Partial charge
Encyclopedia
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
).
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.
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.
The following is a detailed list of methods, partly based on Meister and Schwarz (1994).
References:
[1] Theory of vibrational rotational strengths: comparison of a priori theory and approximate models. P.J. Stephens, K.J. Jalkanen and R.W. Kawiecki. J. Am. Chem. Soc. 112 (1990) 6518-6529.
Absolute value
In mathematics, the absolute value |a| of a real number a is the numerical value of a without regard to its sign. So, for example, the absolute value of 3 is 3, and the absolute value of -3 is also 3...
of less than one elementary charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
unit (that is, smaller than the charge of the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
).
Partial atomic charges
Partial charges are created due to the asymmetric distribution of electrons in chemical bonds. For example, in a polar covalent bond like HCl, the shared electron oscillates between the bonded atoms. 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 nucleusAtomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
of an atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
: When an electrically neutral atom bonds chemically
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
to another neutral atom that is more electronegative
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
, 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
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...
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
Uncertainty principle
In quantum mechanics, the Heisenberg uncertainty principle states a fundamental limit on the accuracy with which certain pairs of physical properties of a particle, such as position and momentum, can be simultaneously known...
of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
. 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 mechanicsMolecular mechanics
Molecular mechanics uses Newtonian mechanics to model molecular systems. The potential energy of all systems in molecular mechanics is calculated using force fields...
force fields
Force field (chemistry)
In the context of molecular modeling, a force field refers to the form and parameters of mathematical functions used to describe the potential energy of a system of particles . Force field functions and parameter sets are derived from both experimental work and high-level quantum mechanical...
to compute the electrostatic interaction energy using Coulomb's law
Coulomb's law
Coulomb's law or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. It was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism...
. 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, partly based on Meister and Schwarz (1994).
- Population analysis of wavefunctionWavefunctionNot to be confused with the related concept of the Wave equationA wave function or wavefunction is a probability amplitude in quantum mechanics describing the quantum state of a particle and how it behaves. Typically, its values are complex numbers and, for a single particle, it is a function of...
s- Mulliken population analysis
- Coulson's charges
- Natural charges
- CM1, CM2, CM3 charge models
- Partitioning of electron densityElectron densityElectron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds...
distributions- Bader charges (obtained from an atoms in moleculesAtoms in MoleculesThe atoms in molecules or atoms-in-molecules or quantum theory of atoms in molecules approach is a quantum chemical model that characterizes the chemical bonding of a system based on the topology of the quantum charge density...
analysis) - Density fitted atomic charges
- Hirshfeld charges
- Maslen's corrected Bader charges
- Politzer's charges
- Voronoi Deformation DensityVoronoi Deformation DensityVoronoi Deformation Density is a method employed in computational chemistry to compute the atomic charge distribution of a molecule in order to provide information about its chemical properties. The method is based on the partitioning of space into non-overlapping atomic areas modelled as Voronoi...
charges
- Bader charges (obtained from an atoms in molecules
- Charges derived from density-dependent properties
- Partial derived charges
- DipoleDipoleIn physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
charges - Dipole derivative charges, also called atomic polar tensor (APT) derived charges http://dx.doi.org/10.1021/ja00174a011
- Charges derived from electrostatic potential
- Chelp
- ChelpG (Breneman model)ChelpgCHELPG is an due to , in which atomic charges are fitted to reproduce the molecular electrostatic potential at a number of points around the molecule....
- Merz-Singh-Kollman (also known as Merz-Kollman, or MK)
- Charges derived from spectroscopic data
- Charges from infrared intensities
- Charges from X-ray photoelectron spectroscopyX-ray photoelectron spectroscopyX-ray photoelectron spectroscopy is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material...
(ESCA) - Charges from X-ray emission spectroscopy
- Charges from X-ray absorption spectra
- Charges from ligand-field splittings
- Charges from UV-vis intensitiesUltraviolet-visible spectroscopyUltraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent ranges...
of transition metal complexes - Charges from other spectroscopies, such as NMRNMR spectroscopyNuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
, EPR, EQR
- Charges from other experimental data
- Charges from bandgaps or dielectric constantDielectric constantThe relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
s - Apparent charges from the piezoelectric effect
- Charges derived from adiabatic potential energy curvesPotential energy surfaceA potential energy surface is generally used within the adiabatic or Born–Oppenheimer approximation in quantum mechanics and statistical mechanics to model chemical reactions and interactions in simple chemical and physical systems...
- ElectronegativityElectronegativityElectronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
-based charges - Other physicochemical data, such as equilibrium and reaction rate constants, thermochemistryThermochemistryThermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the...
, and liquid densities. - Formal chargeFormal chargeIn chemistry, a formal charge is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity....
s
- Charges from bandgaps or dielectric constant
References:
[1] Theory of vibrational rotational strengths: comparison of a priori theory and approximate models. P.J. Stephens, K.J. Jalkanen and R.W. Kawiecki. J. Am. Chem. Soc. 112 (1990) 6518-6529.