How is dipole moment measured experimentally?

The dipole moment of a molecule is measured experimentally by determining the dielectric constant (ε_r) of the substance, usually in the gas phase or when dissolved in a non-polar solvent.

The entire process is based on the Debye equation, which relates the total polarizability of a substance (P_T) to its permanent dipole moment (p) and its temperature (T).

• Experimental Setup: A capacitor (two parallel plates) is used. Its capacitance is first measured with a vacuum (C₀) and then measured again after the substance is placed between the plates (C). The ratio C/C₀ gives the dielectric constant.
• The Concept (Two Types of Polarization):
  1. Induced Polarization (P_i): The external electric field of the capacitor slightly distorts the molecule's electron cloud. This happens to all molecules (polar or not) and is independent of temperature.
  2. Orientation Polarization (P_o): This only happens for polar molecules. The electric field exerts a torque that tries to align the permanent dipoles with the field. This alignment is highly dependent on temperature.
     • At high temperatures, random thermal motion (kinetic energy) prevents the dipoles from aligning, so P_o is low.
     • At low temperatures, the dipoles align more easily, so P_o is high.
• Measurement Steps:
  1. The dielectric constant of the substance is measured at several different temperatures.
  2. The total molar polarization (P_T = P_i + P_o) is calculated at each temperature using a formula (the Clausius-Mossotti equation).
  3. A graph is plotted of the total polarization (P_T) on the y-axis against the inverse of the absolute temperature (1/T) on the x-axis.
  4. This graph will be a straight line. The slope of this line is directly proportional to the square of the dipole moment (p²). The y-intercept (where 1/T = 0, i.e., infinite temperature) gives the value of the induced polarization (P_i).

By measuring the slope of this graph, the permanent electric dipole moment (p) of the molecule can be accurately calculated.