB Afterwards we may choose the following partitionings Atomic partitioning N qA qA PrsSrs A re B seB Mulliken where q are called the Mulliken charges. They are often calculated in practical ap-charges plications and serve to provide information on how much of the electronic den- sity p is concentrated on atom A. Such a quantity is of interest because it may be directly linked to the reactivity of atom A often identified with its ability to be attacked by nucleophilic or electrophilic agents. 2 Also if we measure the dipole moment we would like to know why this moment is large in a molecule. By performing Mulliken analysis we might be able to identify those atoms that are responsible for this. This might be of value when interpreting experimental data. Atomic and bond partitioning The summation may also be performed in a slightly different way N PrsSrs E 2 EE PrsSrs E A E qAB-A r seA A B reA seB A A B The first term represents the contributions qA of the atoms the second pertains to the atomic pairs qAB. The latter populations are large and positive for those pairs of atoms for which chemists assign chemical bonds. The bond population qAB may be treated as a measure of whether in the A - B atomic interaction bonding or antibonding character If for two atoms qAB 0 we may say that they are not bound by any chemical bond if qAB is large then we may treat it as an indication that these two atoms are bound by a chemical bond or bonds. 2We have to remember that besides electrons this atom has a nucleus. This has to be taken into account when calculating the atomic net charge. 3Prs is the sum over the occupied orbitals of the products of the LCAO coefficients of two atoms in each of the occupied molecular orbitals. The equal signs of these coefficients with Srs 0 means a bonding interaction recall Chapter 8 and Appendix R on p. 1009 and such a contribution increases Prs. The opposite signs of the coefficients .

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