In
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
, pyramidal inversion (also umbrella inversion) is a
fluxional process in compounds with a
pyramid
A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrilat ...
al molecule, such as
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
(NH
3) "turns inside out".
It is a
rapid oscillation of the atom and substituents, the molecule or ion passing through a
planar transition state
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked wi ...
. For a compound that would otherwise be
chiral due to a
stereocenter, pyramidal inversion allows its
enantiomer
In chemistry, an enantiomer ( /ɪˈnænti.əmər, ɛ-, -oʊ-/ ''ih-NAN-tee-ə-mər''; from Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''(méros)'' 'part') – also called optical isomer, antipode, or optical ant ...
s to
racemize. The general phenomenon of pyramidal inversion applies to many types of molecules, including
carbanion
In organic chemistry, a carbanion is an anion in which carbon is trivalent (forms three bonds) and bears a formal negative charge (in at least one significant resonance form).
Formally, a carbanion is the conjugate base of a carbon acid:
:R3C ...
s,
amines,
phosphines,
arsine
Arsine (IUPAC name: arsane) is an inorganic compound with the formula As H3. This flammable, pyrophoric, and highly toxic pnictogen hydride gas is one of the simplest compounds of arsenic. Despite its lethality, it finds some applications in th ...
s,
stibine
Stibine (IUPAC name: stibane) is a chemical compound with the formula SbH3. A pnictogen hydride, this colourless, highly toxic gas is the principal covalent hydride of antimony, and a heavy analogue of ammonia. The molecule is pyramidal with H–S ...
s, and
sulfoxide
In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...
s.
Energy barrier
![NvPinvn](https://upload.wikimedia.org/wikipedia/commons/a/a7/NvPinvn.png)
The identity of the inverting atom has a dominating influence on the barrier.
Inversion of ammonia is rapid at
room temperature
Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
. In contrast,
phosphine
Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...
(PH
3) inverts very slowly at room temperature (energy barrier: 132
kJ/mol
The joule per mole (symbol: J·mol−1 or J/mol) is the unit of energy per amount of substance in the International System of Units (SI), such that energy is measured in joules, and the amount of substance is measured in moles.
It is also an SI ...
). Consequently, amines of the type RR′R"N usually are not optically stable (enantiomers racemize rapidly at room temperature), but
''P''-chiral phosphines are.
Appropriately substituted
sulfonium
In organic chemistry, a sulfonium ion, also known as sulphonium ion or sulfanium ion, is a positively-charged ion (a " cation") featuring three organic substituents attached to sulfur. These organosulfur compounds have the formula . Together wi ...
salts,
sulfoxide
In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...
s,
arsine
Arsine (IUPAC name: arsane) is an inorganic compound with the formula As H3. This flammable, pyrophoric, and highly toxic pnictogen hydride gas is one of the simplest compounds of arsenic. Despite its lethality, it finds some applications in th ...
s, etc. are also optically stable near room temperature.
Steric effects
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
can also influence the barrier.
The ammonia interconversion is rapid at room
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
, inverting 30 billion times per second. Two factors contribute to the rapidity of the inversion: a low
energy barrier
In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...
(24.2
kJ/mol
The joule per mole (symbol: J·mol−1 or J/mol) is the unit of energy per amount of substance in the International System of Units (SI), such that energy is measured in joules, and the amount of substance is measured in moles.
It is also an SI ...
; 5.8 kcal/mol) and a narrow width of the barrier itself, which allows for frequent
quantum tunnelling
Quantum tunnelling, also known as tunneling ( US) is a quantum mechanical phenomenon whereby a wavefunction can propagate through a potential barrier.
The transmission through the barrier can be finite and depends exponentially on the barrier h ...
(see below). In contrast,
phosphine
Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...
(PH
3) inverts very slowly at room temperature (energy barrier: 132 kJ/mol).
Nitrogen inversion
Pyramidal inversion in
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituen ...
s is known as nitrogen inversion. It is a
rapid oscillation of the nitrogen atom and substituents, the nitrogen "moving" through the plane formed by the substituents (although the substituents also move - in the other direction); the molecule passing through a
planar transition state
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked wi ...
. For a compound that would otherwise be
chiral due to a nitrogen
stereocenter, nitrogen inversion provides a low energy pathway for
racemization, usually making
chiral resolution impossible.
Quantum effects
Ammonia exhibits a
quantum tunnelling
Quantum tunnelling, also known as tunneling ( US) is a quantum mechanical phenomenon whereby a wavefunction can propagate through a potential barrier.
The transmission through the barrier can be finite and depends exponentially on the barrier h ...
due to a narrow tunneling barrier, and not due to thermal excitation. Superposition of two states leads to
energy level splitting
In quantum physics, energy level splitting or a split in an energy level of a quantum system occurs when a perturbation changes the system. The perturbation changes the corresponding Hamiltonian and the outcome is change in eigenvalues; several di ...
, which is used in ammonia
maser
A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, Ja ...
s.
Examples
The inversion of ammonia was first detected by
microwave spectroscopy in 1934.
In one study the inversion in an
aziridine
Aziridine is an organic compound consisting of the three-membered heterocycle . It is a colorless, toxic, volatile liquid that is of significant practical interest. Aziridine was discovered in 1888 by the chemist Siegmund Gabriel. Its derivati ...
was slowed by a factor of 50 by placing the nitrogen atom in the vicinity of a
phenol
Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it req ...
ic alcohol group compared to the oxidized
hydroquinone
Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. It has two hydroxyl groups bonded to a benzene ring in a ''para'' ...
.
The system interconverts by oxidation by
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
and reduction by
sodium dithionite
Sodium dithionite (also known as sodium hydrosulfite) is a white crystalline powder with a sulfurous odor. Although it is stable in dry air, it decomposes in hot water and in acid solutions.
Structure
The structure has been examined by Raman s ...
.
Exceptions
Conformational strain and structural rigidity can effectively prevent the inversion of amine groups.
Tröger's base
Tröger's base is a white solid tetracyclic organic compound. structure and formula of (CH3C6H3NCH2)2CH2. Tröger's base and its analogs are soluble in various organic solvents and strong acidic aqueous solutions due to their protonation.
History ...
analogs (including the Hünlich's base
) are examples of compounds whose nitrogen atoms are chirally stable
stereocenters and therefore have significant
optical activity
Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...
.
References
{{Reflist
Physical chemistry
Stereochemistry
Organic chemistry