View source for Isomer

{{Short description|Chemical compounds with the same molecular formula but different atomic arrangements}}
{{About|the chemical concept|"isomerism" of atomic nuclei|nuclear isomer|part of the body of the proarticulates|isomer (Proarticulata)}}
{{pp-move-indef}}
{{Use dmy dates|date=May 2022}}

In [[chemistry]], '''isomers''' are [[molecule]]s or [[polyatomic ion]]s with identical [[molecular formula]] – that is, the same number of [[atom]]s of each [[element (chemistry)|element]] – but distinct arrangements of atoms in space.<ref>{{cite book |last1 = Petrucci |first1 = Ralph H. |last2 = Harwood |first2 = William S. |last3 = Herring |first3 = F. Geoffrey |date=2002 |title = General chemistry: principles and modern applications |url = https://archive.org/details/generalchemistry00hill |url-access = registration |edition=8th |location=Upper Saddle River, N.J |publisher=Prentice Hall |isbn = 978-0-13-014329-7 |lccn=2001032331 |oclc=46872308 |page=91]}}</ref>  ''Isomerism'' refers to the existence or possibility of isomers.

Isomers do not necessarily share similar [[chemical property|chemical]] or [[physical property|physical properties]]. Two main forms of isomerism are [[structural isomerism|structural]] (or constitutional) isomerism, in which ''[[chemical bond|bond]]s'' between the atoms differ; and [[stereoisomerism]] (or spatial isomerism), in which the bonds are the same but the ''relative positions'' of the atoms differ.

Isomeric relationships form a [[hierarchy]]. Two chemicals might be the same constitutional isomer, but upon deeper analysis be stereoisomers of each other. Two molecules that are the same stereoisomer as each other might be in different conformational forms or be different [[Isotopologue|isotopologues]]. The depth of analysis depends on the field of study or the chemical and physical properties of interest.

The English word "isomer" ({{IPAc-en|ˈ|aɪ|s|əm|əɹ}}) is a [[back-formation]] from "isomeric",<ref name=MW.isomer>Merriam-Webster: [https://www.merriam-webster.com/dictionary/isomer "isomer"] {{Webarchive|url=https://web.archive.org/web/20201021040204/https://www.merriam-webster.com/dictionary/isomer |date=21 October 2020 }} online dictionary entry. Accessed on 2020-08-26</ref> which was borrowed through [[German language|German]] ''isomerisch''<ref name=MW.isomeric>Merriam-Webster: [https://www.merriam-webster.com/dictionary/isomeric "isomeric"] {{Webarchive|url=https://web.archive.org/web/20201026100040/https://www.merriam-webster.com/dictionary/isomeric |date=26 October 2020 }} online dictionary entry. Accessed on 2020-08-26</ref> from [[Swedish language|Swedish]] {{lang|sv|isomerisk}}; which in turn was coined from [[Greek language|Greek]] ἰσόμερoς {{transliteration|grc|isómeros}}, with roots {{transliteration|grc|isos}} = "equal",  {{transliteration|grc|méros}} = "part".<ref name=berz1830/>
[[File:IsomerOverview.svg|thumb|center|Two broad types of isomers]]

==Structural isomers==
{{main|Structural isomer}}
'''Structural isomers''' have the same number of atoms of each element (hence the same [[molecular formula]]), but the atoms are connected in distinct ways.<ref>{{cite book |last=Smith |first=Janice Gorzynski |title=General, Organic and Biological Chemistry |publisher=McGraw-Hill |edition=1st |year=2010 |isbn=978-0-07-302657-2 |page=450 |url=https://books.google.com/books?id=oGz9XwAACAAJ&pg=PA450 }}</ref>

===Example: {{chem|C|3|H|8|O}} ===

For example, there are three distinct compounds with the molecular formula <chem>C3H8O</chem>:

[[File:Structural isomers.svg|class=skin-invert-image|center|500px|Structural isomers of {{chem|C|3|H|8|O}}: '''I''' 1-propanol, '''II''' 2-propanol, '''III''' ethyl-methyl-ether.]]

