Weak hydrogen bonds: theoretical studies

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 ...pi, with phenyl rings, C=C 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.