Editing Partial charge

{{short description|Electric charge which is not an integer multiple of elementary charge}}

In [[atomic physics]], a '''partial charge''' (or '''net atomic charge''') is a non-[[integer]] charge value when measured in [[electric charge|elementary charge]] units. It is represented by the Greek lowercase [[delta (letter)|delta]] (𝛿), namely 𝛿− or 𝛿+.

Partial charges are created due to the asymmetric distribution of [[electron]]s in [[chemical bond]]s. 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 [[atomic nucleus|nucleus]] of an [[atom]]: When an electrically neutral atom [[covalent bond|bonds chemically]] to another neutral atom that is more [[electronegativity|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.

{{Image frame|width=230|caption=Polarity of [[chloromethane]] (left)<br/> and of the related [[Grignard compound]]<br/> with indication of the partial charge.|content=<chem>H3\overset{\delta+}C-\overset{\delta-}Cl \qquad H3\overset{\delta-}C-\overset{\delta+}Mg\overset{\delta-}Cl</chem>}}
In such a situation, the distributed charges taken as a group always carries a [[integer|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 field (chemistry)|force fields]] to compute the electrostatic interaction energy using [[Coulomb's law]], even though this leads to substantial failures for anisotropic charge distributions.<ref>{{cite journal |last1=Kramer |first1=Christian |last2=Spinn |first2=Alexander |last3=Liedl |first3=Klaus R. |title=Charge Anisotropy: Where Atomic Multipoles Matter Most |journal=Journal of Chemical Theory and Computation |year=2014 |volume=10 |issue=10 |pages=4488–4496 |doi=10.1021/ct5005565|pmid=26588145 }}</ref> Partial charges are also often used for a qualitative understanding of the structure and reactivity of molecules.

Occasionally, δδ+ is used to indicate a partial charge that is less positively charged than δ+ (likewise for δδ−) in cases where it is relevant to do so.<ref>{{Cite web|title=Basic principles in organic chemistry: Steric and electronic effects in a covalent bond – Open Teaching Project|url=http://padakshep.org/otp/subjects/chemistry/organic-chemistry/steric-electronic-effects/|access-date=2020-10-11|language=en-US}}</ref> This can be extended to δδδ± to indicate even weaker partial charges as well. Generally, a single δ+ (or δ−) is sufficient for most discussions of partial charge in organic chemistry.

==Determining partial atomic charges==

Partial atomic charges can be used to quantify the degree of ionic versus covalent bonding of any compound across the periodic table. The necessity for such quantities arises, for example, in molecular simulations to compute bulk and surface properties in agreement with experiment. Evidence for chemically different compounds shows that available experimental data and chemical understanding lead to justified atomic charges.<ref>{{cite journal | doi = 10.1021/jp048142t | author1 = H. Heinz | author2 = U. W. Suter | title = Atomic Charges for Classical Simulations of Polar Systems | journal = J. Phys. Chem. B | year = 2004 | volume = 108 | issue = 47 | pages = 18341–18352}}</ref> Atomic charges for a given compound can be derived in multiple ways, such as:
# extracted from electron densities measured using high resolution x-ray, gamma ray, or electron beam diffraction experiments
# measured dipole moments
# the Extended Born thermodynamic cycle, including an analysis of covalent and ionic bonding contributions
# spectroscopically measured properties, such as core-electron binding energy shifts
# the relationship of atomic charges to melting points, solubility, and cleavage energies for a set of similar compounds with similar degree of covalent bonding
# the relationship of atomic charges to chemical reactivity and reaction mechanisms for similar compounds reported in the literature.

The discussion of individual compounds in prior work has shown convergence in atomic charges, i.e., a high level of consistency between the assigned degree of polarity and the physical-chemical properties mentioned above. The resulting uncertainty in atomic charges is ±0.1e to ±0.2e for highly charged compounds, and often <0.1e for compounds with atomic charges below ±1.0e. Often, the application of one or two of the above concepts already leads to very good values, especially taking into account a growing library of experimental benchmark compounds and compounds with tested force fields.<ref>{{cite journal | doi = 10.1021/la3038846 | author1 = H. Heinz | author2 = T. Z. Lin | author3 = R. K. Mishra | author4 = F. S. Emami | title = Thermodynamically Consistent Force Fields for the Assembly of Inorganic, Organic, and Biological Nanostructures: The INTERFACE Force Field | journal = Langmuir | year = 2013 | volume = 29 | issue = 6 | pages = 1754–1765 | pmid = 23276161}}</ref>

