pi-H...O hydrogen bonds: multicenter covalent pi-H interaction acts as the proton-donating system.

The MP2 method and the Pople-style basis sets 6-311++G(d,p), 6-311++G(2df,2pd), and 6-311++G(3df,3pd) were used to perform calculations on H3O+...C2H2 and C2H3+...C2H2 complexes and related species. Hydrogen bonds existing for the analyzed complexes were investigated as well as related pi-H...O --> pi...H-O and pi-H...pi --> pi...H-pi proton-transfer processes. For some of the complexes analyzed the multicenter pi-H interaction possessing the properties of a covalent bond acts as a proton donor; more generally it is classified as the Lewis acid. The quantum theory of "atoms in molecules" (QTAIM) was also applied to deepen the nature of these interactions in terms of characteristics of bond critical points. The pi-H...O, O-H...pi, and pi-H...pi interactions analyzed here may be classified as hydrogen bonds since their characteristics are the same as or at least similar to those of typical hydrogen bonds. H...pi interactions are common in crystal structures of organic and organometallic compounds. The analyses performed here show a continuum of such interactions since there are H...pi contacts possessing the characteristics of weak intermolecular interactions on the one hand and pi-H multicenter covalent bonds on the other. Ab initio and QTAIM results support the latter statements.

Hydrogen bonds with pi and sigma electrons as the multicenter proton acceptors: high level ab initio calculations.

Pi-electrons of acetylene and sigma-electrons of molecular hydrogen were investigated as Lewis bases in different complexes. Hence high level ab initio calculations were performed up to the MP2/6-311++G(3df,3pd) level of approximation. It was found that species analyzed possess characteristics typical for H-bonded systems. The Bader theory was additionally applied; bond paths between proton and pi-electrons of acetylene or sigma-electrons of molecular hydrogen were detected with the corresponding bond critical points attributed to the proton-acceptor interactions. Numerous correlations between topological, geometrical and energetic parameters were also found. For example, the H...pi or H...sigma interaction is stronger for the shorter corresponding distance between the proton and the middle of C[triple bond]C or H-H bond. It is connected with the greater elongation of C[triple bond]C or H-H bonds and the greater transfer of electron charge from the Lewis base to the Lewis acid.

Intramolecular H-bonds: DFT and QTAIM studies on 3-(aminomethylene)pyran-2,4-dione and its derivatives.

Intramolecular N-H...O hydrogen bonds in 3-(aminomethylene)pyran-2,4-dione and its simple derivatives (F, Li, and BeH substituents) were analyzed theoretically. The systems were optimized at the B3LYP/6-311++G(d,p) level of approximation. For some fluorine derivatives the corresponding tautomers with O-H...N intramolecular H-bonds were investigated, and for such pairs of tautomers, the calculations on transition states of the N-H...O <--> N...H-O proton-transfer reaction were carried out. The geometrical and energetic parameters for these species were characterized. The topological parameters derived from Bader theory were also analyzed; these are characteristics of H-bond critical points and also of ring critical points. Besides N-H...O and O-H...N intramolecular hydrogen bonds, there are the other intramolecular interactions, mostly ionic such as Be(+delta)...(-delta)O, Li(+delta)...(-delta)O, and Li(+delta)...(-delta)F. The F...O interactions also exist for some of species investigated. They may be classified as energetically stabilizing ones since the corresponding bond paths and critical points exist. The numerous correlations and dependencies between geometrical, topological, and energetic parameters were detected and described.