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Abstract: (Chemical Abstract 2000, 132:222558)
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The electron pair density in conjunction with the AIM theory and calculated
NMR chemical shifts have been 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 (W) 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 one, due to electrostatic repulsion.
When the X atom has similar electronegativity than 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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