The first two isomers shown of  <chem>C3H8O</chem> are [[propanol]]s, that is, [[alcohol (chemistry)|alcohols]] derived from [[propane]].  Both have a chain of three carbon atoms connected by single bonds, with the remaining carbon [[valence (chemistry)|valences]] being filled by seven [[hydrogen]] atoms and by a [[hydroxyl]] group <chem>-OH</chem> comprising the [[oxygen]] atom bound to a hydrogen atom. These two isomers differ on which carbon the hydroxyl is bound to: either to an extremity of the carbon chain [[propan-1-ol]] (1-propanol, ''n''-propyl alcohol, ''n''-propanol; '''I''') or to the middle carbon [[propan-2-ol]] (2-propanol, isopropyl alcohol, isopropanol; '''II'''). These can be described by the [[Structural formula#Condensed formula|condensed structural formulas]] <chem>H3C-CH2-CH2OH</chem> and <chem>H3C-CH(OH)-CH3</chem>.

The third isomer of <chem>C3H8O</chem> is the [[ether]] [[methoxyethane]] (ethyl-methyl-ether; '''III'''). Unlike the other two, it has the oxygen atom connected to two carbons, and all eight hydrogens bonded directly to carbons. It can be described by the condensed formula <chem>H3C-CH2-O-CH3</chem>.

The alcohol "3-propanol" is not another isomer, since the difference between it and 1-propanol is not real; it is only the result of an arbitrary choice in the direction of numbering the carbons along the chain.  For the same reason, "ethoxymethane" is the same molecule as methoxyethane, not another isomer.

1-Propanol and 2-propanol are examples of [[Structural isomer#Position isomerism (regioisomerism)|positional isomers]], which differ by the position at which certain features, such as [[double bond]]s or [[functional group]]s, occur on a "parent" molecule (propane, in that case).

===Example: {{chem|C|3|H|4}}===
There are also three structural isomers of the [[hydrocarbon]] <chem>C3H4</chem>:

{| style="margin-left:auto; margin-right:auto; border:none;" class="skin-invert-image"
|-
|style="padding: 0px 20px 0px;"|[[File:Propadiene structure.svg|150px]]
|style="padding: 0px 20px 0px;"|[[File:Propyne-2D-flat.png|150px]]
|style="padding: 0px 20px 0px;"|[[File:cyclopropene.png|100px]]
|-
|style="text-align:center;padding-left=20px;"|'''I''' Propadiene
|style="text-align:center;padding-left=20px;"|'''II''' Propyne
|style="text-align:center;padding-left=20px;"|'''III''' Cyclopropene
|-
|}

In two of the isomers, the three carbon atoms are connected in an open chain, but in one of them ([[propadiene]] or allene; '''I''') the carbons are connected by two [[double bond]]s, while in the other ([[propyne]] or methylacetylene; '''II''') they are connected by a single bond and a [[triple bond]].  In the third isomer ([[cyclopropene]]; '''III''') the three carbons are connected into a ring by two single bonds and a double bond. In all three, the remaining valences of the carbon atoms are satisfied by the four hydrogens.

Again, note that there is only one structural isomer with a triple bond, because the other possible placement of that bond is just drawing the three carbons in a different order. For the same reason, there is only one cyclopropene, not three.

===Tautomers===
[[Tautomer]]s are structural isomers which readily interconvert, so that two or more species co-exist in equilibrium such as

<chem>H-X-Y=Z <=> X=Y-Z-H</chem>.<ref>{{cite journal |title=tautomerism |url=http://goldbook.iupac.org/html/T/T06252.html |website=IUPAC Gold Book |year=2014 |publisher=IUPAC |doi=10.1351/goldbook.T06252 |access-date=21 April 2019 |doi-access=free |archive-date=6 April 2019 |archive-url=https://web.archive.org/web/20190406145801/http://goldbook.iupac.org/html/T/T06252.html |url-status=live }}</ref>

Important examples are [[keto-enol tautomerism]] and the equilibrium between neutral and [[zwitterionic]] forms of an [[amino acid]].

==Stereoisomers==
[[File:Stereoisomer are of two types, enantiomers vs diastereomers.svg|thumb|center|Two kinds of stereoisomers|200px]]

{{main|Stereoisomerism}}
Stereoisomers have the same atoms or isotopes connected by bonds of the same type, but differ in the relative positions of those atoms in space.  Two broad types of stereoisomers exist, enantiomers and diastereomers.  Enantiomers have identical physical properties but diastereomers do not.<ref>{{March6th|page=136}}</ref>

===Enantiomers===
Two compounds are said to be [[enantiomer]]s if their molecules are mirror images of each other and cannot be made to coincide only by rotations or translations – like a left hand and a right hand. The two shapes are said to be [[chirality|chiral]].

