Debye

The debye (symbol: D) is a non-SI, CGS unit of electrical dipole moment. It is defined as 1×10⁻¹⁸ statcoulomb centimeter (or 1×10⁻²⁰ esu m, or 1×10⁻¹⁸ Fr cm). In SI units, 1 D equals approximately 3.33564×10⁻³⁰ coulomb-meter (exactly 1×10⁻²¹ C m²/s divided by c, the speed of light in vacuum). Conversely 1 C m = 2.9979×10²⁹ D. It is named after the physicist Peter J. W. Debye.

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 esu in older literature), which were separated by 1 angstrom (10^-8 cm or 10^-10m).

Dipole moment is defined:

1 debye = (10^-10)(10^-8) statcoulomb centimeter

Note that 10^-10 statcoulomb is 0.48 units of elementary charge.

This gave a convenient unit for molecular dipole moments. Typical dipole moments for simple diatomic molecules are in the range of 0 - 11D, where symmetric homoatomic species, e.g. chlorine, Cl₂, have a dipole moment of 0D and highly ionic molecular species such as gas phase potassium bromide, KBr have a dipole moment of 10.5D.^[1]
The debye is still used in atomic physics and chemistry because SI units are inconveniently large, particularly since the smallest prefix is ×10⁻²⁴ (e.g., 2.54 D = 8.47×10⁻⁶ yCm). Note that SI disallows the application of prefixes to both members of a compound unit (e.g., 2.54 D = 8.47 fC·fm) or the compounding of prefixes (e.g., 2.54 D = 8.47 µyCm), so there is currently no satisfactory solution to this problem of notation.

[edit] References

• Atomic unit of electric dipole moment NIST
• CGS units R. Rowlett (University of North Carolina at Chapel Hill)

1. ^ Physical chemistry 2d Edition (1966) G.M. Barrow McGraw Hill