1 research outputs found
Quest for Insight into Ultrashort C–H···π Proximities in Molecular “Iron Maidens”
Molecular iron maidens are a strained
type of cyclophane in which
a methine hydrogen, by the action of the bridges, is placed closer
to the center of an aromatic ring. Such constrained molecular frameworks
are in fact a noteworthy synthetic challenge. The present study provides
a comprehensible theoretical analysis that elucidates unique structural
and energetic aspects of this class of molecules, evaluating, in the
light of quantum chemistry, both the influence of the aromatic moiety,
from π-basic to π-acid, and the nature of the heteroatoms
located at the bridges. Our results not only propose the shortest
intramolecular centered C–H···π distance
to date, which is supported by calculated <sup>1</sup>H chemical shifts,
but also shed light on the main factors that rationalize and justify
such proximity. QTAIM, NBO, and NCI analyses allow us prematurely
to conclude that the ultrashort C–H···π
distance is sustained by an interplay between a large stabilizing
electrostatic component with a non-negligible covalent character.
However, the energetics involving such strained molecular scaffolds,
addressed by means of isodesmic reactions, revealed that the C–H···π
proximity is modulated mainly by the capacity of the bridges to support
the strain imposed by the whole structure, hence compressing the C–H
bond against the π-system