A classic example is [[bromochlorofluoromethane]] (<chem>CHFClBr</chem>). The two enantiomers can be distinguished, for example, by whether the path <chem>F->Cl->Br  </chem> turns clockwise or counterclockwise as seen from the hydrogen atom. In order to change one conformation to the other, at some point those four atoms would have to lie on the same plane – which would require severely straining or breaking their bonds to the carbon atom.  The corresponding energy barrier between the two conformations is so high that there is practically no conversion between them at room temperature, and they can be regarded as different configurations.

The compound [[chlorofluoromethane]] <chem>CH2ClF</chem>, in contrast, is not chiral; the mirror image of its molecule is also obtained by a half-turn about a suitable axis.

Another example of a chiral compound is [[Pentadiene|2,3-pentadiene]] <chem>H3C-CH=C=CH-CH3</chem>, a hydrocarbon that contains two overlapping double bonds. The double bonds are such that the three middle carbons are in a straight line, while the first three and last three lie on perpendicular planes. The molecule and its mirror image are not superimposable, even though the molecule has an axis of symmetry.  The two enantiomers can be distinguished, for example, by the [[right-hand rule]]. This type of isomerism is called [[axial isomer]]ism.

Enantiomers behave identically in chemical reactions, except when reacting with chiral compounds or in the presence of chiral [[catalysts]], such as most [[enzyme]]s. For this latter reason, the two enantiomers of most chiral compounds usually have markedly different effects and roles in living organisms. In [[biochemistry]] and [[food science]], the two enantiomers of a chiral molecule – such as [[glucose]] – are usually identified and treated as very different substances.

Each enantiomer of a chiral compound typically rotates the plane of [[polarized light]] that passes through it. The rotation has the same magnitude but opposite senses for the two isomers, and can be a useful way of distinguishing and measuring their concentration in a solution. For this reason, enantiomers were formerly called "optical isomers".{{sfn|Petrucci|Harwood|Herring|2002|pp=996-997}}<ref>Whitten K.W., Gailey K.D. and Davis R.E. "General Chemistry" (4th ed., Saunders College Publishing 1992), p. 976–7 {{ISBN|978-0-03-072373-5}}</ref> However, this term is ambiguous and is discouraged by the [[IUPAC]].<ref>{{GoldBookRef |title=optical isomers |file=O04308}}</ref><ref name=Eliel2>{{cite book | author = [[Ernest L. Eliel]] and Samuel H. Wilen | title = Stereochemistry of Organic Compounds | publisher = Wiley Interscience | date = 1994 | pages = 1203}}</ref>

Some enantiomer pairs (such as those of [[trans-Cyclooctene|''trans''-cyclooctene]]) can be interconverted by internal motions that change bond lengths and angles only slightly. Other pairs (such as CHFClBr) cannot be interconverted without breaking bonds, and therefore are different configurations.

===Diastereomers===
Stereoisomers that are not enantiomers are called [[diastereomer]]s. Some diastereomers may contain [[chiral center|chiral centers]], and some may not.<ref name=Eliel>{{cite book | author = [[Ernest L. Eliel]] and Samuel H. Wilen | title = Stereochemistry of Organic Compounds | publisher = Wiley Interscience | date = 1994 | pages = 52–53}}</ref>

====Cis-trans isomerism====
A double bond between two carbon atoms forces the remaining four bonds (if they are single) to lie on the same plane, perpendicular to the plane of the bond as defined by its [[π bond|π orbital]]. If the two bonds on each carbon connect to different atoms, two distinct conformations are possible that differ from each other by a twist of 180 degrees of one of the carbons about the double bond.

The classical example is dichloroethene <chem>C2H2Cl2</chem>, specifically the structural isomer <chem>Cl-HC=CH-Cl</chem> that has one chlorine bonded to each carbon. It has two conformational isomers, with the two chlorines on the same side or on opposite sides of the double bond's plane. They are traditionally called ''cis'' (from Latin meaning "on this side of") and ''trans'' ("on the other side of"), respectively, or ''Z'' and ''E'' in the [[IUPAC]] recommended nomenclature.  Conversion between these two forms usually requires temporarily breaking bonds (or turning the double bond into a single bond), so the two are considered different configurations of the molecule.

