- Industrie: Chemistry
- Number of terms: 1965
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The International Union of Pure and Applied Chemistry (IUPAC) serves to advance the worldwide aspects of the chemical sciences and to contribute to the application of chemistry in the service of people and the environment. As a scientific, international, non-governmental and objective body, IUPAC ...
The ease of distortion of the electron cloud of a molecular entity by an electric field (such as that due to the proximity of a charged reagent). It is experimentally measured as the ratio of induced dipole moment (uind) to the field E which induces it:
<center>α = μ<sub>ind</sub>/E</center>
The units of α are C<sup>2</sup> m<sup>2 </sup>V<sup>-1</sup>. In ordinary usage the term refers to the "mean polarizability", i.e., the average over three rectilinear axes of the molecule. Polarizabilities in different directions (e.g. along the bond in Cl<sub>2</sub>, called "longitudinal polarizability", and in the direction perpendicular to the bond, called "transverse polarizability") can be distinguished, at least in principle. Polarizability along the bond joining a substituent to the rest of the molecule is seen in certain modern theoretical approaches as a factor influencing chemical reactivity, etc., and parametrization thereof has been proposed.
Industry:Chemistry
A geometric hypersurface on which the potential energy of a set of reactants is plotted as a function of the coordinates representing the molecular geometries of the system.
For simple systems two such coordinates (characterizing two variables that change during the progress from reactants to products) can be selected, and the potential energy plotted as a contour map.
For simple elementary reactions, e.g. A-B + C → A + B-C, the surface can show the potential energy for all values of the A, B, C geometry, providing that the ABC angle is fixed.
For more complicated reactions a different choice of two coordinates is sometimes preferred, e.g. the bond orders of two different bonds. Such a diagram is often arranged so that reactants are located at the bottom left corner and products at the top right. If the trace of the representative point characterizing the route from reactants to products follows two adjacent edges of the diagram, the changes represented by the two coordinates take place in distinct succession; if the trace leaves the edges and crosses the interior of the diagram, the two changes are concerted. In many qualitative applications it is convenient (although not strictly equivalent) for the third coordinate to represent the standard Gibbs energy rather than potential energy.
Using bond orders is, however, an oversimplification, since these are not well-defined, even for the transition state. (Some reservations concerning the diagrammatic use of Gibbs energies are noted under Gibbs energy diagram.)
The energetically easiest route from reactants to products on the potential-energy contour map defines the potential-energy profile.
Industry:Chemistry
A curve describing the variation of the potential energy of the system of atoms that make up the reactants and products of a reaction as a function of one geometric coordinate, and corresponding to the "energetically easiest passage" from reactants to products (i.e. along the line produced by joining the paths of steepest descent from the transition state to the reactants and to the products). For an elementary reaction the relevant geometric coordinate is the reaction coordinate; for a stepwise reaction it is the succession of reaction coordinates for the successive individual reaction steps. (The reaction coordinate is sometimes approximated by a quasi-chemical index of reaction progress, such as "degree of atom transfer" or bond order of some specified bond.)
Industry:Chemistry
A kinetic isotope effect attributable to isotopic substitution of an atom to which a bond is made or broken in the rate-controlling step or in a pre-equilibrium step of a specified reaction is termed a primary isotope effect. The corresponding isotope effect on the equilibrium constant of a reaction in which one or more bonds to isotopic atoms are broken, is called a "primary equilibrium isotope effect".
Industry:Chemistry
One of the conceptually simpler molecular changes into which an elementary reaction can be notionally dissected. Such changes include bond rupture, bond formation, internal rotation, change of bond length or bond angle, bond migration, redistribution of charge, etc.
The concept of primitive changes is helpful in the detailed verbal description of elementary reactions, but a primitive change does not represent a process that is by itself necessarily observable as a component of an elementary reaction.
Industry:Chemistry
When equilibrium is reached in a reaction system (containing an arbitrary number of components and reaction paths), as many atoms, in their respective molecular entities, will pass forward, as well as backwards, along each individual path in a given finite time interval. Accordingly, the reaction path in the reverse direction must in every detail be the reverse of the reaction path in the forward direction (provided always that the system is at equilibrium).
The principle of detailed balancing is a consequence for macroscopic systems of the principle of microscopic reversibility.
Industry:Chemistry
The hypothesis that, for given reactants, the reactions involving the smallest change in nuclear positions will have the lowest energy of activation . (It is also often simply referred to as principle of least motion.)
Industry:Chemistry
In a reversible reaction, the mechanism in one direction is exactly the reverse of the mechanism in the other direction. This does not apply to reactions that begin with a photochemical excitation.
Industry:Chemistry
This principle applies to reactions in which there is a lack of synchronization between bond formation or bond rupture and other primitive changes that affect the stability of products and reactants, such as resonance, solvation, electrostatic, hydrogen bonding and polarizability effects. The principle states that a product-stabilizing factor whose development lags behind bond changes at the transition state, or a reactant-stabilizing factor whose loss is ahead of bond changes at the transition state, increases the intrinsic barrier and decreases the "intrinsic rate constant" of a reaction. For a product-stabilizing factor whose development is ahead of bond changes, or reactant factors whose loss lags behind bond changes, the opposite relations hold. The reverse effects are observable for factors that destabilize a reactant or product.
Industry:Chemistry
The term is used for reactions under kinetic control where the selectivity parallels the relative (thermodynamic) stabilities of the products. Product development control is usually associated with a transition state occurring late on the reaction coordinate.
Industry:Chemistry
