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| In chemistry '''Ion-association''' is a [[chemical reaction]] whereby [[ion]]s of opposite [[electrical charge]] come together in [[solution]] to form a distinct chemical entity. Ion-associates are classified according to the number of ions that associate with each other, and the nature of the interaction. The most important factor to determine the extent of ion-association is the [[dielectric constant]] of the [[solvent]]. Ion-associates have been characterized by means of [[vibrational spectroscopy]]. The concept has been proposed by [[Niels Bjerrum]].
| | e - Shop Word - Press is a excellent cart for your on the web shopping organization. Online available for hiring are most qualified, well knowledgeable and talented Wordpress developer India from offshore Wordpress development services company. Step-4 Testing: It is the foremost important of your Plugin development process. After confirming the account, login with your username and password at Ad - Mob. In the most current edition you can customize your retailer layout and display hues and fonts similar to your site or blog. <br><br>Generally, for my private income-making market websites, I will thoroughly research and discover the leading 10 most worthwhile niches to venture into. Some of the Wordpress development services offered by us are:. Our Daily Deal Software plugin brings the simplicity of setting up a Word - Press blog to the daily deal space. If you cherished this article and you simply would like to obtain more info relating to [http://mmservice.dk/wordpress_backup_plugin_111653 backup plugin] kindly visit our own webpage. So if you want to create blogs or have a website for your business or for personal reasons, you can take advantage of free Word - Press installation to get started. Word - Press makes it possible to successfully and manage your website. <br><br>Here are a few reasons as to why people prefer Word - Press over other software's. The following piece of content is meant to make your choice easier and reassure you that the decision to go ahead with this conversion is requited with rich benefits:. Possibly the most downloaded Word - Press plugin, the Google XML Sitemaps plugin but not only automatically creates a site map linking to everyone your pages and posts, it also notifies Google, Bing, Yahoo, and Ask. You can allow visitors to post comments, or you can even allow your visitors to register and create their own personal blogs. Article Source: Stevens works in Internet and Network Marketing. <br><br>If all else fails, please leave a comment on this post with the issue(s) you're having and help will be on the way. As an example, if you are promoting a product that cures hair-loss, you most likely would not wish to target your adverts to teens. Specialty about our themes are that they are easy to load, compatible with latest wordpress version and are also SEO friendly. Word - Press is the most popular open source content management system (CMS) in the world today. See a product, take a picture, and it gives you an Amazon price for that product, or related products. <br><br>Yet, overall, less than 1% of websites presently have mobile versions of their websites. I'm a large fan of using Word - Press to create pretty much any sort of web page. In simple words, this step can be interpreted as the planning phase of entire PSD to wordpress conversion process. It is a fact that Smartphone using online customers do not waste much of their time in struggling with drop down menus. Customers within a few seconds after visiting a site form their opinion about the site. |
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| In chemistry the term ''Ion-pairs'' is used when a [[cation]] and [[anion]] come together.
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| However in Physics [[ion|"ion pairs"]] are formed by ion impact, such as in the formation of a [[Townsend avalanche]]. See [[ion]] article for more information.
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| == Classification ==
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| ''Ion-pairs'' are formed when a [[cation]] and [[anion]] come together.
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| : A<sup>n+</sup> + B<sup>m-</sup> {{eqm}} AB<sup>(n-m)+</sup>
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| There are three distinct types of ion-pair, depending on the extent of [[solvation]] of the two ions.
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| <gallery caption="Schematic representations of ion-pairs" widths="200px" heights="100px" perrow="3">
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| Image:solvent separated.png| fully solvated
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| Image:solvent shared.png| solvent-shared or solvent-separated
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| Image:contact ion pair.png| contact
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| </gallery>
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| In the schematic representation above, the circles represent spheres. The sizes are arbitrary and not necessarily similar as illustrated. The cation is coloured red and the anion is coloured blue. The green area represents solvent molecules in a primary solvation shell: secondary solvation is ignored. When both ions have a complete primary solvation sphere the ion-pair may be termed ''fully solvated''. When there is about one solvent molecule between cation and anion, the ion-pair may be termed ''solvent-shared''. Lastly when the ions are in contact with each other the ion-pair is termed a ''contact'' ion-pair. Even in a contact ion-pair, however, the ions retain most of their solvation shell. The nature of this solvation shell is generally not known with any certainty. In aqueous solution and in other donor solvents metal cations are surrounded by between four and nine solvent molecules in the primary [[solvation shell]],<ref name=Burgess>{{cite book |title=Metal Ions in Solution
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| |last=Burgess |first=J. |year=1978 |publisher=Ellis Horwood |isbn=0-85312-027-7}} Chapter 5, "Solvation numbers".</ref> but the nature of solvation of anions is mostly unknown.
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| An alternative name for a solvent-shared ion-pair is an ''outer-sphere complex''. This usage is common in [[Coordinate covalent bond|co-ordination]] chemistry and denotes a complex between a solvated metal cation and an anion. Similarly a contact ion-pair may be termed an ''inner-sphere complex''. The essential difference between the three types is the closeness with which the ions approach each other fully solvated > solvent-shared > contact. With fully solvated and solvent shared ion-pairs the interaction is primarily electrostatic, but in a contact ion-pair there will also be some covalent character in the bond between cation and anion.