More generally, [[cis–trans isomerism|''cis''–''trans'' isomerism]] (formerly called "geometric isomerism") occurs in molecules where the relative orientation of two distinguishable functional groups is restricted by a somewhat rigid framework of other atoms.<ref>{{GoldBookRef |title=geometric isomerism |file=G02620}}</ref>

For example, in the cyclic alcohol [[inositol]] <chem>(CHOH)6</chem> (a six-fold alcohol of cyclohexane), the six-carbon cyclic backbone largely prevents the hydroxyl <chem>-OH</chem> and the hydrogen <chem>-H</chem> on each carbon from switching places.  Therefore, one has different configurational isomers depending on whether each hydroxyl is on "this side" or "the other side" of the ring's mean plane.  Discounting isomers that are equivalent under rotations, there are nine isomers that differ by this criterion, and behave as different stable substances (two of them being enantiomers of each other). The most common one in nature (''myo''-inositol) has the hydroxyls on carbons 1, 2, 3 and 5 on the same side of that plane, and can therefore be called ''cis''-1,2,3,5-''trans''-4,6-cyclohexanehexol. And each of these ''cis''-''trans'' isomers can possibly have stable "chair" or "boat" conformations (although the barriers between these are significantly lower than those between different ''cis''-''trans'' isomers).

[[Image:Cisplatin and transplatin.svg|class=skin-invert-image|thumb|right|320px|The two isomeric complexes, [[cisplatin]] and [[transplatin]], are examples of square planar MX<sub>2</sub>Y<sub>2</sub> molecules with M = Pt.]]
[[Coordination complex#Cis–trans isomerism and facial–meridional isomerism|''Cis'' and ''trans'' isomers]] also occur in inorganic [[coordination compound]]s, such as [[square planar molecular geometry|square planar]] <chem>MX2Y2</chem> complexes and [[octahedral molecular geometry|octahedral]] <chem>MX4Y2</chem> complexes.

For more complex organic molecules, the ''cis'' and ''trans'' labels can be ambiguous. In such cases, a more precise labeling scheme is employed based on the [[Cahn-Ingold-Prelog priority rules]].<ref>{{GoldBookRef |title=cis, trans |file=C01092}}</ref><ref name=Eliel/>

<!-- but these examples do not exist, PF4Cl does not exist as isomers, etc===Centers with non-equivalent bonds===
More generally, atoms or atom groups that can form three or more non-equivalent single bonds (such as the [[transition metal]]s in coordination compounds) may give rise to multiple stereoisomers when different atoms or groups are attached at those positions. The same is true if a center with six or more equivalent bonds has two or more substituents.

For instance, in the compound <chem>PF4Cl</chem>, the bonds from the [[phosphorus]] atom to the five [[halogen]]s have approximately [[trigonal bipyramidal geometry]]. Thus two stereoisomers with that formula are possible, depending on whether the chlorine atom occupies one of the two "axial" positions, or one of the three "equatorial" positions.

For the compound <chem>PF3Cl2</chem>, three isomers are possible, with zero, one, or two chlorines in the axial positions.

As another example, a complex with a formula like <chem>MX3Y3</chem>, where the central atom M forms six bonds with [[octahedral geometry]], has at least two [[Coordination complex#Cis–trans isomerism and facial–meridional isomerism|facial–meridional isomers]], depending on whether the three <chem>X</chem> bonds (and thus also the three <chem>Y</chem> bonds) are directed at the three corners of one face of the octahedron (''fac'' isomer), or lie on the same equatorial or "meridian" plane of it (''mer'' isomer).-->
<!--

[[File:Cyclohexane chair form AND boat form 8254 TR.png|400px|thumb|Molecular models of cyclohexane in boat and chair conformations.  The carbon atoms are colored amber or blue according to whether they lie above or below the mean plane of the ring. The C–C bonds on the ring are light green.]]
A classic example of conformational isomerism is [[cyclohexane]].  [[Alkanes]] generally have minimum energy when the <chem>C-C-C</chem> angles are close to 110 degrees. Conformations of the cyclohexane molecule with all six carbon atoms on the same plane have a higher energy, because some or all the <chem>C-C-C</chem> angles must be far from that value (120 degrees for a regular hexagon). Thus the conformations which are local energy minima have the ring twisted in space, according to one of two patterns known as [[Cyclohexane conformation#Chair conformation|chair]] (with the carbons alternately above and below their mean plane) and [[Cyclohexane conformation#Boat and twist-boat conformations|boat]] (with two opposite carbons above the plane, and the other four below it).
===Rotamers and atropisomers===
Two parts of a molecule that are connected by just one single bond can rotate about that bond. While the bond itself is indifferent to that rotation, attractions and repulsions between the atoms in the two parts normally cause the energy of the whole molecule to vary (and possibly also the two parts to deform) depending on the relative angle of rotation φ between the two parts. Then there will be one or more special values of φ for which the energy is at a local minimum. The corresponding conformations of the molecule are called rotational isomers or [[rotamer]]s.

