- المجال: 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 ...
Control of stereoselectivity of a reaction by steric hindrance towards attack of the reagent, which is directed to the less hindered face of the molecule. Partial bond making is strong enough at the transition state for steric control to take place. This suggests that the transition state should not be close to products.
Industry:Chemistry
A technique for following the kinetics of reactions in solution (usually in the millisecond time range) in which two reactant solutions are rapidly mixed by being forced through a mixing chamber. The flow of the mixed solution along a uniform tube is then suddenly arrested. At a fixed position along the tube the solution is monitored (as a function of time following the stoppage of the flow) by some method with a rapid response (e.g. photometry).
Industry:Chemistry
Strain is present in a molecular entity or transition structure if the energy is enhanced because of unfavorable bond lengths, bond angles, or dihedral angles ("torsional strain") relative to a standard.
It is quantitatively defined as the standard enthalpy of a structure relative to a strainless structure (real or hypothetical) made up from the same atoms with the same types of bonding. (The enthalpy of formation of cyclopropane is 53.6 kJ mol<sup>-1</sup>, whereas the enthalpy of formation based on three "normal" methylene groups, from acyclic models, is -62 kJ mol<sup>-1</sup>. On this basis cyclopropane is destabilized by ca. 115 kJ mol<sup>-1</sup> of strain energy.)
Industry:Chemistry
The Next-to-Highest Occupied Molecular Orbital ("NHOMO", also called "HOMO<sup>-</sup>") and the Second Lowest Unoccupied Molecular Orbital (SLUMO). Subjacent orbitals are sometimes found to play an important role in the interpretation of molecular interactions according to the frontier orbital approach.
Industry:Chemistry
An atom or group of bonded atoms that can be considered to have replaced a hydrogen atom (or two hydrogen atoms in the special case of bivalent groups) in a parent molecular entity (real or hypothetical).
Industry:Chemistry
A reaction, elementary or stepwise, in which one atom or group in a molecular entity is replaced by another atom or group. For example,
<center>CH<sub>3</sub>Cl + OH<sup>-</sup> → CH<sub>3</sub>OH + Cl<sup>-</sup></center>
Industry:Chemistry
A chemical species, the reaction of which with some other chemical reagent is under observation (e.g., a compound that is transformed under the influence of a catalyst). The term should be used with care. Either the context or a specific statement should always make it clear which chemical species in a reaction is regarded as the substrate.
Industry:Chemistry
The radical ion pair which forms by the transfer of an electron from the donor D to the acceptor A after these species have diffused together to form the precursor or encounter complex:
<center>A + D → (A D) → (A<sup>. -</sup> D<sup>.+</sup>)</center>
Industry:Chemistry
A medium having a high acidity, generally greater than that of 100 wt.-% sulfuric acid. The common superacids are made by dissolving a powerful Lewis acid (e.g. SbF<sub>5</sub>) in a suitable Brønsted acid, such as HF or HSO<sub>3</sub>F. (An equimolar mixture of HSO<sub>3</sub>F and SbF<sub>5</sub> is known by the trade name "magic acid".)
In a biochemical context "superacid catalysis" is sometimes used to denote catalysis by metal ions analogous to catalysis by hydrogen ions.
By analogy, a compound having a very high basicity, such as lithium diisopropylamide, is called a "superbase".
Industry:Chemistry
A system of two or more molecular entities held together and organized by means of intermolecular (noncovalent) binding interactions. LEHN (1993).
Industry:Chemistry
