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.
Highlights
There are three issues connected with the physical meaning of chemical bonds, and with their definition:(1) More than one electron pair may be located between two atomic centers, double and triple bonds are observed
Two groups of species were analyzed here: the proton-bound complexes connected by the π-H···π and π-H···σ links, and their counterparts with a lithium cation instead of an H-center
The connections with the H-center possess numerous characteristics of hydrogen bonds. They may be classified as hydrogen bonds, as in line with the definition introduced recently
Summary
(1) More than one electron pair may be located between two atomic centers, double and triple bonds are observed This issue was taken into account by Butlerow in his studies on structural chemistry [1];. (3) According to the Lewis definition of the chemical bond, it is a strong two-center– two-electrons interaction, 2c/2e. It seems that multi-center bonds exist, such as three-center ones [3]. The 3c/4e (three-center–four-electron) term is related to hypervalency, where the octet rule is not obeyed and the center considered is characterized by the excess of electrons It concerns three-center ω-bonds, and this kind of arrangement was discussed recently for various types of interactions [4]
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