The published research literature on partial atomic charges varies in quality from extremely poor to extremely well-done. Although a large number of different methods for assigning partial atomic charges from quantum chemistry calculations have been proposed over many decades, the vast majority of proposed methods do not work well across a wide variety of material types.<ref name = Manz2016>{{cite journal | doi = 10.1039/c6ra04656h | journal = RSC Adv. | title = Introducing DDEC6 atomic population analysis: part 1. Charge partitioning theory and methodology | author1 = T. A. Manz | author2 = N. Gabaldon-Limas | year = 2016 | volume = 6 | issue = 53 | pages = 47771–47801| bibcode = 2016RSCAd...647771M | s2cid = 102206475 | url = https://zenodo.org/record/895508 }}</ref><ref name = Limas2016>{{cite journal | doi = 10.1039/c6ra05507a | journal = RSC Adv. | title = Introducing DDEC6 atomic population analysis: part 2. Computed results for a wide range of periodic and nonperiodic materials | author1 = N. Gabaldon-Limas | author2 = T. A. Manz | year = 2016 | volume = 6 | issue = 51 | pages = 45727–45747| bibcode = 2016RSCAd...645727L | s2cid = 102242157 | doi-access = free }}</ref> Only as recently as 2016 was a method for theoretically computing partial atomic charges developed that performs consistently well across an extremely wide variety of material types.<ref name = Manz2016 /><ref name = Limas2016 /> All of the earlier methods had fundamental deficiencies that prevented them from assigning accurate partial atomic charges in many materials.<ref name = Manz2016 /><ref name = Limas2016 /> Mulliken and Löwdin partial charges are physically unreasonable, because they do not have a mathematical limit as the basis set is improved towards completeness.<ref name = Reed1985>{{cite journal | doi = 10.1063/1.449486 | journal = J. Chem. Phys. | title = Natural population analysis | author1 = A. E. Reed | author2 = R. B. Weinstock | author3 = F. Weinhold |year = 1985 | volume = 83 | issue = 2 | pages = 735–746| bibcode = 1985JChPh..83..735R }}</ref> Hirshfeld partial charges are usually too low in magnitude.<ref>{{cite journal | doi = 10.1007/BF01113058 | journal = Theor. Chim. Acta | title = A test of the Hirshfeld definition of atomic charges and moments | author1 = E. R. Davidson | author2 = S. Chakravorty | year = 1992 | volume = 83 | issue = 5–6 | pages = 319–330| s2cid = 93652756 }}</ref> Some methods for assigning partial atomic charges do not converge to a unique solution.<ref name = Manz2016 /> In some materials, [[atoms in molecules]] analysis yields non-nuclear attractors describing electron density partitions that cannot be assigned to any atom in the material; in such cases, [[atoms in molecules]] analysis cannot assign partial atomic charges.<ref>{{cite journal | doi = 10.1007/BF01192234 | journal = Theor. Chim. Acta | title = Charge density topological study of bonding in lithium clusters | author1 = C. Gatti | author2 = P. Fantucci | author3 = G. Pacchioni | year = 1987 | volume = 72 | issue = 5–6 | pages = 433–458| s2cid = 101073677 }}</ref>

According to Cramer (2002), partial charge methods can be divided into four classes:<ref>{{cite book | author = C. J. Cramer | title = Essentials of Computational Chemistry: Theories and Methods | publisher = Wiley | year = 2002 | pages = 278–289}}</ref>

*''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).<ref>{{cite journal | author1 = J. Meister | author2 = W. H. E. Schwarz | title = Principal Components of Ionicity | journal = J. Phys. Chem. | year = 1994 | volume = 98 | issue = 33 | pages = 8245–8252 | doi=10.1021/j100084a048}}</ref>

