Definitions

# 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).

## 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. 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, based on Meister and Schwarz (1994).

## 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.

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