|Posted on November 11, 2015 at 6:25 PM|
The formation of molecules through the harpoon mechanism occurs from the interaction of two fragments, one with a low ionization potential (IP) and another that has a large electron affinity (EA). The reactants approach each other and, at a certain distance, an electron from the fragment with low IP harpoons the fragment with large EA, giving rise to a rapid electron-transfer process that is triggered by the Coulomb attraction exerted by the two fragments. Michael Polanyi, the father of the Nobel laureate John Polanyi, suggested the mechanism in 1932 to explain the large reaction cross sections observed in alkali atoms with halogen molecules. The reaction has been widely studied but, despite the intriguing reaction mechanism, its chemical bonding has not been so carefully analyzed.
In this work we perform a more exhaustive analysis of the bonding in the formation of molecules through harpoon mechanism using the electron sharing indices (ESI), the electron localization function (ELF) and the Laplacian of the electron density. Several diatomics are analyzed but the focus is put into the two lowest-lying singlet sigma states of LiH. The LiH in its ground state is formed by means of a harpoon mechanism, going through two avoided crossings, that change twice its bonding character. The single sigma excited state, shows a very peculiar behavior and likewise changes its bonding nature twice, as it also passes through two avoided crossings.
The work is part of Mauricio's thesis and it will be published in a special issue of Molecular Physics dedicated to the 65th birthday of Andreas Savin, who (aside from its important contributions to the density functional theory [DFT]) also contributed to the popularization, the understanding and interpretation of the ELF (e.g. the DFT version of the ELF) and introduce significant ideas in the field of the chemical bonding analysis. Happy birthday Andreas!