Hydrogen and Lithium Bonds—Lewis Acid Units Possessing Multi-Center Covalent Bonds

Abstract

MP2/aug-cc-pVTZ calculations were carried out on complexes wherein the proton or the lithium cation is located between π-electron systems, or between π-electron and σ-electron units. The acetylene or its fluorine and lithium derivatives act as the Lewis base π-electron species similarly to molecular hydrogen, which acts as the electron donor via its σ-electrons. These complexes may be classified as linked by π-H∙∙∙π/σ hydrogen bonds and π-Li∙∙∙π/σ lithium bonds. The properties of these interactions are discussed, and particularly the Lewis acid units are analyzed, because multi-center π-H or π-Li covalent bonds may occur in these systems. Various theoretical approaches were applied here to analyze the above-mentioned interactions—the Quantum Theory of Atoms in Molecules (QTAIM), the Symmetry-Adapted Perturbation Theory (SAPT) and the Non-Covalent Interaction (NCI) method.

Proton and Lithium Cations Linked to π‐Electron and σ‐Electron Systems: Are Such Interactions beyond or within the Definition of Hydrogen/Lithium Bond?

“…[39] The E int energies were corrected for the Basis Set Superposition Error (BSSE). [40] The complexes that were analyzed in the recent study [19] are characterized here in terms of the ELF and NBO approaches. These are complexes with the proton located between π-electron systems of the acetylene molecules, C 2 H 2 , or the acetylene derivatives, FCCF and LiCCLi.…”

“…The geometries of the systems analyzed here were discussed in the former study. [19] In the case of complexes containing the lithium cation in the central part of the system the lower electron charge shifts are observed. The charge of the lithium center is close to unity, between + 0.94 au and + 0.96 au.…”

“…The specific complexes which are not classified as the hydrogen bonded systems, according to the majority of definitions, are the subject of this study; the lithium analogues of these complexes are also analyzed. However in the former studies the numerous characteristics typical for hydrogen bonds were found for such complexes as [17][18][19] Besides, these are not only systems analyzed theoretically. For example, the C 2 H 2 •••H + •••C 2 H 2 complex and other clusters containing the proton and the acetylene molecules were investigated with the use of the infrared photodissociation spectroscopy.…”

“…Such systems were investigated in the former studies from time to time. [17][18][19] For example, it was found that the proton inserted between two acetylene molecules is moved closer to one of these molecules for the energetic global minimum. [17] One may consider that in this complex the π•••H + •••π charge assisted hydrogen bond occurs where the πelectrons of acetylene are the proton acceptor.…”

“…were also analysed, even their lithium counterparts, complexes linked by the π [17][18][19] None of the former definitions of the hydrogen bond, [2,3,6,7] covers the above-mentioned π…”