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Chemical bonding is the fundamental feature/concern of any chemical species, which are
the building blocks of our nature. A line symbol was introduced to represent a chemical bond
in 15th century. The nature of a chemical bond was first illustrated in 1016 by Lewis, in the
form of the famous formulation of Lewis dot structure. The true nature of a chemical bond
was presented by Heitler and London in 1927 surprising everybody including Lewis. Over a
century, many new aspects of different chemical bonds have been addressed and illustrated.
However, the much things have not changed from the point of view of representation of a
chemical bond as originally suggested by Lewis. A covalent-coordinate bond is believed to
be the closed shell orbital interaction, while covalent electron-sharing bond is an open-shell
orbital interaction. The former is represented by an arrow as per suggestion of Sidwick
nearly a century ago. Chemical bonding, nature of orbital interactions, pairwise orbital
interactions can be analysed by energy decomposition analysis coupled with natural orbital
for chemical valence (EDA-NOCV). Quantification of ionic, covalent interaction energies and
dispersion energies can be estimated by EDA-NOCV analysis. This also provides the Pauli
repulsion energy between the fragments at the bonding distance. This total interaction
energy can be correlated with bond dissociation energy if the fragment preparation energy is
added to the total interaction energy. Additionally, EDA-NOCV calculations provide valuable
information on orbital interaction in the space, which provides captivating visualisation of drift
of the electron density as the deformation density plot (red to blue). In this presentation, we
will take different examples, and discuss different aspects of EDA-NOCV not only to explain
the bonding, but also to explain the reactivity of chemical species of complex systems. |