Electronic effect
Encyclopedia
An electronic effect influences the structure
, reactivity, or properties of molecule
but is neither a traditional bond
nor a steric effect. 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.
is the redistribution of electron density
through a traditional sigma bond
ed 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 also affected by the position of electron lone pairs with respect to the pi-system. Electronic effects can be transmitted throughout a pi-system allowing their influence to extend further than induction.
Hyperconjugation
is the stabilizing interaction that results from the interaction of the electron
s 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. Hyperconjugation can be used to explain phenomena such as the gauche effect and anomeric effect
.
Orbital symmetry is important when dealing with orbitals that 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 reaction
s 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 borders 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 that hold like charge
s.
Electronic spin state
at it simplest describes the number of unpaired electrons in a molecule. Most molecules including the protein
s, carbohydrate
s, and lipid
s that make up the majority of life have no unpaired electrons even when charged. Such molecules are called singlet molecules, since their paired electrons have only 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 spin-forbidden
in quantum mechanics. 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 states are more complex for transition metal
s. To understand the reactivity of transition metals, it is essential to understand the concept of d electron configuration
as well as high-spin and low-spin configuration. For example, a low-spin d8 transition metal complex is usually square planar substitutionally inert with no unpaired electrons. In contrast, a high-spin d8 transition metal complex is usually octahedral, substitutionally labile, with two unpaired electrons.
Jahn-Teller effect
is the geometrical distortion of non-linear molecule
s 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.
s, ionic bond
s, hydrogen bond
s, and other 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.
Special computational procedure was developed to separate steric and electronic effects of an arbitrary group in the molecule and to reveal their influence on structure and reactivity.
Chemical structure
A chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as...
, reactivity, or properties of molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
but is neither a traditional bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
nor a steric effect. 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
InductionInductive effect
In chemistry and physics, the inductive effect is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic induction...
is the redistribution of electron density
Electron density
Electron density is the measure of the probability of an electron being present at a specific location.In molecules, regions of electron density are usually found around the atom, and its bonds...
through a traditional sigma bond
Sigma bond
In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is...
ed structure according to the electronegativity
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
of the atoms involved. The inductive effect drops across every sigma bond involved limiting its effect to only a few bonds.
Conjugation
Conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
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 also affected by the position of electron lone pairs with respect to the pi-system. Electronic effects can be transmitted throughout a pi-system allowing their influence to extend further than induction.
Hyperconjugation
Hyperconjugation
In organic chemistry, hyperconjugation is the interaction of the electrons in a sigma bond with an adjacent empty 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...
is the stabilizing interaction that results from the interaction of the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s in a sigma bond
Sigma bond
In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is...
(usually C-H or C-C) with an adjacent empty (or partially filled) non-bonding p-orbital or antibonding
Antibonding
Antibonding is a type of chemical bonding. An antibonding orbital is a form of molecular orbital that is located outside the region of two distinct nuclei...
π orbital
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
or filled π orbital to give an extended molecular orbital
Molecular orbital
In chemistry, a molecular orbital is a mathematical function describing the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region. The term "orbital" was first...
that increases the stability of the system. Hyperconjugation can be used to explain phenomena such as the gauche effect and anomeric effect
Anomeric effect
In organic chemistry, the anomeric effect or Edward-Lemieux effect is a stereoelectronic effect that describes the tendency of heteroatomic substituents adjacent to a heteroatom within a cyclohexane ring to prefer the axial orientation instead of the less hindered equatorial orientation that would...
.
Orbital symmetry is important when dealing with orbitals that 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 reaction
Pericyclic reaction
In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions...
s such as the Diels-Alder reaction
Diels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...
, 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
Van der Waals radius
The van der Waals radius, r, of an atom is the radius of an imaginary hard sphere which can be used to model the atom for many purposes. It is named after Johannes Diderik van der Waals, winner of the 1910 Nobel Prize in Physics, as he was the first to recognise that atoms had a finite size and to...
. Hydrogen bonding borders 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 that hold like charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
s.
Electronic spin state
Spin quantum number
In atomic physics, the spin quantum number is a quantum number that parameterizes the intrinsic angular momentum of a given particle...
at it simplest describes the number of unpaired electrons in a molecule. Most molecules including the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
s, and lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
s that make up the majority of life have no unpaired electrons even when charged. Such molecules are called singlet molecules, since their paired electrons have only one spin state. In contrast, dioxygen under ambient conditions has two unpaired electrons. Dioxygen is a triplet molecule
Triplet oxygen
Triplet oxygen is the ground state of the oxygen molecule. The electron configuration of the molecule has two unpaired electrons occupying two degenerate molecular orbitals...
, since the two unpaired electrons allow for three spin states. The reaction of a triplet molecule with a singlet molecule is spin-forbidden
Selection rule
In physics and chemistry a selection rule, or transition rule, formally constrains the possible transitions of a system from one state to another. Selection rules have been derived for electronic, vibrational, and rotational transitions...
in quantum mechanics. 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 states are more complex for transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
s. To understand the reactivity of transition metals, it is essential to understand the concept of d electron configuration
Spin states (d electrons)
Spin states when describing transition metal coordination complexes refers to the potential spin configurations of the metal centers d electrons. In many molecules these spin states vary between high-spin and low-spin configurations...
as well as high-spin and low-spin configuration. For example, a low-spin d8 transition metal complex is usually square planar substitutionally inert with no unpaired electrons. In contrast, a high-spin d8 transition metal complex is usually octahedral, substitutionally labile, with two unpaired electrons.
Jahn-Teller effect
Jahn-Teller effect
The Jahn–Teller effect, sometimes also known as Jahn–Teller distortion, or the Jahn–Teller theorem, describes the geometrical distortion of non-linear molecules under certain situations. This electronic effect is named after Hermann Arthur Jahn and Edward Teller, who proved, using group theory,...
is the geometrical distortion of non-linear molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s under certain situations. Any non-linear molecule with a degenerate
Degenerate energy level
In physics, two or more different quantum states are said to be degenerate if they are all at the same energy level. Statistically this means that they are all equally probable of being filled, and in Quantum Mechanics it is represented mathematically by the Hamiltonian for the system having more...
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 bondCovalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s, ionic bond
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
s, hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s, and other forms of bonding. This bonding supplies a basic molecular skeleton that is modified by repulsive forces generally considered steric effects
Steric effects
Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape and reactivity.-Steric...
. 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.
Special computational procedure was developed to separate steric and electronic effects of an arbitrary group in the molecule and to reveal their influence on structure and reactivity.