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Theories in Chemistry

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Richard Bader. Richard F.

Richard Bader

W. Bader (October 15, 1931 – January 15, 2012) was a Canadian quantum chemist, noted for his work on the Atoms in molecules theory. Benson group increment theory. Benson Group Increment Theory (BGIT), or Group Increment Theory, uses the experimentally calculated heat of formation for individual groups of atoms to calculate the entire heat of formation for a molecule under investigation.

Benson group increment theory

This can be a quick and convenient way to determine theoretical heats of formation without conducting tedious experimentation. The technique was developed by Professor Sidney William Benson of the University of Southern California. Heats of formations are intimately related to bond dissociation energies and thus are important in understanding chemical structure and reactivity.[1] Furthermore, although the theory is old, it still is practically useful as one of the best group additivity methods aside from computational methods such as molecular mechanics.

However, the BGIT has its limitations, and thus cannot always predict the precise heat of formation. Origin[edit] A Limiting Law for Additivity Rules.Zero-Order Approximation. Brønsted–Lowry acid–base theory. Chemical graph theory. See also[edit] References[edit] Jump up ^ Danail Bonchev, D.H.

Chemical graph theory

Rouvray (eds.) (1991) "Chemical Graph Theory: Introduction and Fundamentals", ISBN 0-85626-454-7Jump up ^ Nenad Trinajstic – Pioneer of Chemical Graph Theory, by Milan RandićJump up ^ A review of the book by Ivan Gutman, Oskar E. Polansky, "Mathematical Concepts in Organic Chemistry" in SIAM Review Vol. 30, No. 2 (1988), pp. 348-350Jump up ^ D.H. Rouvray, "Combinatorics in Chemistry", pp. 1955-1982, in: Ronald Graham, Martin Grötschel, László Lovász (Eds.) (1996) Handbook of Combinatorics, vol. Collision theory. When a catalyst is involved in the collision between the reactant molecules, less energy is required for the chemical change to take place, and hence more collisions have sufficient energy for reaction to occur.

Collision theory

The reaction rate therefore increases. Crystal field theory. Overview of crystal field theory analysis[edit] According to CFT, the interaction between a transition metal and ligands arises from the attraction between the positively charged metal cation and negative charge on the non-bonding electrons of the ligand.

Crystal field theory

Debye–Hückel theory. Overview[edit] In an ideal electrolyte solution the activity coefficients of all the ions are equal to one.

Debye–Hückel theory

Non-ideality arises principally (but not exclusively) because ions of opposite charge attract each other due to electrostatic forces, while ions of the same charge repel each other. In consequence ions are not randomly distributed throughout the solution, as they would be in an ideal solution. Activity coefficients of single ions cannot be measured experimentally because an electrolyte solution must contain both positively charged ions and negatively charged ions. Instead, a mean activity coefficient, is defined. Density functional theory. DFT has been very popular for calculations in solid-state physics since the 1970s.

Density functional theory

However, DFT was not considered accurate enough for calculations in quantum chemistry until the 1990s, when the approximations used in the theory were greatly refined to better model the exchange and correlation interactions. In many cases the results of DFT calculations for solid-state systems agree quite satisfactorily with experimental data. Computational costs are relatively low when compared to traditional methods, such as Hartree–Fock theory and its descendants based on the complex many-electron wavefunction. Flory–Huggins solution theory. For mixing a polymer with a solvent.

Flory–Huggins solution theory

Although it makes simplifying assumptions, it generates useful results for interpreting experiments. The thermodynamic equation for the Gibbs free energy change accompanying mixing at constant temperature and (external) pressure is A change, denoted by and The result obtained by Flory[1] and Huggins[2] is and volume fraction. Frontier molecular orbital theory.

In chemistry, frontier molecular orbital theory is an application of MO theory describing HOMO / LUMO interactions.

Frontier molecular orbital theory

History[edit] In 1952, Kenichi Fukui published a paper in the Journal of Chemical Physics titled "A molecular theory of reactivity in aromatic hydrocarbons. "[1] Though widely criticized at the time, he later shared the Nobel Prize in Chemistry with Roald Hoffmann for his work on reaction mechanisms. Hoffman's work focused on creating a set of four pericyclic reactions in organic chemistry, based on orbital symmetry, which he coauthored with Robert Burns Woodward entitled, "The Conservation of Orbital Symmetry.

" HSAB theory. The HSAB concept is an initialism for "hard and soft (Lewis) acids and bases".

HSAB theory

Also known as the Pearson acid base concept, HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species. 'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are strongly polarizable.[1] The concept is a way of applying the notion of orbital overlap to specific chemical cases. [citation needed] The theory is used in contexts where a qualitative, rather than quantitative, description would help in understanding the predominant factors which drive chemical properties and reactions. Kinetic theory. This article is about kinetic theory of gases.

For branch of classical mechanics, see Kinematics. Lewis acids and bases. Diagram of Lewis acids and bases Diagram of Lewis acid and base bond types. For example, an s-LUMO Lewis acid such as the sodium ion (Na+), interacts with a Lobe-HOMO Lewis base such as the hydroxide ion (OH–), to give sodium hydroxide, a Type 7 complex.

The term Lewis acid refers to a definition of acid published by Gilbert N. Ligand field theory. Ligand field theory (LFT) describes the bonding, orbital arrangement, and other characteristics of coordination complexes.[1] It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valence atomic orbitals - consisting of five (n)d, one (n+1)s, and three (n+1)p orbitals. These orbitals are of appropriate energy to form bonding interaction with ligands. The LFT analysis is highly dependent on the geometry of the complex, but most explanations begin by describing octahedral complexes, where six ligands coordinate to the metal.

Other complexes can be determined by reference to crystal field theory.[2] Marcus theory. Marcus theory is a theory originally developed by Rudolph A. Marcus, starting in 1956, to explain the rates of electron transfer reactions – the rate at which an electron can move or jump from one chemical species (called the electron donor) to another (called the electron acceptor).[1] It was originally formulated to address outer sphere electron transfer reactions, in which the two chemical species only change in their charge with an electron jumping (e.g. the oxidation of an ion like Fe2+/Fe3+), but do not undergo large structural changes.

It was extended to include inner sphere electron transfer contributions, in which a change of distances or geometry in the solvation or coordination shells of the two chemical species is taken into account (the Fe-O distances in Fe(H2O)2+ and Fe(H2O)3+ are different).[2][3] G0 of the redox reaction. G0 domain. R.A. Molecule. Molecular orbital theory. Møller–Plesset perturbation theory. Polyhedral skeletal electron pair theory. Polymer field theory. Canonical ensemble[edit] Reptation. Reptation is the thermal motion of very long linear, entangled macromolecules in polymer melts or concentrated polymer solutions. Derived from the word reptile, reptation suggests the movement of entangled polymer chains as being analogous to snakes slithering through one another. "The forward and backward diffusion is reminiscent of the motion of a snake and for this reason the motion was given the name reptation by de Gennes, after the Latin word reptare, which means to creep.

" Ring strain. In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. RRKM theory. Rubber elasticity. Rubber elasticity, a well-known example of hyperelasticity, describes the mechanical behavior of many polymers, especially those with Cross-link. Specific ion interaction theory. Transition state theory. Figure 1: Reaction coordinate diagram for the bimolecular nucleophilic substitution (SN2) reaction between bromomethane and the hydroxide anion. Valence bond theory. Variational transition-state theory.