- Settore: 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 energy released when an additional electron (without excess energy) attaches itself to a molecular entity (usually an electrically neutral molecular entity). (The direct measurement of this quantity involves molecular entities in the gas phase.)
Industry:Chemistry
If P(x,y,z) dx dy dz is the probability of finding an electron in the volume element dx dy dz at the point of a molecular entity with coordinates x,y,z, then P(x,y,z) is the electron density at this point. For many purposes (e.g., X-ray scattering, forces on atoms) the system behaves exactly as if the electrons were spread out into a continuously distributed charge. The term has frequently been wrongly applied to negative charge population.
Industry:Chemistry
(1) A molecular entity that can transfer an electron to another molecular entity, or to the corresponding chemical species.
+ (2) A Lewis base. This use is discouraged.
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The transfer of an electron from one molecular entity to another, or between two localized sites in the same molecular entity.
Industry:Chemistry
A measure of the power of an atom or a group of atoms to attract electrons from other parts of the same molecular entity. The concept has been quantified by a number of authors, including especially Pauling and Mulliken.
Industry:Chemistry
The principle expresses the fact that all pure substances carry a net charge of zero.
Industry:Chemistry
A distribution of the electrons of an atom or a molecular entity over a set of one-electron wavefunctions called orbitals, according to the Pauli principle. From one configuration several states with different multiplicities may result. For example, the ground electronic configuration of the oxygen molecule (O<sub>2</sub>) is
<center>1σ<sub>g</sub><sup>2</sup>, 1σ<sub>u</sub><sup>2</sup>, 2σ<sub>g</sub><sup>2</sup>, 2σ<sub>u</sub><sup>2</sup>, 1π<sub>u</sub><sup>4</sup>, 3σ<sub>g</sub><sup>2</sup>, 1π<sub>g</sub><sup>2</sup></center>
resulting in the
<center><sup>3</sup>Σ<sub>g</sub>, <sup>1</sup>Δ<sub>g</sub>, and <sup>3</sup>Σ<sub>g</sub><sup>+</sup> multiplets</center>
Industry:Chemistry
The inductive effect has universally been represented by the symbol I. This is now commonly taken to include both through-bonds and through-space transmission, but I is also used specifically for through-bonds transmission; through-space transmission is then symbolized as F (for field effect). The symbols for the influence of substituents exerted through electron delocalization have variously been M (mesomeric), E (electromeric), T (tautomeric), C (conjugative), K (konjugativ), and R (resonance). Since the present fashion is to use the term resonance effect, R is the most commonly used symbol, although M is still seen quite often.
Both the possible sign conventions are in use. The Ingold sign convention associates electronegativity (relative to hydrogen atom) with a negative sign, electropositivity with a positive sign. Thus the nitro group is described as electron- withdrawing by virtue of its - I and - M effects; chloro is described as a - I, +M substituent, etc. For correlation analysis and linear free-energy relationships this convention has been found inconvenient, for it is in contradiction to the sign convention for polar substituent constants (σ-constants). Authors concerned with these fields often avoid this contradiction by adopting the opposite sign convention originally associated with Robinson, for electronic effects. This practice is almost always associated with the use of R for the electron delocalization effect: thus the nitro group is a +I, +R substituent; chloro a +I, - R substituent, etc.
Industry:Chemistry
