Editing Crown ether (section)

==Affinity for cations==
Due to the [[chelate effect]] and [[macrocyclic effect]], crown ethers exhibit stronger affinities for diverse cations than their divided or [[Open-chain compound|acyclic]] analogs. Hereby, the cation selectivity for alkali metal ions is mainly dependent on the size and charge density of the ion and the cavity size of the crown ether.<ref name=":0">{{Cite journal|last1=Liou|first1=Chien-Chung|last2=Brodbelt|first2=Jennifer S.|author-link1=Jennifer S. Brodbelt|date=July 1992|title=Determination of orders of relative alkali metal ion affinities of crown ethers and acyclic analogs by the kinetic method|journal=Journal of the American Society for Mass Spectrometry|volume=3|issue=5|pages=543–548|doi=10.1016/1044-0305(92)85031-e|pmid=24234497|s2cid=36106963 |issn=1044-0305|doi-access=|bibcode=1992JASMS...3..543L }}</ref> 
{| class="wikitable"
|+Comparison of Cavity Size with Effective Ion Radii of Alkali Metals
!Crown Ether
!Cavity Size/Å<ref>{{Citation|last1=Christensen|first1=J.J.|title=PREFACE|date=1978|work=Synthetic Multidentate Macrocyclic Compounds|pages=ix–x|publisher=Elsevier|isbn=978-0-12-377650-1|last2=Izatt|first2=R.M.|doi=10.1016/b978-0-12-377650-1.50005-8|doi-access=}}</ref>
!Favored Alkali Ion<ref>{{Cite journal|last=Frensdorff|first=Hans K.|date=February 1971|title=Stability constants of cyclic polyether complexes with univalent cations|journal=Journal of the American Chemical Society|language=en|volume=93|issue=3|pages=600–606|doi=10.1021/ja00732a007|bibcode=1971JAChS..93..600F |issn=0002-7863}}</ref>
!Effective Ion Radius/Å<ref>{{Cite journal|last=Shannon|first=R. D.|date=1976-09-01|title=Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides|journal=Acta Crystallographica Section A|volume=32|issue=5|pages=751–767|doi=10.1107/s0567739476001551|bibcode=1976AcCrA..32..751S|issn=0567-7394}}</ref>
|-
|12-crown-4
|0.6-0.75
|Li<sup>+</sup>
|0.76
|-
|15-crown-5
|0.86-0.92
|Na<sup>+</sup>
|1.02
|-
|18-crown-6
|1.34-1.55
|K<sup>+</sup>
|1.38
|-
|21-crown-7
|1.7-2.1
|Cs<sup>+</sup>
|1.67
|}
Affinities of a given crown ether towards the cations of [[lithium]], [[sodium]], and potassium can change by multiple magnitudes, which is attributed to the high differences in their charge density. Between the cations of potassium, [[rubidium]], and cesium changes in affinities are less notable, as their charge density varies less than the alkali metals in earlier periods.<ref name=":0" />

Apart from its high affinity for potassium cations, [[18-crown-6]] can also bind to protonated amines and form very stable complexes in both solution and the gas phase. Some [[amino acids]], such as [[lysine]], contain a primary [[amine]] on their side chains. Those protonated amino groups can bind to the cavity of 18-crown-6 and form stable complexes in the gas phase. Hydrogen-bonds are formed between the three hydrogen atoms of protonated amines and three oxygen atoms of 18-crown-6. These hydrogen-bonds make the complex a stable adduct. By incorporating luminescent substituents into their backbone, these compounds have proved to be sensitive ion probes, as changes in the absorption or fluorescence of the photoactive groups can be measured for very low concentrations of metal present.<ref>{{cite book|last1=Fabbrizzi|first1= L.|last2= Francese|first2= G.|last3= Licchelli|first3= M.|last4= Pallavicini|first4= P.|last5=Perotti |first5=A.|last6= Poggi|first6= A.|last7= Sacchi|first7= D.|last8= Taglietti|first8= A.|title= Chemosensors of Ion and Molecule Recognition|series= NATO ASI Series C |volume=492|editor1-last= Desvergne|editor1-first= J. P.|editor2-last= Czarnik|editor2-first= A. W.|publisher= Kluwer Academic Publishers|location= Dordrecht|date= 1997|page= 75}}</ref> Some attractive examples include macrocycles, incorporating oxygen and/or nitrogen donors, that are attached to polyaromatic species such as [[anthracenes]] (via the 9 and/or 10 positions)<ref>{{cite book|last1=Bouas-Laurent|first1= H.|last2= Desvergne|first2= J. P.|last3= Fages|first3= F.|last4= Marsau|first4= P. |title=Fluorescent Chemosensors for Ion and Molecule Recognition |url=https://archive.org/details/fluorescentchemo1992czar|url-access=limited|series=ACS Symposium Series 538|editor-first= Czarnik|editor-last= A. W. |publisher=American Chemical Society |location=Washington, DC|date= 1993| page= [https://archive.org/details/fluorescentchemo1992czar/page/n66 59]|isbn= 9780841227286}}</ref> or [[naphthalenes]] (via the 2 and 3 positions).<ref>{{cite journal|last1=Sharghi|first1=Hashem|last2=Ebrahimpourmoghaddam|first2=Sakineh|title=A Convenient and Efficient Method for the Preparation of Unique Fluorophores of Lariat Naphtho-Aza-Crown Ethers|journal=Helvetica Chimica Acta|volume=91|issue=7|year=2008|pages=1363–1373 |doi=10.1002/hlca.200890148}}</ref> Some modifications of dye ionophores by crown ethers exhibit [[Molar attenuation coefficient|extinction coefficients]] that are dependent on the chain lengths of chained cations.<ref>{{Cite journal|last1=Fuji|first1=Kaoru|last2=Tsubaki|first2=Kazunori|last3=Tanaka|first3=Kiyoshi|last4=Hayashi|first4=Noriyuki|last5=Otsubo|first5=Tadamune|last6=Kinoshita|first6=Takayoshi|date=April 1999|title=Visualization of Molecular Length of α,ω-Diamines and Temperature by a Receptor Based on Phenolphthalein and Crown Ether|journal=Journal of the American Chemical Society|volume=121|issue=15|pages=3807–3808|doi=10.1021/ja9836444|bibcode=1999JAChS.121.3807F |issn=0002-7863}}</ref>