Pyramidal inversion: Difference between revisions

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{{Short description|Fluxional process in trigonal-pyramidal molecules}}

In [[chemistry]], '''pyramidal inversion''' is a [[fluxional molecule|fluxional process]] in compounds with a [[pyramid]]al molecule, such as [[ammonia]] (NH₃) "turns inside out".<ref>{{ cite journal | authors = Arvi Rauk, Leland C. Allen, [[Kurt Mislow]] | title = Pyramidal Inversion | journal = [[Angewandte Chemie International Edition|Angew. Chem. Int. Ed.]] | year = 1970 | volume = 9 | pages = 400-414 | doi = 10.1002/anie.197004001 }}</ref><ref> {{GoldBookRef|file=P04956|title=Pyramidal inversion}}</ref> It is a [[Molecular vibration|rapid oscillation]] of the atom and substituents, the molecule or ion passing through a [[Trigonal planar molecular geometry|planar]] [[transition state]].<ref>{{ cite journal | author = J. M. Lehn | authorlink = Jean-Marie Lehn | title = Nitrogen Inversion: Experiment and Theory | journal = Fortschr. Chem. Forsch. | year = 1970 | volume = 15 | pages = 311-377 | doi = 10.1007/BFb0050820}}</ref> For a compound that would otherwise be [[Chirality (chemistry)|chiral]] due to a [[stereocenter]], pyramidal inversion allows its [[enantiomer]]s to [[racemize]].

{| class=floatright valign=center width="272px" style="margin-left:2em; margin-bottom:1ex; border: 1px solid darkgray; font-size:88.5%"

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|[[Image:Amine R-N.svg|96px]]

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|[[Image:Amine N-R.svg|96px]]

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| colspan=3 |'''Nitrogen inversion:''' The amine C₃ axis is horizontal; the pair of dots represent the lone pair (on that axis). Note that the two amine molecules are [[mirror symmetry|symmetric across a mirror plane]]. If the three R groups attached are all unique, then the amine is chiral; isolability depends on the [[Gibbs energy|free energy]] required to invert the molecule.

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In [[chemistry]], '''pyramidal inversion''' (also '''umbrella inversion''') is a [[fluxional molecule|fluxional process]] in compounds with a [[pyramid]]al molecule, such as [[ammonia]] (NH₃) "turns inside out".<ref>{{ cite journal | authorsauthor1 = Arvi Rauk, |author2=[[Leland C. Allen, ]]|author3=[[Kurt Mislow]] | title = Pyramidal Inversion | journal = [[Angewandte Chemie International Edition|Angew. Chem. Int. Ed.]] | year = 1970 | volume = 9 | issue = 6 | pages = 400-414400–414 | doi = 10.1002/anie.197004001 }}</ref><ref> name="ReferenceA">{{GoldBookRef|file=P04956|title=Pyramidal inversion}}</ref> It is a [[Molecular vibration|rapid oscillation]] of the atom and substituents, the molecule or ion passing through a [[Trigonal planar molecular geometry|planar]] [[transition state]].<ref>{{ cite journal | author = J. M. Lehn | authorlinkauthor-link = Jean-Marie Lehn | title = Nitrogen Inversion: Experiment and Theory | journal = Fortschr. Chem. Forsch. | year = 1970 | volume = 15 | pages = 311-377311–377 | doi = 10.1007/BFb0050820}}</ref> For a compound that would otherwise be [[Chirality (chemistry)|chiral]] due to a [[stereocenter]], pyramidal inversion allows its [[enantiomer]]s to [[racemize]]. The general phenomenon of pyramidal inversion applies to many types of molecules, including [[carbanion]]s, [[amines]], [[phosphane|phosphines]], [[arsine]]s, [[stibine]]s, and [[sulfoxide]]s.<ref>{{cite journal |author=Arvi Rauk |author2=Leland C. Allen |author3=Kurt Mislow |year=1970 |title=Pyramidal Inversion |journal=Angewandte Chemie International Edition |volume=9 |issue=6 |pages=400–414 |doi=10.1002/anie.197004001}}</ref><ref name="ReferenceA" />

==Energy barrier==

[[File:NvPinvn.png|thumb|244px|Qualitative [[reaction coordinate]] for inversion of an amine and a phosphine. The y-axis is energy.]]

