Partial charge

A partial charge is a charge with an absolute value of less than one elementary charge unit (that is, smaller than the charge of the electron).

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Partial atomic charges

Partial charges are created due to the asymmetric distribution of electrons in chemical bonds. The resulting partial charges are a property only of zones within the distribution, and not the assemblage as a whole. For example, chemists often choose to look at a small space surrounding the nucleus of an atom: When an electrically neutral atom bonds chemically to another neutral atom that is more electronegative, its electrons are partially drawn away. This leaves the region about that atom's nucleus with a partial positive charge, and it creates a partial negative charge on the atom to which it is bonded.

In such a situation, the distributed charges taken as a group always carries a whole number of elementary charge units. Yet one can point to zones within the assemblage where less than a full charge resides, such as the area around an atom's nucleus. This is possible in part because particles are not like mathematical points--which must be either inside a zone or outside it--but are smeared out by the uncertainty principle of quantum mechanics. Because of this smearing effect, if one defines a sufficiently small zone, a fundamental particle may be both partly inside and partly outside it.

Uses

Partial atomic charges are used in molecular mechanics force fields to compute the electrostatic interaction energy using Coulomb's law. They are also often used for a qualitative understanding of the structure and reactivity of molecules.

Methods of determining partial atomic charges

Despite its usefulness, the concept of a partial atomic charge is somewhat arbitrary, because it depends on the method used to delimit between one atom and the next (in reality, atoms have no clear boundaries). As a consequence, there are many methods for estimating the partial charges. The following list is taken from Meister and Schwarz, 1994 (see the article for details and references about each method).

• Population analysis of wavefunctions
  • Mulliken population analysis
  • Coulson's charges
  • Natural charges
  • CM1, CM2, CM3 charge models

• Partitioning of electron density distributions
  • Hirshfield charges
  • Density fitted atomic charges
  • Bader charges (obtained from an atoms in molecules analysis)
  • Maslen's corrected Bader charges
  • Politzer's charges

• Charges derived from density-dependent properties
  • Partial derived charges
  • Dipole charges
  • Dipole derivative charges

• Charges derived from spectroscopic data
  • Charges from infrared intensities
  • Charges from X-ray photoelectron spectroscopy (ESCA)
  • Charges from X-ray emission spectroscopy
  • Charges from X-ray absorption spectra
  • Charges from ligand-field splittings
  • Charges from UV-vis intensities of transition metal complexes
  • Charges from other spectroscopies, such as NMR, EPR, EQR

• Charges from other experimental data
  • Charges from bandgaps or dielectric constants
  • Apparent charges from the piezoelectric effect
  • Charges derived from adiabatic potential energy curves
  • Electronegativity-based charges
  • Other physicochemical data, such as equilibrium and reaction rate constants, thermochemistry, and liquid densities.
  • Formal charges

Fundamental particles with non-integer charge

An "up-type quark" has an intrinsic charge of +²/₃ of a unit and a "down-type quark" has an intrinsic charge –¹/₃ of a unit.

References

J. Meister, W. H. E. Schwarz. Principal Components of Ionicity. J. Phys. Chem. 1994, 98, 8245-8252.