- Settore: Chemistry
- Number of terms: 1965
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Alternative (and etymologically more correct) name for cheletropic reaction.
Industry:Chemistry
The breakdown of a single entity (normal molecule, reaction intermediate, etc.) into two or more fragments.
Industry:Chemistry
Reversible processes (processes which may be made to proceed in the forward or reverse direction by the (infinitesimal) change of one variable, ultimately reach a point where the rates in both directions are identical, so that the system gives the appearance of having a static composition at which the Gibbs energy, G, is a minimum. At equilibrium the sum of the chemical potentials of the reactants equals that of the products, so that
<center>ΔG<sub>r</sub> = ΔG<sub>r</sub><sup>0</sup> + RT ln K = 0
ΔG<sub>r</sub><sup>0</sup> = -RT ln K</center>
The equilibrium constant, K, is given by the mass-law effect.
Industry:Chemistry
A process that results in the interconversion of chemical species. Chemical reactions may be elementary reactions or stepwise reactions. (It should be noted that this definition includes experimentally observable interconversions of conformers.)
Detectable chemical reactions normally involve sets of molecular entities, as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. "microscopic chemical events").
Industry:Chemistry
If the equilibrium mixture of a chemical reaction is disturbed by a sudden change, especially of some external parameter (such as temperature, pressure, or electrical field strength), the system will readjust itself to a new position of the chemical equilibrium or return to the original position, if the perturbation is temporary. The readjustment is known as chemical relaxation.
In many cases, and in particular when the displacement from equilibrium is slight, the progress of the system towards equilibrium can be expressed as a first-order law
<center>(C<sub>t</sub> - (C<sub>eq</sub>)<sub>2</sub>) = ((C<sub>eq</sub>)<sub>1</sub> - (C<sub>eq</sub>)<sub>2</sub>)exp(-t/ τ)</center>
where (C<sub>eq</sub>)<sub>1</sub> and (C<sub>eq</sub>)<sub>2</sub> are the equilibrium concentrations of one of the chemical species involved in the reaction before and after the change in the external parameter, and C<sub>t</sub> is its concentration at time t. The time parameter t, named relaxation time, is related to the rate constants of the chemical reaction involved.
Measurements of the relaxation times by relaxation methods (involving a temperature jump (T-jump), pressure jump, electric field jump or a periodic disturbance of an external parameter, as in ultrasonic techniques) are commonly used to follow the kinetics of very fast reactions.
Industry:Chemistry
The variation of the resonance frequency of a nucleus in nuclear magnetic resonance (NMR) spectroscopy in consequence of its magnetic environment. The chemical shift of a nucleus, δ, is expressed in ppm by its frequency, ν<sub>cpd</sub>, relative to a standard, νref, and defined as
<center>δ = 10<sup>6</sup>( ν<sub>cpd</sub> - ν<sub>ref</sub>)/ ν<sub>o</sub></center>
where ν<sub>o</sub> is the operating frequency of the spectrometer. For <sup>1</sup>H and <sup>13</sup>C NMR the reference signal is usually that of tetramethylsilane (SiMe<sub>4</sub>). Other references are used in the older literature and in other solvents, such as D<sub>2</sub>O.
If a resonance signal occurs at lower frequency or higher applied field than an arbitrarily selected reference signal, it is said to be upfield, and if resonance occurs at higher frequency or lower applied field, the signal is downfield. Resonance lines upfield from SiMe<sub>4</sub> have positive, and resonance lines downfield from SiMe<sub>4</sub> have negative δ-values.
Industry:Chemistry
An ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on the time scale of the experiment. The term is applied equally to a set of chemically identical atomic or molecular structural units in a solid array.
For example, two conformational isomers may be interconverted sufficiently slowly to be detectable by separate NMR spectra and hence to be considered to be separate chemical species on a time scale governed by the radiofrequency of the spectrometer used. On the other hand, in a slow chemical reaction the same mixture of conformers may behave as a single chemical species, i.e. there is virtually complete equilibrium population of the total set of molecular energy levels belonging to the two conformers.
Except where the context requires otherwise, the term is taken to refer to a set of molecular entities containing isotopes in their natural abundance.
The wording of the definition given in the first paragraph is intended to embrace both cases such as graphite, sodium chloride, or a surface oxide, where the basic structural units may not be capable of isolated existence, as well as those cases where they are.
In common chemical usage, and in this Glossary, generic and specific chemical names (such as radical or hydroxide ion) or chemical formulae refer either to a chemical species or to a molecular entity.
Industry:Chemistry
Non-Boltzmann nuclear spin state distribution produced in thermal or photochemical reactions, usually from colligation and diffusion, or disproportionation of radical pairs, and detected by NMR spectroscopy by enhanced absorption or emission signals.
Industry:Chemistry
Chemoselectivity is the preferential reaction of a chemical reagent with one of two or more different functional groups. A reagent has a high chemoselectivity if reaction occurs with only a limited number of different functional groups. For example, sodium tetrahydroborate is a more chemoselective reducing agent than is lithium tetrahydroaluminate. The concept has not been defined in more quantitative terms. The term is also applied to reacting molecules or intermediates which exhibit selectivity towards chemically different reagents.
Some authors use the term chemospecificity for 100% chemoselectivity. However, this usage is discouraged.
Industry:Chemistry
The part (atom or group of atoms) of a molecular entity in which the electronic transition responsible for a given spectral band is approximately localized. The term arose in the dyestuff industry, referring originally to the groupings in the molecule that are responsible for the dye's color.
Industry:Chemistry