Thus, for example, in an [[ethane]] molecule <chem>H3C-CH3</chem>, all the bond angles and length are narrowly constrained, except that the two [[methyl]] groups can independently rotate about the <chem>C-C</chem> axis. Thus, even if those angles and distances are assumed fixed, there are infinitely many conformations for the ethane molecule, that differ by the relative angle φ of rotation between the two groups.  The feeble repulsion between the hydrogen atoms in the two methyl groups causes the energy to minimized for three specific values of φ, 120° apart. In those configurations, the six planes <chem>H-C-C</chem> or <chem>C-C-H</chem> are 60° apart. Discounting rotations of the whole molecule, that configuration is a single isomer – the so-called ''staggered'' conformation.

Rotation between the two halves of the molecule [[1,2-dichloroethane]] (<chem>ClH2C-CH2Cl</chem> also has three local energy minima, but they have different energies due to differences between the <chem>H-H</chem>, <chem>Cl-Cl</chem>, and <chem>H-Cl</chem> interactions.  There are therefore three rotamers: a ''trans'' isomer where the two chlorines are on the same plane as the two carbons, but with oppositely directed bonds; and two ''gauche'' isomers, mirror images of each other, where the two <chem>-CH2Cl</chem> groups are rotated about 109° from that position.  The computed energy difference between ''trans'' and ''gauche'' is ~1.5 kcal/mol, the barrier for the ~109° rotation from ''trans'' to ''gauche'' is ~5 kcal/mol, and that of the ~142° rotation from one ''gauche'' to its enantiomer is ~8 kcal/mol.<ref name=wibe1987>Kenneth B. Wiberg and Mark A. Murcko (1987): "Rotational barriers. 1. 1,2-Dihaloethanes". ''Journal of Physical Chemistry'', volume 91, issue 13, pages 3616–3620. {{doi|10.1021/j100297a030}}</ref> The situation for [[butane]] is similar, but with sightly lower ''gauche'' energies and barriers.<ref name=wibe1987/>

If the two parts of the molecule connected by a single bond are bulky or charged, the energy barriers may be much higher. For example, in the compound [[biphenyl]] – two [[phenyl]] groups connected by a single bond  – the repulsion between hydrogen atoms closest to the central single bond gives the fully planar conformation, with the two rings on the same plane, a higher energy than conformations where the two rings are skewed.  In the gas phase, the molecule has therefore at least two rotamers, with the ring planes twisted by ±47°, which are mirror images of each other. The barrier between them is rather low (~8 [[kilojoule|kJ]]/mol).<ref name=lens1994>A. T. H. Lenstra, C. Van Alsenoy, K. Verhulst and H. J. Geise (1994): "Solids modelled by crystal field ab initio methods. 5. The phase transitions in biphenyl from a molecular point of view". ''Acta Crystallographica Section B'', volume B50, pages 96-106. {{doi|10.1107/S0108768193007931}}</ref> This [[steric hindrance]] effect is more pronounced when those four hydrogens are replaced by larger atoms or groups, like chlorines or [[carboxyl]]s.  If the barrier is high enough for the two rotamers to be separated as stable compounds at room temperature, they are called [[atropisomer]]s.

===Topoisomers===
Large molecules may have isomers that differ by the [[topology]] of their overall arrangement in space, even if there is no specific geometric constraint that separate them.  For example, long chains may be twisted to form topologically distinct [[knot theory|knot]]s, with interconversion prevented by bulky substituents or [[cyclic compound|cycle]] closing (as in circular [[DNA]] and [[RNA]] [[plasmid]]s). Some knots may come in mirror-image enantiomer pairs. Such forms are called topological isomers or [[topoisomer]]s.

