Topic 1.5. Application of Density Functional Theory to Chemical Properties and Reactivity

Topic 1.5. Application of Density Functional Theory to Chemical Properties and Reactivity 

The qualitative ideas of valence bond (VB) theory provide a basis for understanding the relationships between structure and reactivity. Molecular orbital (MO) theory offers insight into the origin of the stability associated with delocalization and also the importance of symmetry. As a central premise of density functional theory (DFT) is that the electron density distribution determines molecular properties, there has be an effort to apply DFT to numerical evaluation of the qualitative concepts such as electronegativity, polarizability, hardness, and softness. The sections that follow explore the relationship of these concepts to the description of electron density provided by DFT. 

DFT suggests quantitative expressions and interrelation of certain properties such as electronegativity and polarizability, and the related concepts of hardness and sotfness introduced in Sections 1.1.3 through 1.1.6. DFT calls the escaping tendency of an electron from a particular field its chemical potential, p, defined by 

A stable system, such as a molecule, attains a common chemical potential among its components. That is, there is no net force to transfer electron density from one point to another. The idea that chemical potential is equivalent throughout a molecule, and specifically between bonded atoms, accords with the concept of electronegativity equalization (see Section 1.1.4). Chemical potential is related to electrophilicity and nucleophilicity. A system with an attraction toward electrons is electrophilic, whereas a system that can donate electrons is nucleophilic. Chemical potential is considered to be the opposite of absolute (Mulliken) electronegativity and can be approximated by 

---