The dipole moment indicates how charge is unevenly distributed in a molecule and strongly influences properties such as solubility and boiling point. By measuring the dielectric constant of liquids at different temperatures, Debye formulated the orientation polarization of molecules and devised the Debye equation to derive dipole moments indirectly. He also pioneered the analysis of scattering angles when X-rays or electron beams hit gas molecules, extracting bond lengths and bond angles. These techniques became the foundation for obtaining molecular ‘blueprints,’ essential in drug design and advanced materials research.

Debye’s achievements tightly linked theory with experiment. Using the dielectric constant ε and refractive index n, he derived the relation (ε−1)(2ε+1)/(n^2−1)(2n^2+1) ∝ T that allows quantitative extraction of the permanent dipole moment μ. This greatly advanced the statistical-thermodynamic treatment of polar molecules. His gas-phase electron diffraction technique transformed diffraction patterns, via Fourier methods, into bond lengths and angles for freely rotating molecules, providing data complementary to X-ray crystal analysis of solids and liquids. Modern GC/electron-diffraction hybrid instruments and gas-phase electron imaging trace their origin to his work.

Debye expressed the complex dielectric function via a Laurent expansion and, by taking the ω→0 limit, extracted molecular dipole moments from the orientation polarization term. He pioneered the introduction of a correlation factor g that modifies the Clausius-Mossotti approximation for polar liquids with non-independent dipoles. In gas-phase electron diffraction he applied the Born approximation for the λr ≪ 1 regime, presented the Debye equation that separates cosine and sine scattering amplitudes, and performed flat tree-averaging over molecular orientations. This enabled Å-scale determination of orientationally averaged structures, providing calibration standards for subsequent vibrational analyses and cold-beam spectroscopy.