Also, two or more such molecules may be bound together in a [[catenane]] by such topological linkages, even if there is no chemical bond between them.  If the molecules are large enough, the linking may occur in multiple topologically distinct ways, constituting different isomers. [[Cage compound]]s, such as [[helium]] enclosed in [[dodecahedrane]] (He@{{chem|C|20|H|20}}) and [[carbon peapod]]s, are a similar type of topological isomerism involving molecules with large internal voids with restricted or no openings.<ref name=iwam2011>Takahiro Iwamoto, Yoshiki Watanabe, Tatsuya Sadahiro, Takeharu Haino, and Shigeru Yamago (2011): "Size-selective encapsulation of C60 by [10]cycloparaphenylene: Formation of the shortest fullerene-peapod". ''Angewandte Chemie International Edition'', volume 50, issue 36, pages 8342–8344. {{doi|10.1002/anie.201102302}}</ref>-->

==Isotopes and spin==
===Isotopomers===
Different isotopes of the same element can be considered as different kinds of atoms when enumerating isomers of a molecule or ion. The replacement of one or more atoms by their isotopes can create multiple structural isomers and/or stereoisomers from a single isomer.

For example, replacing two atoms of common [[hydrogen]] (<chem>^1 H </chem>) by [[deuterium]] (<chem>^2 H </chem>, or <chem>D</chem>) on an [[ethane]] molecule yields two distinct structural isomers, depending on whether the substitutions are both on the same carbon (1,1-dideuteroethane, <chem>HD2C-CH3</chem>) or one on each carbon (1,2-dideuteroethane, <chem>DH2C-CDH2</chem>); as if the substituent was [[chlorine]] instead of deuterium.  The two molecules do not interconvert easily and have different properties, such as their [[microwave spectroscopy|microwave spectrum]].<ref name=hiro2012>Eizi Hirota (2012): "Microwave spectroscopy of isotope-substituted non-polar molecules".  Chapter 5 in ''Molecular Spectroscopy: Modern Research'', volume 3. 466 pages. {{isbn|9780323149327}}</ref>

Another example would be substituting one atom of deuterium for one of the hydrogens in [[chlorofluoromethane]] (<chem>CH2ClF</chem>).  While the original molecule is not chiral and has a single isomer, the substitution creates a pair of chiral enantiomers of <chem>CHDClF</chem>, which could be distinguished (at least in theory) by their optical activity.<ref>{{cite journal | last1=Cameron | first1=Robert P. | last2=Götte | first2=Jörg B. | last3=Barnett | first3=Stephen M. | title=Chiral rotational spectroscopy | journal=Physical Review A | volume=94 | issue=3 | date=2016-09-08 | issn=2469-9926 | doi=10.1103/physreva.94.032505 | page=032505| arxiv=1511.04615 | bibcode=2016PhRvA..94c2505C | doi-access=free }}</ref>

When two isomers would be identical if all isotopes of each element were replaced by a single isotope, they are described as [[isotopomer]]s or isotopic isomers.<ref name=Seeman>{{cite journal  |last1=Seeman |first1=Jeffrey I.  |last2=Paine, III |first2=J. B. |title=Letter to the Editor: 'Isotopomers, Isotopologs' |journal=Chemical & Engineering News |date=7 December 1992|publisher=American Chemical Society |volume=70 |issue=2 |doi=10.1021/cen-v070n049.p002 |doi-access=free }}</ref> In the above two examples if all <chem>D</chem> were replaced by <chem>H</chem>, the two dideuteroethanes would both become ethane and the two deuterochlorofluoromethanes would both become <chem>CH2ClF</chem>.

The concept of isotopomers is different from [[isotopologue|isotopologs]] or isotopic homologs, which differ in their isotopic composition.<ref name=Seeman/> For example, <chem>C2H5D</chem> and <chem>C2H4D2</chem> are isotopologues and not isotopomers, and are therefore not isomers of each other.

