Abstract:
Chemical bonding is at heart but, not being a quantum mechanical-defined
physical property of a system, is a subject of endless and often fruitless debates.
Having so many and very different models of chemical bonding without knowing
what this really is does not make it easier. There is, however, a general
agreement that concentrating electron density (ED) in and delocalizing ED to
internuclear region is always associated with minimizing system's energy and
synonymous with chemical bonding. Fragment, atomic, localized, delocalized,
and interatomic (FALDI)-based density analysis involves entire space occupied
by a molecule. From this molecular-wide and density-based methodology, it is
possible to quantify localized and delocalized by all atoms ED at any coordinate
r, including critical points on Bader's molecular graphs. Each atom and
atom-pair contributions of delocalized density are quantified to reveal major
players in the all-atom and molecular-wide chemical bonding. Partitioning the
total ED to individual molecular or natural orbital's contributions using
MO-ED and MO-DI methods, in conjunction with one dimensional (1D) cross
section methodology, generates an orbital-based molecular-wide picture. This
provides, besides reproducing results from FALDI, qualitative description of
orbitals' nature that correlates well with classical understanding of bonding, nonbonding, and antibonding orbitals. A qualitative and quantitative impact
of an immediate, distant, or molecular-wide molecular environment on intraand
intermolecular di-atomic, intra- and interfragment interactions is the
domain of the Fragment Attributed Molecular System Energy Change
(FAMSEC) family of methods. The FALDI, FAMSEC, MO-ED, MO-DI, and 1D
cross section methodologies provide consistent and quantifiable physics-based
picture of molecular-wide chemical bonding without invoking unicorns, such
as a chemical bond.