The identity of the inverting atom has a dominating influence on the barrier. [[Nitrogen inversion|Inversion of ammonia]] is rapid at [[room temperature]], inverting 30 billion times per second. Three factors contribute to the rapidity of the inversion: a low [[temperatureactivation energy|energy barrier]] (24.2&nbsp;[[kJ/mol]]; 5.8 kcal/mol), a narrow barrier width (distance between geometries), and the low mass of hydrogen atoms, which combine to give a further 80-fold rate enhancement due to [[quantum tunnelling]].<ref name="HalpernRamachandranGlendening2007">{{cite journal | last1 = Halpern | first1 = Arthur M. | last2 = Ramachandran | first2 = B. R. | last3 = Glendening | first3 = Eric D. | title = The Inversion Potential of Ammonia: An Intrinsic Reaction Coordinate Calculation for Student Investigation | journal = Journal of Chemical Education | date = June 2007 | volume = 84 | issue = 6 | page = 1067 | issn = 0021-9584 | eissn = 1938-1328 | doi = 10.1021/ed084p1067 | pmid = | bibcode = 2007JChEd..84.1067H | url = }}</ref> In contrast, [[phosphine]] (PH₃) inverts very slowly at room temperature (energy barrier: 132&nbsp;[[kJ/mol]]).<ref>{{cite journal | last1 = Kölmel | first1 = C. | last2 = Ochsenfeld | first2 = C. | last3 = Ahlrichs | first3 = R. | year = 1991 | title = An ab initio investigation of structure and inversion barrier of triisopropylamine and related amines and phosphines | url = | journal = Theor. Chim. Acta. | volume = 82 | issue = 3-43–4| pages = 271–284 | doi = 10.1007/BF01113258 | s2cid = 98837101 }}</ref> Consequently, amines of the type RR′R"N usually are not optically stable (enantiomers racemize rapidly at room temperature), but [[P-Chiral phosphine|''P''-chiral phosphines]] are.<ref name=Kwon>{{cite journal|authorsauthor1=Xiao, Y.; |author2=Sun, Z.; |author3=Guo, H.; |author4=Kwon, O. |title=Chiral Phosphines in Nucleophilic Organocatalysis|journal =Beilstein Journal of Organic Chemistry|year=2014|volume=10|pages=2089-21212089–2121|doi=10.3762/bjoc.10.218|pmid=25246969|pmc=4168899}}</ref> Appropriately substituted [[sulfonium]] salts, [[sulfoxide]]s, [[arsine]]s, etc. are also optically stable near room temperature. [[Steric effects]] can also influence the barrier.

==Nitrogen inversion==

[[Image:Nitrogen-inversion-3D-balls.png|thumb|250px|Nitrogen inversion in ammonia]]

Pyramidal inversion in [[nitrogen]] and [[amine]]s is known as '''nitrogen inversion'''.<ref>{{Cite journal|last1=Ghosh|first1=Dulal C.|last2=Jana|first2=Jibanananda|last3=Biswas|first3=Raka|date=2000|title=Quantum chemical study of the umbrella inversion of the ammonia molecule|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/1097-461X%282000%2980%3A1%3C1%3A%3AAID-QUA1%3E3.0.CO%3B2-D|journal=International Journal of Quantum Chemistry|language=en|volume=80|issue=1|pages=1–26|doi=10.1002/1097-461X(2000)80:1<1::AID-QUA1>3.0.CO;2-D|issn=1097-461X|url-access=subscription}}</ref> It is a [[Molecular vibration|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);<ref>{{Greenwood&Earnshaw2nd|page=423}}</ref> the molecule passing through a [[Trigonal planar molecular geometry|planar]] [[transition state]].<ref>{{ cite journal | author = J. M. Lehn | author-link = Jean-Marie Lehn | title = Nitrogen Inversion: Experiment and Theory | journal = [[Fortschritte der Chemischen Forschung|Fortschr. Chem. Forsch.]] | year = 1970 | volume = 15 | pages = 311–377 | doi = 10.1007/BFb0050820 }}</ref> For a compound that would otherwise be [[Chirality (chemistry)|chiral]] due to a nitrogen [[stereocenter]], nitrogen inversion provides a low energy pathway for [[racemization]], usually making [[chiral resolution]] impossible.<ref>{{March6th|pages=142–145}}</ref>

