Pyramidal inversion: Difference between revisions

Revision as of 03:07, 28 January 2022

Revision as of 22:22, 26 June 2022

Line 3:

==Energy barrier==

[[File:NvPinvn.png|thumb|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]]. 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 | journal = Theor. Chim. Acta. | volume = 82 | issue = 3–4| 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|authors=Xiao, Y.; Sun, Z.; Guo, H.; Kwon, O.|title=Chiral Phosphines in Nucleophilic Organocatalysis|journal =Beilstein Journal of Organic Chemistry|year=2014|volume=10|pages=2089–2121|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.

The ammonia interconversion is rapid at room [[temperature]], inverting 30 billion times per second. Two factors contribute to the rapidity of the inversion: a low [[activation energy|energy barrier]] (24.2&nbsp;[[kJ/mol]]; 5.8 kcal/mol) and a narrow width of the barrier itself{{clarify|date=November 2014}}, which allows for frequent [[quantum tunnelling]] (see below). 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 | journal = Theor. Chim. Acta | volume = 82 | issue = 3–4| pages = 271–284 | doi = 10.1007/BF01113258 | s2cid = 98837101 }}</ref>

Line 27:

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) {{DOIdoi|10.1021/ja065325f}}</ref>

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

Line 34:

===Exceptions===

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 |url=https://semanticscholar.org/paper/bfc9c874c3cde0d61f6bb27e9fcedfd49924dfe8 }}</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>

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