- 行业: Chemistry
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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 Brønsted acid BH<sup>+</sup> formed on protonation of a base B is called the conjugate acid of B, and B is the conjugate base of BH<sup>+</sup>. (The conjugate acid always carries one unit of positive charge more than the base, but the absolute charges of the species are immaterial to the definition.) For example: the Brønsted acid HCl and its conjugate base Cl<sup>-</sup> constitute a conjugate acid-base pair.
Industry:Chemistry
In the original meaning a conjugated system is a molecular entity whose structure may be represented as a system of alternating single and multiple bonds: e.g.
<center>CH<sub>2</sub>=CH=CH=CH<sub>2</sub> CH<sub>2</sub>=CH-C≡N</center>
In such systems, conjugation is the interaction of one p-orbital with another across an intervening sigma bond in such structures. (In appropriate molecular entities d-orbitals may be involved.) The term is also extended to the analogous interaction involving a p-orbital containing an unshared electron pair, e.g.
<center>:Cl-CH=CH<sub>2</sub></center>
Industry:Chemistry
In a chemical context, the information content of a line formula, but omitting any indication of bond multiplicity.
Industry:Chemistry
The description of the identity and connectivity (and corresponding bond multiplicities) of the atoms in a molecular entity (omitting any distinction from their spatial arrangement).
Industry:Chemistry
The definition is based on the valence-bond formulation of the quantum mechanical idea of the wavefunction of a molecule as composed of a linear combination of wavefunctions, each representative of a formula containing bonds that are only single, double or triple with a particular pairing of electron spins. Each such formula represents a contributing structure, also called "resonance structure" to the total wavefunction, and the degree to which each contributes is indicated by the square of its coefficient in the linear combination. The contributing structures, also called "canonical forms", themselves thus have a purely formal significance: they are the components from which wavefunctions can be built. Structures may be covalent (or non-polar) or ionic (or polar). The representation is frequently kept qualitative so that we speak of important or major contributing structures and minor contributing structures. For example, two major non-equivalent contributing structures for the conjugate base of acetone are
<center>CH<sub>2</sub>=C(CH<sub>3</sub>)-O<sup>-</sup> ↔ H<sub>2</sub>C<sup>-</sup>-C(CH<sub>3</sub>)=O</center>
Industry:Chemistry
The formation of a covalent bond, the two shared electrons of which have come from only one of the two parts of the molecular entity linked by it, as in the reaction of a Lewis acid and a Lewis base to form a Lewis adduct; alternatively, the bonding formed in this way. In the former sense, it is the reverse of unimolecular heterolysis. "Coordinate covalence" and "coordinate link" are synonymous (obsolescent) terms. The synonym "dative bond" is obsolete.
(The origin of the bonding electrons has by itself no bearing on the character of the bond formed. Thus, the formation of methyl chloride from a methyl cation and a chloride ion involves coordination; the resultant bond obviously differs in no way from the C-Cl bond in methyl chloride formed by any other path, e.g. by colligation of a methyl radical and a chlorine atom.)
The term is also used to describe the number of ligands around a central atom without necessarily implying two-electron bonds.
Industry:Chemistry
The coordination number of a specified atom in a chemical species is the number of other atoms directly linked to that specified atom. For example, the coordination number of carbon in methane is four, and it is five in protonated methane, CH<sub>5</sub><sup>+</sup>. (The term is used in a different sense in the crystallographic description of ionic crystals.)
Industry:Chemistry
A transition metal complex that has formally 18 outer shell electrons at the central metal atom.
Industry:Chemistry
A transition metal complex that possesses fewer ligands than exist in the coordinatively saturated complex. These complexes usually have fewer than 18 outer shell electrons at the central metal atom.
Industry:Chemistry
The use of empirical correlations relating one body of experimental data to another, with the objective of finding quantitative estimates of the factors underlying the phenomena involved. Correlation analysis in organic chemistry often uses linear free-energy relations for rates or equilibria of reactions, but the term also embraces similar analysis of physical (most commonly spectroscopic) properties and of biological activity.
Industry:Chemistry
