Weak hydrogen bonds: theoretical studies
Abstract
Hydrogen bonds A–H⋯A′, where A and A′
are electronegative atoms have been widely discussed. Weak hydrogen bonds
involving such different arrangements as X–H⋯A, where X can be
C; X–H⋯π, with phenyl rings, CC bonds;
X–H⋯M, where M is a transition metal; X–H⋯H–M
and X–H⋯H–B, have also been described in recent years.
While the first types are typical of organic and inorganic compounds, as
well as biological molecules, those involving transition metal atoms are
special to organometallic chemistry. Theoretical calculations of different
kinds and at several levels have been performed for many systems, revealing
that a similar geometrical arrangement can hide another type of
interaction. This happens for N–H⋯M close contacts which can be
agostic interactions or hydrogen bonds, not so easily distinguishable for
16-electron complexes. M–H⋯H–X interactions also exhibit
a different behavior, depending on whether the complexes are neutral or
ionic. The AIM approach, by analysing the topological properties of the
charge density with the determination of critical points, provides another
way of looking for bonds, as discussed in several examples.