Dipole Moments

Dipole moments (DM) arise in molecules which have an "asymmetric electron distribution", i.e. the molecule has polar covalent bonding present. Each polar covalent bond produces a "bond moment" due to the unequal sharing of electrons between the two atoms. The electrons are drawn towards, and spend more time around, the more electronegative of the two atoms. A molecular dipole is the vector sum of all bond moment in the molecule. We will demonstrate this with several examples below. Note: in the images below the red arrows are bond moments and the green arrows the molecular dipole moment. This is also displayed in 3D. The "asymmetric electron distribution" is also clearly visible in the Jmol Electron Density - ESP mapped surfaces. The red end of the molecule has an excess of electron density while the blue end has had its electron density decreased. The result is a dipole moment which goes from low electron density (blue) to high electron density (red).

Note: in CCl₄, even though there are polar covalent bonds (producing bond moments), the symmetry of the molecule means that the bond moments cancel when their vectors are added to produce the molecular dipole moment. It is also apparent from the Electron Density - ESP plot that there is an "asymmetric charge distribution" (Cl more negative than C) but the symmetry of the molecule cancels the effect.

Note: by comparison the C-H bond moments are very small and have little effect on the molecular dipole moment.

MF	Dipole 2D	Dipole 3D	Electron Density - ESP Surface
CH3Cl		DM = 2.0832 Debye
CH2Cl2		DM = 1.8524 Debye
CHCl3		DM = 1.2758 Debye
CCl4		DM = 0 Debye