Debye

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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:
\mathbf{p} = q \, \mathbf{r}
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

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