Advances in Charge Displacement Analysis

We
define new general density-based descriptors for the quantification
of charge transfer and polarization effects associated with the interaction
between two fragments and the formation of a chemical bond. Our aim
is to provide a simple yet accurate picture of a chemical interaction
by condensing the information on the charge rearrangement accompanying
it into a few chemically meaningful parameters. These charge displacement
(CD) parameters quantify the total charge displaced upon bond formation
and decompose it into a charge transfer component between the fragments
and charge rearrangements taking place within the fragments. We then
show how the new parameters can be easily calculated using the well-known
CD function, which describes the charge flow along a chosen axis accompanying
the formation of a bond. The approach presented here can be useful
in a wide variety of contexts, ranging from weak interactions to electronic
excitations to coordination chemistry. In particular, we discuss here
how the scheme can be used for the characterization of the donation
and back-donation components of metal–ligand bonds, in combination
with the natural orbitals for chemical valence (NOCV) theory. In doing
so, we discuss the interesting relationship between the proposed parameters
and the corresponding NOCV eigenvalues, commonly used as a measure
of the electron charge displacement associated with a given bonding
contribution. As a prototype case study, we investigate the bond between
a N-heterocyclic carbene and different metallic fragments. Finally,
we show that our approach can be used in combination with the energy
decomposition of the extended transition state method, providing an
estimate of both charge transfer and polarization contributions to
the interaction energy.