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| An ''Ion-triplet'' may be formed from one cation and two anions, or from one anion and two cations.<ref>{{cite journal|doi=10.1021/ja01304a001|last=Fuoss|first=R.M.|coauthors=Kraus, C.A.|year=1935|title=Properties of Electrolytic Solutions. XV. Thermodynamic Properties of Very Weak Electrolytes|journal=J. Amer. Chem. Soc.|volume=57|pages=1–4}}</ref> Higher aggregates, such as a tetramer, (AB)<sub>4</sub> may be formed.
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| Ternary ion-associates involve the association of three species.<ref>{{cite journal|last=Alexandrov|first=A.|coauthors=Kostova, S.|year=1984 |title= Extraction-spectrophotometric and radiometric investigation of the ternary ion-association complex of niobium(V) with pyrocatechol and triphenyl-tetrazolium chloride |journal=Journal of Radioanalytical and Nuclear Chemistry |publisher=Springer|volume=83|issue=2|pages=247–255 |doi= 10.1007/BF02037138}}</ref> Another type, named ''intrusion ion-pair'' has also been characterized.<ref>{{cite journal| last=Fletcher| first=R.J.|coauthors=Gans, P.; Gill, J.B.; Geyer, C.|year=1997|title=Spectrochemistry of solutions. part 29. Intrusion ion pairing: identification of a new form of ion pair in transition metal salt solutions in pyridine through their visible spectra |journal=J. Mol. Liquids|volume=73-74 |pages=99–106|doi=10.1016/S0167-7322(97)00060-3 }}</ref>
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| == Theory ==
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| Ions of opposite charge are naturally attracted to each other by the [[electrostatic force]]. This is described by Coulomb's law.
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| :<math>F = \frac{q_1q_2}{\epsilon r^2},</math>
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| ''F'' is the force of attraction, ''q''<sub>1</sub> and ''q''<sub>2</sub> are the magnitudes of the electrical charges, ε is the [[dielectric constant]] of the medium and ''r'' is the distance between the ions. For ions in solution this is an approximation because the ions exert a polarizing effect on the solvent molecules that surround them which attenuates the electric field somewhat. Nevertheless some general conclusions can be inferred.
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| :Ion association will increase as
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| :*the magnitude(s) of the electrical charge(s) ''q<sub>1</sub>'' and ''q<sub>2</sub>'' increase
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| :*the magnitude of the dielectric constant, ε decreases
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| :*the size of the ions decreases so that the distance between cation and anion, ''r'', decreases
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| The [[equilibrium constant]] for ion-pair formation, ''K'', like all equilibrium constants, is related to the standard free energy change.<ref>{{cite book|last=Klotz|first=I.M.|title=Chemical Thermodynamics|publisher=W.A. Benjamin|date=1964}} Chapter 10</ref>
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| :Δ''G''<sup>[[Image:StrikeO.png]]</sup> = -''RT'' ln<sub>e</sub>''K''.
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| ''R'' is the [[gas constant]] and ''T'' is the temperature in [[Kelvin]]. Free energy is made up of an [[enthalpy]] term and an [[entropy]] term.
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| :Δ''G''<sup>[[Image:StrikeO.png]]</sup> = Δ''H''<sup>[[Image:StrikeO.png]]</sup> − ''T''Δ''S''<sup>[[Image:StrikeO.png]]</sup>
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| The coulombic energy released when ions associate contributes to the [[enthalpy]] term, Δ''H''<sup>[[Image:StrikeO.png]]</sup>. In the case of contact ion-pairs, the [[covalent]] interaction energy also contributes to the enthalpy, as does the energy of displacing a solvent molecule from the solvation shell of the cation or anion. The tendency to associate is opposed by the [[entropy]] term which results from the fact that the solution containing un-associated ions is more disordered than a solution containing associates. The entropy term is similar for electrolytes of the same type, with minor differences dues to solvation effects. Therefore it is the magnitude of the enthalpy term that most determines the extent of ion-association for a given electrolyte type. This explains the general rules given above.
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| == Occurrence ==
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| [[Dielectric constant]] is the most important factor in determining the occurrence of ion-association. The following table gives some typical values.
