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).

Contents 1 Partial atomic charges 1.1 Uses 1.2 Methods of determining partial atomic charges 2 References

[edit] 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.

[edit] 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.

[edit] 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. According to Cramer (2002), all methods can be classified in one of four classes:

• Class I charges are those that are not determined from quantum mechanics, but from some intuitive or arbitrary approach. These approaches can be based on experimental data such as dipoles and electronegativities.

• Class II charges are derived from partitioning the molecular wave function using some arbitrary, orbital based scheme.

• Class III charges are based on a partitioning of a physical observable derived from the wave function, such as electron density.

• Class IV charges are derived from a semiempirical mapping of a precursor charge of type II or III to reproduce experimentally determined observables such as dipole moments.

The following is a detailed list of methods, partly based on Meister and Schwarz (1994).

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

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

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

• Charges derived from electrostatic potential
  • Chelp
  • ChelpG, Breneman model
  • MK, Merz-Kollman

• 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

[edit] References

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

C. J. Cramer. Essentials of Computational Chemistry: Theories and Methods. Wiley, 2002, pp. 278-289.

Frank Jensen, Introduction to Computational Chemistry, 2nd Edition, Wiley ISBN: 978-0-470-01187-4