Editing Rotamer (section)

{{Short description|Various molecular structures formed only by rotation about single bonds}}
[[File:Gauche-eclipsed interconversion.svg|thumb|300px|Rotation about single bond of [[butane]] to interconvert one conformation to another.  The ''gauche'' conformation on the right is a conformer, while the ''eclipsed'' conformation on the left is a transition state between conformers. Above: Newman projection; below: depiction of spatial orientation.]]

In [[chemistry]], '''rotamers''' are chemical species that differ from one another primarily due to rotations about one or more [[single bond]]s. Various arrangements of [[atom]]s in a [[molecule]] that differ by rotation about single bonds can also be referred to as '''conformations'''.  Conformers/rotamers differ little in their energies, so they are almost never separable in a practical sense. Rotations about single bonds are subject to small energy barriers.<ref>{{cite journal |title=free rotation (hindered rotation, restricted rotation) |url=https://goldbook.iupac.org/terms/view/F02520 |website=IUPAC Gold Book|doi=10.1351/goldbook.F02520 |doi-access=free }}</ref> When the time scale for interconversion is long enough for isolation of individual rotamers (usually arbitrarily defined as a [[half-life]] of interconversion of 1000 seconds or longer), the species are termed '''atropisomers''' (''see:'' [[atropisomer]]ism).<ref name=":0">{{Cite journal|last=Moss|first=GP|date=1996-01-01|title=Basic terminology of stereochemistry (IUPAC Recommendations 1996)|journal=Pure and Applied Chemistry|volume=68|issue=12|pages=2193–2222|doi=10.1351/pac199668122193|s2cid=98272391|issn=1365-3075|doi-access=free}}</ref><ref>Ōki, Michinori (1983) Recent Advances in Atropisomerism, in ''Topics in Stereochemistry'', Vol. 14 (N. L. Allinger, E. L. Eliel and S. H. Wilen, Eds.), Hoboken, NJ:John Wiley & Sons, pp. 1–82; ''published online in 2007'', DOI: 10.1002/9780470147238.ch1, see [http://onlinelibrary.wiley.com/doi/10.1002/9780470147238.ch1/summary] and [http://onlinelibrary.wiley.com/store/10.1002/9780470147238.fmatter/asset/fmatter.pdf?v=1&t=hwclipn7&s=be733dd54229cc5689e4f5e17777dcb94458748b]{{Dead link|date=July 2019|bot=InternetArchiveBot|fix-attempted=yes}}, accessed 12 June 2014.</ref><ref name=bookatrop>{{cite book|last=Alkorta|first=Ibon|author2=Jose Elguero |author3=Christian Roussel |author4=Nicolas Vanthuyne |author5=Patrick Piras |title=Atropisomerism and Axial Chirality in Heteroaromatic Compounds|series=Advances in Heterocyclic Chemistry|year=2012|doi= 10.1016/B978-0-12-396530-1.00001-2|volume=105 |pages=1–188|isbn=9780123965301|hdl=10261/62060}}</ref> The [[Ring flip|ring-flip]] of substituted [[cyclohexane]]s constitutes a common form of conformers.<ref>{{cite web|last=Hunt|first=Ian|title=Stereochemistry|url=http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch07/ch7-1.html|work=University of Calgary|access-date=28 October 2013}}</ref>

The study of the energetics of bond rotation is referred to as '''conformational analysis'''.<ref name=dougherty>{{cite book|last=Anslyn|first=Eric|title=Modern Physical Organic Chemistry|url=https://archive.org/details/modernphysicalor00ansl|url-access=limited|year=2006|publisher=University Science|isbn=978-1891389313|page=[https://archive.org/details/modernphysicalor00ansl/page/n122 95]|author2=Dennis Dougherty}}</ref> In some cases, conformational analysis can be used to predict and explain product selectivity, mechanisms, and rates of reactions.<ref name="nobel lect">{{cite journal|last=Barton|first=Derek|title=The Principles of Conformational Analysis.|url=https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1969/barton-lecture.html|journal=Nobel Media AB 2013|year=1970|volume=169|issue=3945|pages=539–44|publisher=Elsevier Publishing Co.|doi=10.1126/science.169.3945.539|access-date=10 November 2013|pmid=17746022|bibcode=1970Sci...169..539B|url-access=subscription}}</ref> Conformational analysis also plays an important role in rational, structure-based [[drug design]].