Atoms in Molecules Theory has been applied to analyze bonding properties, in potentially hypervalent
pnicogen (N, P or As)−chalcogen(O or S) bonds within the framework of three plausible models: (i) one σ
bond and two π back-bonds (negative hyperconjugation), (ii) one σ bond and three π back-bonds, and (iii)
three Ω (banana) bonds. The topological analyses (based upon the electron charge density (ρ(r)), its Laplacian
(∇2ρ(r)), bond ellipticity, etc.) and the charges were consistent with a highly polarized σ bond, with bond
strength dependent on the electrostatic interactions. The equilibrium geometries were optimized by both density
functional theory with a hybrid functional (B3LYP) and by ab initio methods at the MP2(full) level, using the
6-311G basis set augmented by polarization and/or diffuse functions.
[1] We present results for the electronic and atomic structures of different size molecular clusters containing sulfuric acid, water, and ammonia molecules. The electronic structure calculations are performed using the DMol 3 quantum chemical program packages and the PW91 density functional. We report implications of our results for atmospheric cluster size distributions. Our calculations indicate that ammonia is bonded to the cluster structures much more strongly than water. The presence of ammonia also strengthens the binding of sulfuric acid molecules to the clusters, and significantly increases the formation of clusters containing two sulfuric acid molecules and 0-2 water molecules. However, owing to the low overall concentration of two-acid clusters, the fraction of ammonia-containing clusters in the calculated atmospheric cluster size distribution was still very low.Citation: Kurtén, T., L. Torpo, C.-G. Ding, H. Vehkamäki, M. R. Sundberg, K. Laasonen, and M. Kulmala (2007), A density functional study on water-sulfuric acid-ammonia clusters and implications for atmospheric cluster formation,
The electron pair density in conjunction with the AIM theory and calculated NMR chemical shifts were used to characterize the bonding properties for nine pnicogen and chalcogen ylide structures. The hybrid B3LYP and MP2 methods were employed with the 6-311+G* basis set. No evidence was found to support a banana (Ω) bonding scheme. Instead, different bonding schemes were found to be dependent on the electronegativity of the X atom in the C-X bond. When X is a highly electronegative atom (N,O), the C-X bond is weaker than a single bond, due to electrostatic repulsion. When the X atom has electronegativity similar to carbon, a covalent, yet significantly polar interaction results, and its strength is determined mainly by electrostatic interactions, with a small contribution of negative hyperconjugation.
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