Polarity

As we have already discussed a polar molecule is one having a permanent electric dipole moment. For instance these are $NaCl$, $O_3$, $NH_3$, and many others. It is known that the rotational absorption transitions can occur only in polar molecules. The group theory give important instructions, how the molecular symmetry is related to the molecular polarity. For instance, if a molecule belongs to the group $C_n$, where $n>1$, then it cannot have a component of the dipole moment perpendicular to the symmetry axis, because a dipole moment which exist in one direction perpendicular to the axis is cancelled by an opposing dipole. A dipole moment in these molecules can be only parallel to the molecular axis. The same is valid for any of the $C_{nv}$ group molecule. The molecules which belongs to all other groups, but $C_s$, cannot have a permanent dipole moment, because they always have symmetry operations transforming one end of the molecule into another. Thus, only the molecules which belong to the $C_n$, $C_nv$, or $C_s$ group can have a permanent dipole moment.

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