MOLECULAR POLARITY
Covalent Compounds  Elmhurst College
Electrostatic Potential Organic Functional Groups Functional Gps vs. Boiling Pt.  Chemistry Department
Simple Inorganics Organic Chain Length  Virtual ChemBook

 

 Molecular Electrostatic Potential

Introduction:

Electrostatic potential correlates with dipole moment, electronegativity, and partial charges. It provides a visual method to understand the relative polarity of a molecule.


Click for larger image 

Electronegativity:

Linus Pauling first defined electronegativity as: "The power of an atom in a molecule to attract electrons to itself." A numerical scale based upon a physical measurements allows a comparison between atoms.

A rough approximation for comparison of atoms is to say, the the closer an atom is to Fluorine in the periodic table, the greater the electronegativity compared to an atom further away.

The greater the electronegativity difference between atoms in a bond, the more polar the bond.

 

Partial Charges:

Electronegativity of atoms in molecules indicates where partial charges are likely to be found - the most electronegative atoms are most negative, the others are less negative or more positive.

Quantum mechanical calculations generate values for partial charges for the atoms in a molecule. These are related to electron densities around various atoms resulting from bonding and lone pairs of electrons.

The calculated partial charges represented as spheres (yellow is negative, red is positive) show how the molecule would interact with an approaching proton.

The greater the difference in partial charges, the more polar the molecule.


Click for larger image 

Electrostatic Potential:

Hypochlorous Acid - Chime in new window

The graphic on the left shows both partial charge and contours of electrostatic potential.

The molecular electrostatic potential is the potential energy of a proton at a particular location near a molecule.

Negative electrostatic potential corresponds to a attraction of the proton by the concentrated electron density in the molecules (from lone pairs, pi-bonds, etc.) (colored in shades of red).

Positive electrostatic potential corresponds to repulsion of the proton by the atomic nuclei in regions where low electron density exists and the nuclear charge is incompletely shielded(colored in shades of blue).

The calculated partial charges represented as spheres (yellow is negative, red is positive) show how the molecule would interact with approaching protons or positive charges.

When a unit of positive charge (proton) approaches a positive region of the molecule, the repulsive interaction results in an increasing positive potential energy (colored in shades of blue). As a proton approaches a negative region an attractive interaction results in negative potential energy (colored in shades of red).

The electron density isosurface is a surface on which the molecule's electron density has a particular value and that encloses a specified fraction of the molecule's electron probability density. The electrostatic potential at different points on the electron density isosurface is shown by coloring the isosurface with contours.

The more red / blue differences, the more polar the molecule. If the surface is largely white or lighter color shades, the molecule is mostly non-polar.