===Quantum effects===

Ammonia exhibits a [[quantum tunnelling]] due to a narrow tunneling barrier,<ref>{{cite book | last = Feynman | first = Richard P. | author-link = Richard Feynman |author2=Robert Leighton |author3=Matthew Sands | title = The Feynman Lectures on Physics |volume=III |chapter=The Hamiltonian matrix | publisher = Addison-Wesley | year = 1965 | ___location = Massachusetts, USA | isbn = 0-201-02118-8| title-link = The Feynman Lectures on Physics }}</ref> and not due to thermal excitation. Superposition of two states leads to [[energy level splitting]], which is used in ammonia [[maser]]s.

===Examples===

The inversion of ammonia was first detected by [[microwave spectroscopy]] in 1934.<ref name="Cleeton">{{cite journal|last=Cleeton|first=C.E.|author2=Williams, N.H. |title=Electromagnetic waves of 1.1 cm wave-length and the absorption spectrum of ammonia|journal=Physical Review|year=1934|volume=45|pages=234–237|doi=10.1103/PhysRev.45.234|bibcode = 1934PhRv...45..234C|issue=4 }}</ref>

In one study the inversion in an [[aziridine]] was slowed by a factor of 50 by placing the nitrogen atom in the vicinity of a [[phenol]]ic alcohol group compared to the oxidized [[hydroquinone]].<ref>''Control of Pyramidal Inversion Rates by Redox Switching'' Mark W. Davies, Michael Shipman, James H. R. Tucker, and Tiffany R. Walsh [[J. Am. Chem. Soc.]]; '''2006'''; 128(44) pp.&nbsp;14260–14261; (Communication) {{doi|10.1021/ja065325f}}</ref>

[[Image:Nitrogeninversionexample.png|400px|center|Nitrogen inversion Davies 2006]]

The system interconverts by oxidation by [[oxygen]] and reduction by [[sodium dithionite]].

===Exceptions===

[[File:Tröger's base.svg|thumb|272x272px|rigid Tröger's base scaffold prevents nitrogen inversion <ref name=":0" />]]

Conformational strain and structural rigidity can effectively prevent the inversion of amine groups. [[Tröger's base]] analogs<ref>{{Cite journal|last=MRostami |display-authors=etal |date=2017|title=Design and synthesis of Ʌ-shaped photoswitchable compounds employing Tröger's base scaffold|journal=Synthesis|volume=49|issue=6 |pages=1214–1222|doi=10.1055/s-0036-1588913 }}</ref> (including the Hünlich's base<ref>{{Cite journal|last=MKazem |display-authors=etal |date=2017|title=Facile preparation of Λ-shaped building blocks: Hünlich base derivatization|journal=Synlett|volume=28|issue=13 |pages=1641–1645|doi=10.1055/s-0036-1588180 |s2cid=99294625 }}</ref>) are examples of compounds whose nitrogen atoms are chirally stable [[stereocenter]]s and therefore have significant [[Optical rotation|optical activity]].<ref name=":0">{{Cite journal|last=MRostami|first=MKazem|title=Optically active and photoswitchable Tröger's base analogs|doi=10.1039/C9NJ01372E|journal=New Journal of Chemistry|volume=43|issue=20|pages=7751–7755|via=The Royal Society of Chemistry|year=2019|s2cid=164362391 }}</ref>

==References==

{{ReflistClear-right}}

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[[Category:Physical chemistry]]