ABSTRACT
Intermolecular Interaction Cases ..................................................................400 19.5.1 Single Interacting Sites .....................................................................400 19.5.2 Multiple Interacting Sites: Localized Reactive Model and
Smeared Reactive Model ..................................................................403 19.6 Reactivity and Stability of DNA Bases and Their Base Pairs in
Presence of Electric Field .............................................................................405 19.7 Conclusions ................................................................................................... 411 Acknowledgments .................................................................................................. 412 References .............................................................................................................. 412
The challenging problems in chemistry are the understanding of the structure as well as reactivity of the systems at the molecular level and the prediction of a reaction pathway/mechanism.1-6 This is primarily due to the existence of numerous varieties of chemical reactions and phenomena. Many experimental and theoretical groups have shown lots of interest and explained the nature of bonding and reactivity of the systems.1-10 Most of the explanations were based on some intuitive ideas and empirical rules that were essentially derived from several experimental observations and many chemical facts. Among the many seminal works of different theoretical and experimental groups in correlating the structure and reactivity of molecular systems, the well-known and most celebrated principle is the hard-soft acid-base (HSAB) principle introduced by Pearson in 1963.11-13 The principle states that there is an extra stabilization when the soft acid combines with soft base and the hard acid combines with hard base. Based on the large amount of chemical information from the experimental observations, this principle was proposed. The concepts of hardness and softness are, in general, related to the ionic (or electrostatic) and covalent (polarizing power or polarizability) types of interactions, respectively. The HSAB principle has been very successful in rationalizing most of the acid-base types of reactions at a qualitative level and is very popular among the chemist community because of its wider applicability and simplicity.13-15 However, theoretical quantication of this qualitative principle is considered to be a great challenge. As the general concept of hardness and softness parameters lacks the mathematical denition or physical basis, the predictions and explanations made based on these descriptors remained at the qualitative level. Hence, the necessity for an intuitive and correct theoretical approach for these concepts was inevitable. The progress in this direction was realized through the density functional theory (DFT) applications, now known as conceptual DFT, which has been used to explain the hardness and softness parameters along with other useful concepts.16-21
There have been numerous works in terms of monographs and reviews on the conceptual DFT, bringing out the usefulness of these descriptors.22-26 Hence, we will very briey outline the reactivity descriptors without going into the details of their applications. In this review article, the objective is to highlight the quantitative features of the HSAB principle. More importantly, we will (1) illustrate the formal mathematical formulation of the HSAB principle and the local version of the HSAB principle, (2) illustrate the transformation of this qualitative principle into a quantitative formulation, especially in deriving the essential features of the chemical bonding and reaction energies in a generalized way, and (3) validate the applicability of these methods. The reactivity and stability of the DNA base-pair complexes in the presence of external eld have been considered.