===Spin isomers===
Another type of isomerism based on nuclear properties is [[spin isomers of hydrogen|spin isomerism]], where molecules differ only in the relative [[Spin quantum number|spin magnetic quantum numbers]] m<sub>s</sub> of the constituent atomic nuclei. This phenomenon is significant for molecular hydrogen, which can be partially separated into two long-lived states described as spin isomers<ref>{{cite journal | last1 = Matthews | first1 = M.J. | last2 = Petitpas | first2 = G. | last3 = Aceves | first3 = S.M. | date = 23 August 2011 | title = A study of spin isomer conversion kinetics in supercritical fluid hydrogen for cryogenic fuel storage technologies | url = https://aip.scitation.org/doi/10.1063/1.3628453 | journal = Appl. Phys. Lett. | volume = 99 | issue = 8 | page = 081906 | doi = 10.1063/1.3628453 | bibcode = 2011ApPhL..99h1906M | access-date = 1 May 2022 | archive-date = 2 May 2022 | archive-url = https://web.archive.org/web/20220502203722/https://aip.scitation.org/doi/10.1063/1.3628453 | url-status = live }}</ref> or nuclear spin isomers:<ref>{{cite journal |last1=Chen |first1=Judy Y.-C. |last2=Li |first2=Yongjun |last3=Frunzi |first3=Michael |last4=Lei |first4=Xuegong |last5=Murata |first5=Yasujiro |last6=Lawler |first6=Ronald G. |last7=Turro |first7=Nicholas |author-link7=Nicholas Turro |title=Nuclear spin isomers of guest molecules in H2@C60, H2O@C60 and other endofullerenes |journal=Philosophical Transactions of the Royal Society A |date=13 September 2013 |volume=371 |issue=1998 |doi=10.1098/rsta.2011.0628 |pmid=23918710 |bibcode=2013RSPTA.37110628C |s2cid=20443766 |doi-access=free }}</ref> parahydrogen, with the spins of the two nuclei pointing in opposite directions, and orthohydrogen, where the spins point in the same direction.
==Applications==
Isomers having distinct biological properties are common; for example, the placement of [[methyl group]]s. In substituted [[xanthine]]s, [[theobromine]], found in chocolate, is a [[vasodilator]] with some effects in common with [[caffeine]]; but, if one of the two methyl groups is moved to a different position on the two-ring core, the isomer is [[theophylline]], which has a variety of effects, including [[bronchodilator|bronchodilation]] and [[anti-inflammatory]] action. Another example of this occurs in the [[phenethylamine]]-based stimulant drugs. [[Phentermine]] is a [[chirality (chemistry)|non-chiral]] compound with a weaker effect than that of [[amphetamine]]. It is used as an appetite-reducing medication and has mild or no stimulant properties. However, an alternate atomic arrangement gives [[dextromethamphetamine]], which is a stronger stimulant than amphetamine.

In [[medicinal chemistry]] and biochemistry, [[enantiomer]]s are a special concern because they may possess distinct [[biological activity]]. Many preparative procedures afford a mixture of equal amounts of both enantiomeric forms. In some cases, the enantiomers are separated by [[chromatography]] using chiral stationary phases. They may also be separated through the formation of [[diastereomeric crystallization|diastereomeric salts]]. In other cases, [[enantioselective synthesis]] have been developed.

As an inorganic example, [[cisplatin]] (see structure above) is an important drug used in cancer chemotherapy, whereas the trans isomer (transplatin) has no useful pharmacological activity.

==History==
Isomerism was first observed in 1827, when [[Friedrich Wöhler]] prepared [[silver cyanate]] and discovered that, although its elemental composition of <chem>AgCNO</chem> was identical to [[silver fulminate]] (prepared by [[Justus von Liebig]] the previous year),<ref>{{cite journal |title= Fulminic Acid in the History of Organic Chemistry |author= F. Kurzer |journal= [[J. Chem. Educ.]] |year= 2000 |volume= 77 |issue= 7 |pages= 851–857 |url= http://jchemed.chem.wisc.edu/journal/Issues/2000/Jul/abs851.html |doi= 10.1021/ed077p851 |bibcode= 2000JChEd..77..851K |access-date= 27 July 2012 |archive-date= 18 February 2009 |archive-url= https://web.archive.org/web/20090218004148/http://jchemed.chem.wisc.edu/journal/Issues/2000/Jul/abs851.html |url-status= dead }}</ref> its properties were distinct. This finding challenged the prevailing chemical understanding of the time, which held that [[chemical compound]]s could be distinct only when their elemental compositions differ. (We now know that the bonding structures of [[fulminate]] and [[cyanate]] can be approximately described as <chem>O- N+</chem>≡<chem>C-</chem> and <chem>O=C=N-</chem>, respectively.)

