Molecular Electrostatic Potential
Sigma-hole, Pi-hole and Molecular Electrostatic Potential (MESP)
Sigma-hole is the positive electrostatic potential region of a molecule which is located at the outer extension of a sigma bond. For eg., the positive region of Cl-atom in the F3C-Cl molecule located at the outer periphery of C-Cl sigma bond.
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| MESP of F3C-Cl molecule. Red region represents the sigma hole. |
Similarly, Pi-holes are defined as an extension of sigma-holes and are located at the pi-bonding region of a molecule. eg, NO2Br
According to Politzer et.al, who proposed the concept of sigma-hole, when a half-filled p orbital on the halogen participates in forming a covalent bond, there results an electron deficiency in the outer (non-involved) lobe of that p orbital. This was called a σ-hole.
What is the chemical origin of sigma-holes ? How one can correlate it with the molecular electron density ?
What is the chemical origin of sigma-holes ? How one can correlate it with the molecular electron density ?
what is the pi hole bond?
ReplyDeleteIf you look at the equation of the electrostatic potential (almost every author provides it in their paper on sigma/pi hole), you can see that the electron density is there with a negative sign. Although the correlation between the density and the sigma-hole strength is not straightforward, you can get some idea. Sigma/pi hole strength also depends upon the nuclear charge.
ReplyDeleteTriel bond (group 13) shows pi-hole bonding explicitly. Apart from that, chalcogens do show pi-hole and sigma-hole bond as well.