* Population analysis of [[wavefunction]]s
** [[Mulliken population analysis]]
** Löwdin population analysis<ref>{{cite journal | url= https://doi.org/10.1063/1.1747632 | title=On the Non‐Orthogonality Problem Connected with the Use of Atomic Wave Functions in the Theory of Molecules and Crystals | year=1950 | doi=10.1063/1.1747632 | accessdate=2021-01-21| last1=Löwdin | first1=Per‐Olov | journal=The Journal of Chemical Physics | volume=18 | issue=3 | pages=365–375 | bibcode=1950JChPh..18..365L | url-access=subscription }}</ref>
** Coulson's charges
** Natural charges<ref name = Reed1985 />
** CM1, CM2, CM3, CM4, and CM5<ref>{{cite journal | doi = 10.1021/ct200866d | journal = J. Chem. Theory Comput. | title = Charge Model 5: An Extension of Hirshfeld Population Analysis for the Accurate Description of Molecular Interactions in Gaseous and Condensed Phases | author1 = A. V. Marenich | author2 = S. V. Jerome | author3 = C. J. Cramer | author4 = D. G. Truhlar | year = 2012 | volume = 8 | issue = 2 | pages = 527–541| pmid = 26596602 | doi-access = free }}</ref> charge models
* Partitioning of [[electron density]] distributions
** Bader charges (obtained from an [[atoms in molecules]] analysis)
** Density fitted atomic charges
** Hirshfeld charges<ref>{{cite journal | doi = 10.1007/BF00549096 | journal = Theor. Chim. Acta | title = Bonded-atom fragments for describing molecular charge densities | author1 = F. L. Hirshfeld | year = 1977 | volume = 44 | issue = 2 | pages = 129–138| s2cid = 98677294 }}</ref>
** Maslen's corrected Bader charges<ref>{{cite journal | doi = 10.1071/PH850273 | journal = Aust. J. Phys. | title = Atomic charges and electron density partitioning | author1 = E. N. Maslen | author2 = M. A. Spackman | year = 1985 | volume = 38 | issue = 3 | pages = 273–287| bibcode = 1985AuJPh..38..273M | doi-access = free }}</ref>
** Politzer's charges
** [[Voronoi Deformation Density]] charges
** Density Derived Electrostatic and Chemical (DDEC) charges, which simultaneously reproduce the chemical states of atoms in a material and the electrostatic potential surrounding the material's electron density distribution<ref>{{cite journal | doi = 10.1021/ct3002199 | journal = J. Chem. Theory Comput. | title = Improved Atoms-in-Molecule Charge Partitioning Functional for Simultaneously Reproducing the Electrostatic Potential and Chemical States in Periodic and Nonperiodic Materials | author1 = T. A. Manz | author2 = D. S. Sholl | year = 2012 | volume = 8 | issue = 8 | pages = 2844–2867| pmid = 26592125 }}</ref><ref name = Manz2016 />
* Charges derived from dipole-dependent properties
** [[Dipole]] charges
** Dipole derivative charges, also called atomic polar tensor (APT) derived charges,<ref>{{cite journal | doi = 10.1021/ja00174a011 | journal = J. Am. Chem. Soc. | title = Theory of vibrational rotational strengths: comparison of a priori theory and approximate models | author1 = P. J. Stephens | author2 = K. J. Jalkanen | author3 = R. W. Kawiecki | year = 1990 | volume = 112 | issue = 18 | pages = 6518–6529}}</ref> or Born, Callen, or Szigeti effective charges <ref>{{cite journal | doi = 10.1103/PhysRevB.58.6224 | journal = Phys. Rev. B | title = Dynamical atomic charges: The case of ABO<sub>3</sub> compounds | author1 = Ph. Ghosez | author2 = J.-P. Michenaud | author3 = X. Gonze | year = 1998 | volume = 58 | issue = 10 | pages = 6224–6240| arxiv = cond-mat/9805013 | bibcode = 1998PhRvB..58.6224G | s2cid = 119089568 }}</ref>
* Charges derived from electrostatic potential
** Chelp
** [[Chelpg|ChelpG (Breneman model)]]
** Merz-Singh-Kollman (also known as Merz-Kollman, or MK)
** RESP (Restrained Electrostatic Potential)<ref>{{cite journal | doi = 10.1021/j100142a004 | journal = J. Phys. Chem. | title = A well-behaved electrostatic potential based method using charge restraints for deriving atomic charges: the RESP model | author1 = C. I. Bayly |  author2 = P. Cieplak | author3 =  W. Cornell | author4 = P. A. Kollman | year = 1993 | volume = 97 | issue = 40 | pages= 10269–10280}}</ref>
* 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 theory|ligand-field]] splittings
** Charges from [[Ultraviolet-visible spectroscopy|UV-vis intensities]] of transition metal complexes
** Charges from other spectroscopies, such as [[NMR spectroscopy|NMR]], [[Electron spin resonance|EPR]], [[EQR]]
* Charges from other experimental data
** Charges from [[bandgap]]s or [[dielectric constant]]s
** Apparent charges from the [[piezoelectric effect]]
** Charges derived from [[potential energy surface|adiabatic potential energy curves]]
** [[Electronegativity]]-based charges
** Other physicochemical data, such as [[equilibrium constant|equilibrium]] and [[reaction rate constant]]s, [[thermochemistry]], and liquid densities.
* [[Formal charge]]s

==References==
* {{cite book | author = Frank Jensen | title = Introduction to Computational Chemistry | date = 29 November 2006 | edition =  2nd | publisher = Wiley | isbn = 978-0-470-01187-4}}
<references/>

[[Category:Computational chemistry]]
[[Category:Electric charge]]