Electronic effect

Electronic effect

An electronic effect influences the structure, reactivity, or properties of molecule but is neither a traditional bond nor a steric effect. [G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.] In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between the electronic structure and the geometry (stereochemistry) of a molecule.

Types of electronic effects

Induction is the redistribution of electron density through a traditional sigma bonded structure according to the electronegativity of the atoms involved. The inductive effect drops across every sigma bond involved limiting its effect to only a few bonds.

Conjugation is a redistribution of electron density similar to induction but transmitted through interconnected pi-bonds. Conjugation is not only affected by electronegativity of the connected atoms but the position of electron lone pairs with respect to the pi-system. Electronic effects can be transmitted through out a pi-system allowing their influence to extend further than induction.

Hyperconjugation is the stabilizing interaction that results from the interaction of the electrons in a sigma bond (usually C-H or C-C) with an adjacent empty (or partially filled) non-bonding p-orbital or antibonding π orbital or filled π orbital to give an extended molecular orbital that increases the stability of the system ["Organic chemistry" John McMurry 2nd edition ISBN 0534079687] . Hyperconjugation can be used to explain phenomena such as the gauche effect and anomeric effect.

Orbital symmetry is important when dealing with orbitals which contain directional components like p and d. An example of such an effect is square planar low spin d8 transition metal complexes. These complexes exist as square planar complexes due to the directionality of the metal center's d orbitals despite fewer steric congestion in a tetrahedral geometric structure. This is simple one example of many varied examples, including aspects of pericyclic reactions such as the Diels-Alder reaction, among others.

Electrostatic interactions include both attractive and repulsive forces associated with the build-up of charge in a molecule. Electrostatic interactions are generally too weak to be considered traditional bonds or are prevented from forming a traditional bond, possibly by a steric effect. A bond is usually defined as two atoms approaching closer than the sum of their Van der Waal radii. Hydrogen bonding boarders on being an actual "bond" and an electrostatic interaction. While an attractive electrostatic interaction is considered a "bond" if it gets too strong, a repulsive electrostatic interaction is always an electrostatic effect regardless of strength. An example of a repulsive effect is a molecule contorting to minimize the coulombic interactions of atoms which hold like charges.

Electronic spin state at it simplest describes the number of unpaired electrons in a molecule. Most molecules including the proteins, carbohydrates, and lipids that make up the majority of life have no unpaired electrons even when charged. Such molecules are called singlet molecules since their paired electrons only have one spin state. In contrast dioxygen under ambient conditions has two unpaired electrons. Dioxygen is a triplet molecule since the two unpaired electrons allow for three spin states. The reaction of a triplet molecule with a singlet molecule is quantum mechanically spin forbidden. This is the major reasons there is a very high reaction barrier for the extremely thermodynamically favorable reaction of singlet organic molecules with triplet oxygen. This kinetic barrier prevents life from bursting into flames at room temperature. Electronic spin state also includes the phenomenon of high spin and low spin transition metal complexes. For example a low spin d8 transition metal complex is usually square planar with no unpaired electrons while a high spin d8 transition metal complex is usually octahedral with two unpaired electrons.

Jahn-Teller effect is the geometrical distortion of non-linear molecules under certain situations. Any non-linear molecule with a degenerate electronic ground state will undergo a geometrical distortion that removes that degeneracy. This has the effect of lowering the overall energy. The Jahn-Teller distortion is especially common in certain transition metal complexes, for example copper(II) complexes with 9 d electrons.

Trans influence is the influence that a ligand in a square or octahedral complex has on the bond to the ligand trans to it. It is caused by electronic effects, and manifests itself as the lengthening of the trans bonds and as an effect on the overall energy of the complex.

Steric effects vs. Electronic effects

The structure, properties, and reactivity of a molecule is dependent on straight forward bonding interactions including covalent bonds, ionic bonds, hydrogen bonds and lesser forms of bonding. This bonding supplies a basic molecular skeleton that is modified by repulsive forces generally considered steric effects. Basic bonding and steric effects are at times insufficient to explain many structures, properties, and reactivity. Thus steric effects are often contrasted and complemented by electronic effects implying the influence of effects such as induction, conjunction, orbital symmetry, electrostatic interactions, and spin state. There are more esoteric electronic effects but these are among the most important when considering chemical structure and reactivity.

References


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