- 行业: 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 ...
A quantitative measure for nuclear spin-spin, nuclear-electron (hyperfine coupling) and electron-electron (fine coupling in EPR) coupling in magnetic resonance spectroscopy. The "indirect" or scalar NMR coupling constants are in a first approximation independent of the external magnetic field and are expressed in Hz.
Industry:Chemistry
A region of relatively high electron density between nuclei which arises at least partly from sharing of electrons and gives rise to an attractive force and characteristic internuclear distance.
Industry:Chemistry
A modification of the Bunnett-Olsen equation of the form
<center>lg ((SH<sup>+</sup>))/(S)) - lg(H<sup>+</sup>) = m<sup>*</sup>X + pK<sub>SH</sub>+</center>
where X is the activity function lg(γSγH<sup>+</sup>)/γSH<sup>+</sup>) for an arbitrary reference base. The function X is called the excess acidity because it gives a measure of the difference between the acidity of a solution and that of an ideal solution of the same concentration. In practice X = - (Ho + lg(H<sup>+</sup>)) and m<sup>*</sup> = 1 - Φ.
Industry:Chemistry
There is a relatively small range of concentrations separating the limit below which virtually no micelles are detected and the limit above which virtually all additional surfactant molecules form micelles. Many properties of surfactant solutions, if plotted against the concentration, appear to change at a different rate above and below this range. By extrapolating the loci of such a property above and below this range until they intersect, a value may be obtained known as the critical micellization concentration (critical micelle concentration), symbol cM, abbreviation cmc (or c.m.c.). As values obtained using different properties are not quite identical, the method by which the cmc is determined should be clearly stated.
Industry:Chemistry
In a system XC<sub>6</sub>H<sub>4</sub>GY this is conjugation involving the substituent X, the benzene ring, and the side-chain connective-plus-reaction site GY, i.e. either X is a +R group and GY is a -R group, or X is a -R group and GY is a +R group. In Hammett correlations this situation can lead to the need to apply exalted substituent constants σ<sup>+</sup> or σ<sup>-</sup>, respectively, as in electrophilic or nucleophilic aromatic substitution, respectively. The term "through resonance" is synonymous. Cross conjugation has also been used to describe the interactions occurring in 2-phenylallyl and similar systems.
Industry:Chemistry
That part of a solution in the vicinity of a solute molecule in which the ordering of the solvent molecules is modified by the presence of the solute molecule. The term solvent "cosphere" of the solute has also been used.
Industry:Chemistry
Formation of a ring compound from a chain by formation of a new bond.
Industry:Chemistry
The reverse of cycloaddition. The term is preferred to the synonyms "cycloreversion", "retro-addition", and "retrocycloaddition".
Industry:Chemistry
Substances having a negative magnetic susceptibility are diamagnetic. They are repelled out of a magnetic field.
Industry:Chemistry
