QTAIM-based comparison of agostic bonds and intramolecular charge-inverted hydrogen bonds.

Abstract

Using DFT-based calculations with seven exchange-correlation functionals (BP86, B3LYP, B3PW91, PBE0, TPSSh, M06-L, M06) we have performed comparative studies on α-, β-, γ-, and δ-agostic bonds (ABs) and intramolecular charge-inverted hydrogen bonds (IMCIHBs). Our detailed analysis of values of QTAIM parameters computed at bond (BCP) and ring critical points (RCP) as well as of the curvatures of bond paths tracing agostic bonds and intramolecular charge-inverted hydrogen bonds gives the opportunity to distinguish between both these types of interactions. In the case of molecules with agostic bonds, the BCP is significantly closer to the agostic hydrogen, whereas in systems with IMCIHB the BCP is, instead, somewhat closer to the metal atom. Agostic bonds are characterized by H···M bond paths being straight in the BCP···M section and then highly curved near the agostic hydrogen, whereas in the case of IMCIHB any substantial curvature of BP in the vicinity of hydrogen is not present. Quite the contrary, the significant curvature of BP near the metal atom can be obtained, instead. One can also distinguish IMCIHBs and ABs on the basis of values of bond ellipticity at BCP and the electron density at RCP which are either somewhat (PBE0) or considerably (M06) greater for the latter type of interaction. It has also been shown that, in general, the exchange-correlation functional has small influences on most of QTAIM parameters computed at BCP and RCP. More significant influences have only been obtained for Laplacian of the electron density, some its components, and the bond ellipticity.