Chemistry Reference
In-Depth Information
of the electron density” [
12
]. The physical interpretations of the source function
discussed in the previous section foresee this function as capable to mirror in some
way the effects that typical
chemistry changes
, such as chemical bonding, chemical
substitution, and chemical environment, or
paradigms
, such as chemical transfer-
ability and the notion of chemical groups, bring into the electron density. We
challenge, in the following, whether such an anticipated ability is warranted.
We start by considering how the SF may provide an innovative and peculiar
view of how transferability realizes in chemistry; we next move to the SF analysis
of very simple and well-established prototypical chemical bond patterns so as to get
some feeling on how the SF describe and possibly distinguish their different nature.
Later, hydrogen-bonded systems and metal-metal or metal-ligand interactions in
organometallics are inspected to challenge the information the SF is able to provide
in less conventional bonding cases.
3.1 Chemical Transferability and the Source Function
Usually, chemical transferability of a given piece of matter - e.g., a group of atoms
or a molecule - is examined in terms of the constancy, to a given extent, of a
number of its properties despite the different chemical environment in which it is
placed.
Perfect transferability
is achieved when the electron density of such a piece
of matter is fully transferable [
6
], while (partial) transferability of only some of its
properties may realize through what has been termed
compensatory transferability
[
25
,
26
]. This might be for instance the case of a constant electron population for an
atomic group, which realizes either because of a compensation of charge transfers
within the atoms of the group, or even through self-charge polarization mechanisms
within one or more atoms of the group. The group population remains the same, but
the electron density of the group is not fully transferable in such a case.
The source function is of use in determining individual group contributions to
the density in the study of transferability and may also serve to reveal the con-
sequences of the transferability of the properties of a functional group, since, as said
earlier, the extent of transfer of these properties from one molecule to another is a
consequence of a corresponding transferability of the group's electron density. For
instance, the SF enables one to determine the extent to which changes in atoms
neighboring the group in question selectively contribute to the change in its density.
Using the SF, one may also see transferability from a
new and deeper perspective
.
The electron density decomposition afforded by (2) reveals that the perfect transfer-
ability of a group property, expressible in terms of its density, implies a constancy not
only in the electron density of the group but also in the sum of contributions to this
density from the remaining atoms in the system. Concisely, if on passing from one
system to another, a group's electron density remains constant, so need to be both the
“internal” and “external” contributions to this density, at any of its points.
In what follows, we briefly review a case where an almost perfect transferability
realizes and other two where the SF approach reveals interesting compensatory
transferability mechanisms.
