Debye

The debye (/dɪˈbaɪ/ dib-EYE, Dutch: [dəˈbɛiə]; symbol: D) is a CGS unit (a non-SI metric unit) of electric dipole moment named in honour of the physicist Peter J. W. Debye. Likely the earliest recommendation for the Debye as a unit was in the Physikalisch-chemisches Taschenbuch (physical chemistry handbook) written by Carl Drucker and Erich Proskauer in early 1932. It is defined as 10⁻¹⁸ statcoulomb-centimetres. Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10⁻¹⁰ statcoulomb (generally called e.s.u. (electrostatic unit) in older scientific literature), which were separated by 1 ångström. This gave a convenient unit for molecular dipole moments.

1 D	= 10−18 statC·cm
	= 10−18 cm5/2⋅g1/2⋅s−1
	= 10−10 statC·Å
	≘ ⁠1/299792458⁠×10−21 C·m
	≈ 3.33564×10−30 C·m
	≈ 0.3934303 e·a0
	≈ 0.2081943 e⋅Å
	≈ 0.02081943 e·nm

Typical dipole moments for simple diatomic molecules are in the range of 0 to 11 D. Molecules with symmetry point groups or containing inversion symmetry do not have a permanent dipole moment, while highly ionic molecular species have a very large dipole moment, e.g. gas-phase potassium bromide, KBr, with a dipole moment of 10.41 D. A proton and an electron 1 Å apart have a dipole moment of 4.8 D.

The debye is still used in atomic physics and chemistry because SI units have until recently been inconveniently large. The smallest SI unit of electric dipole moment is the quectocoulomb-metre, which corresponds closely to 0.3 D.