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Cycles, Intermediates (Chemistry), Chemical tests and reagents, Density functionals, Chemistry, Chemical reactions, Computational, Identity, Reaction, ADF, Hard-soft theory, HSAB, Hard-soft/acid-base
In 2010, Breugst, Mayr, et al. published a study that demonstrated a failure of Pearson's HSAB theory to predict ambidentate reactivity. Instead, the group proposed that reactivity, as expressed by activation energy, could be calculated through a Marcus approach. The data used in this method were values for the intrinsic barrier height of SN2 identity (substitution) reactions, studied by Hoz and associates in 1999. Hoz established the existence of these barrier heights, as well as periodicity related to the ligand element, but did not offer explanation for the periodicity. This study attempted to investigate the periodicity of identity reaction energies on multiple substrate types, with a consistent range of ligands. All calculations shown were performed with the Amsterdam Density Functional (ADF2012) program. The substrates chosen were elements from Group IV (carbon, silicon, tin, lead) as well as certain transition metals (rhodium, cobalt), which form five-coordinate intermediates similar to those of the Group IV element substrates. In this study, possible explanations were offered for the periodic trends in the barrier height or well depth of a substitution on a Group IV substrate, and an analysis was made of the differences between carbon and the rest of the group. It was demonstrated that HSAB does not successfully predict the energy of identity reactions, and therefore other determining factors were explored. These included, generally, electrostatic forces and steric (Pauli) repulsion. Each factor contributes to the overall trends in barrier height/well depth, but it is the repulsion that is distinctly higher for the carbon substrate and is therefore the cause of that element's unique behavior.
Kay, Sarah Frances, "Five-coordinate intermediates : why carbon will not behave" (2013). Honors Project, Smith College, Northampton, MA.
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