Additional examples were found in succeeding years, such as Wöhler's 1828 discovery that [[urea]] has the same atomic composition (<chem>CH4N2O</chem>) as the chemically distinct [[ammonium cyanate]]. (Their structures are now known to be <chem>(H2N-)2C=O</chem> and <chem>[NH+4] [O=C=N^ -] </chem>, respectively.) In 1830 [[Jöns Jacob Berzelius]] introduced the term ''isomerism'' to describe the phenomenon.<ref name=berz1830>Jac. Berzelius (1830): "[https://books.google.com/books?id=2OY4AAAAMAAJ&pg=RA1-PA70 Om sammansättningen af vinsyra och drufsyra (John's säure aus den Voghesen), om blyoxidens atomvigt, samt allmänna anmärkningar om sådana kroppar som hafva lika sammansättning, men skiljaktiga egenskaper]" ("On the composition of tartaric acid and racemic acid (John's acid of the Vosges), on the molecular weight of lead oxide, together with general observations on those bodies that have the same composition but distinct properties"). ''Kongliga Svenska Vetenskaps Academiens Handling'' (''Transactions of the Royal Swedish Science Academy''), volume 49, pages 49–80</ref><ref name=berz1831a>J. J. Berzelius (1831): "[https://books.google.com/books?id=7G8EAAAAYAAJ&pg=PA326 Über die Zusammensetzung der Weinsäure und Traubensäure (John's säure aus den Voghesen), über das Atomengewicht des Bleioxyds, nebst allgemeinen Bemerkungen über solche Körper, die gleiche Zusammensetzung, aber ungleiche Eigenschaften besitzen]".  ''Annalen der Physik und Chemie'', volume 19, pages 305–335</ref><ref name=berz1831b>J. J. Berzelius (1831): "[https://books.google.com/books?id=mOQ3AAAAMAAJ&pg=PA136 Composition de l'acide tartarique et de l'acide racémique (traubensäure); poids atomique de l'oxide de plomb, et remarques générals sur les corps qui ont la même composition, et possèdent des proprietés différentes]". ''Annales de Chimie et de Physique'', volume 46, pages 113–147.</ref><ref>{{Cite journal |author=Esteban, Soledad |journal=J. Chem. Educ. |year=2008 |volume=85 |page=1201 |title=Liebig–Wöhler Controversy and the Concept of Isomerism |url=http://jchemed.chem.wisc.edu/Journal/Issues/2008/Sep/abs1201.html |doi=10.1021/ed085p1201 |issue=9 |bibcode=2008JChEd..85.1201E |access-date=9 September 2008 |archive-date=23 August 2008 |archive-url=https://web.archive.org/web/20080823172737/http://jchemed.chem.wisc.edu/Journal/Issues/2008/Sep/abs1201.html |url-status=live }}</ref>

In 1848, [[Louis Pasteur]] observed that [[tartaric acid]] crystals came into two kinds of shapes that were mirror images of each other. Separating the crystals by hand, he obtained two version of tartaric acid, each of which would crystallize in only one of the two shapes, and rotated the plane of polarized light to the same degree but in opposite directions.<ref>L. Pasteur (1848) "Mémoire sur la relation qui peut exister entre la forme cristalline et la composition chimique, et sur la cause de la polarisation rotatoire" (Memoir on the relationship which can exist between crystalline form and chemical composition, and on the cause of rotary polarization)," ''Comptes rendus de l'Académie des sciences'' (Paris), vol. 26, pages 535–538.</ref><ref>L. Pasteur (1848) [https://books.google.com/books?id=gJ45AAAAcAAJ&pg=PA442 "Sur les relations qui peuvent exister entre la forme cristalline, la composition chimique et le sens de la polarisation rotatoire"] ("On the relations that can exist between crystalline form, chemical composition, and the sense of rotary polarization"), ''Annales de Chimie et de Physique'', 3rd series, volume 24, issue 6, pages 442–459.</ref> In 1860, Pasteur explicitly hypothesized that the molecules of isomers might have the same composition but different arrangements of their atoms.<ref>[[#refPullman1998|Pullman (1998). ''The Atom in the History of Human Thought'', p. 230]]</ref>

==See also==
{{col div|colwidth=30em}}
* [[Allotropy]] (of elements)
* [[Chirality (chemistry)]]
* [[Cis-trans isomerism]]
* [[Cyclohexane conformation]]
* [[Descriptor (chemistry)]]
* [[Electromerism]]
* [[Isomerization]]
* [[Merosity|Isomery (botany)]]
* [[Ligand isomerism]]
* [[Nuclear isomer]]
* [[Stereocenter]]
* [[Structural isomerism]]
* [[Tautomer]]
* [[Vitamer]]
{{colend}}

==References==
{{Reflist}}

==External links==
{{wikiquote}}
{{Commons category|Isomerism}}

[[Category:Isomerism| ]]
[[Category:1827 introductions]]

[[ga:Isiméir]]