- 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 terms are symmetry designations, pi molecular orbitals being antisymmetric with respect to a defining plane containing at least one atom (e.g. the molecular plane of ethene), and sigma molecular orbitals symmetric with respect to the same plane. In practice the terms are used both in this rigorous sense (for orbitals encompassing the entire molecule) and also for localized two-center orbitals or bonds, and it is necessary to make a clear distinction between the two usages.
In the case of two-center bonds, a pi bond has a nodal plane that includes the internuclear bond axis, whereas a sigma bond has no such nodal plane. (A delta bond in organometallic or inorganic molecular species has two nodes.) Radicals are classified by analogy into sigma and pi radicals.
Such two-center orbitals may take part in molecular orbitals of sigma or pi symmetry. For example, the methyl group in propene contains three C-H bonds, each of which is of local sigma symmetry (i.e. without a nodal plane including the internuclear axis), but these three "sigma bonds" can in turn be combined to form a set of group orbitals one of which has pi symmetry with respect to the principal molecular plane and can accordingly interact with the two-center orbital of pi symmetry (pi bond) of the double-bonded carbon atoms, to form a molecular orbital of pi symmetry.
Such an interaction between the CH<sub>3</sub> group and the double bond is an example of what is called hyperconjugation. This cannot rigorously be described as "sigma-pi conjugation" since sigma and pi here refer to different defining planes, and interaction between orbitals of different symmetries (with respect to the same defining plane) is forbidden.
Industry:Chemistry
(1) Generic name for H<sub>2</sub>Si: and substitution derivatives thereof, containing an electrically neutral bivalent silicon atom with two non-bonding electrons. (The definition is analogous to that given for carbene.)
(2) The silanediyl group (H<sub>2</sub>Si), analogous to the methylene group (H<sub>2</sub>C).
Industry:Chemistry
A reaction mechanism characterized by the transfer of a single electron between the species occurring on the reaction coordinate of one of the elementary steps.
Industry:Chemistry
A SOMO is a singly occupied molecular orbital such as half-filled HOMO or a half-empty LUMO of a radical.
Industry:Chemistry
An approximate atomic orbital that attempts to allow for electron-electron repulsion by scaling the nuclear charge for each orbital.
Industry:Chemistry
Any stabilizing interaction of a solute (or solute moiety) and the solvent or a similar interaction of solvent with groups of an insoluble material (i.e., the ionic groups of an ion-exchange resin). Such interactions generally involve electrostatic forces and van der Waals forces, as well as chemically more specific effects such as hydrogen bond formation.
Industry:Chemistry
A linear free-energy relationship based on solvatochromism.
Industry:Chemistry
The (pronounced) change in position and sometimes intensity of an electronic absorption or emission band, accompanying a change in the polarity of the medium. Negative (positive) solvatochromism corresponds to a hypsochromic (bathochromic) shift with increasing solvent polarity.
Industry:Chemistry
A kinetic or equilibrium isotope effect resulting from change in the isotopic composition of the solvent.
Industry:Chemistry