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| {|class="wikitable" |border="1"
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| |+Dielectric constants of some common solvents
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| |-
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| !Solvent!!dielectric constant!!temperature /K
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| |-
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| |[[benzene]]||2.3||298
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| |-
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| |[[diethyl ether]]||4.3||293
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| |-
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| |[[tetrahydrofuran]] (THF)||7.6||298
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| |-
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| |[[dichloromethane]]||9.1||293
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| |-
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| |[[ammonia|liquid ammonia]]||17||273
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| |-
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| |[[ethanol]]||24.3||298
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| |-
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| |[[methanol]]||32.7||298
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| |-
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| |[[nitromethane]]||35.9||303
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| |-
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| |[[dimethyl formamide]] (DMF)||36.7||298
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| |-
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| |[[acetonitrile]]||37.5||293
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| |-
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| |[[water]]||78.7||298
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| |-
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| |[[formamide]]||109||293
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| |}
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| Water has a relatively high dielectric constant at 25°C, so in aqueous solutions at ambient temperatures 1:1 electrolytes such as NaCl do not form ion-pairs to an appreciable extent except when the solution is very concentrated.<ref>Assuming that both Na<sup>+</sup> and Cl<sup>−</sup> have six water molecules in the primary solvation shell at ambient temperatures a 5 M solution (5 mol dm<sup>-1</sup>) will consist almost entirely of fully solvated ion-pairs.</ref> 2:2 electrolytes (q<sub>1</sub>=2, q<sub>2</sub>=2) form ion-pairs more readily. Indeed the solvent shared ion-pair [Mg(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup>SO<sub>4</sub><sup>2-</sup> was famously discovered to be present in seawater, in equilibrium with the contact ion-pair [Mg(H<sub>2</sub>O)<sub>5</sub>(SO<sub>4</sub>)]<ref>[http://nobelprize.org/nobel_prizes/chemistry/laureates/1967/eigen-lecture.pdf Manfred Eigen, Nobel lecture]</ref> Trivalent ions such as Al<sup>3+</sup>, Fe<sup>3+</sup> and [[lanthanide]] ions form weak complexes with monovalent anions.
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| The dielectric constant of water decreases with increasing temperature to ca. 55 at 100 °C and ca. 5 at the [[critical temperature]], 217.7 °C.<ref>{{cite web | last= Clifford| first=A.A | title=Changes of water properties with temperature | url=http://www.criticalprocesses.com/Use%20of%20enthalpies%20to%20calculate%20energy%20needed.htm }}</ref> Thus ion-pairing will become more significant in [[superheated water]].
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| Solvents with a dielectric constant in the range, roughly, 20-40, show extensive ion-pair formation. For example, in acetonitrile both contact and solvent-shared ion pairs of Li(NCS) have been observed.<ref>{{cite journal|last=Gans|first=P.|coauthors=Gill, J.B.;Longdon, P.J|year=1989|title=Spectrochemistry of solutions. Part 21.—Inner- and outer-sphere complexes of lithium with thiocyanate in acetonitrile solutions |journal =J. Chem. Soc. Faraday Trans. 1|volume=85|issue=7|pages=1835–1839|doi=10.1039/F19898501835}}</ref> In methanol the 2:1 electrolyte Mg(NCS)<sub>2</sub> is partially dissociated into a contact ion-pair, [Mg(NCS)]<sup>+</sup> and the thiocyanate ion.<ref>{{cite journal |last=Gans |first=P |coauthors=Gill, J.B.;Holden, K.M.L. |year=1994|title=Spectrochemistry of solutions. Part 27.—Formation of [Mg(NCS)]<sup>+</sup> in solutions of Mg(NCS)<sub>2</sub> in methanol|journal=J. Chem. Soc. Faraday Trans.|volume=90|pages=2351–2352|doi=10.1039/FT9949002351 |issue=16}}</ref>
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| The dielectric constant of liquid ammonia decreases from 26 at its [[freezing point]], −80 °C to 17 at 20 °C (under pressure). Many simple 1:1 electrolytes form contact ion-pairs at ambient temperatures. The extent of ion-pairing decreases as temperature decreases. With lithium salts there is evidence to show that both inner-sphere and outer-sphere complexes exist in liquid ammonia solutions.<ref name=JBG>{{cite journal|last=Gill|first=J.B.|year=1981|title=Solute-solute interactions in liquid ammonia solutions: a vibrational spectroscopic view|journal=Pure Appl. Chem.|volume=53|issue=7|pages=1365–1381|doi=10.1351/pac198153071365| url=http://media.iupac.org/publications/pac/1981/pdf/5307x1365}}</ref>
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| Of the solvents with dielectric constant of 10 or less, tetrahydrofuran is particularly relevant in this context as it solvates cations strongly with the result that simple electrolytes have sufficient solubility to make the study of ion-association possible. In this solvent ion association is the rule rather than the exception. Indeed, higher associates such as tetramers are often formed.<ref>{{cite journal|last=Goralski|first=P.|coauthors=Chabanel, M.|year=1987|title=Vibrational study of ionic association in aprotic solvents. 11. Formation and structure of mixed aggregates between lithium halides and lithium thiocyanate|journal=Inorg. Chem.|volume=26|issue=13|pages=2169–2171|doi=10.1021/ic00260a032 }}</ref> Triple cations and triple anions have also been characterized in THF solutions.<ref>{{cite journal|last=Bacelon|first=P.|coauthors=Corset, J.; de Loze , C.|year=2004|title=Triple ion formation in solutions of alkaline sulfocyanides |journal=J.Solution Chem.|pages=129–139|doi=10.1007/BF00644484|volume=9|issue=2 }} (sulfocyanides = thiocyanates)</ref>
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| Ion-association is an important factor in [[phase-transfer catalysis]] since a species such as R<sub>4</sub>P<sup>+</sup>Cl<sup>−</sup> is formally neutral and so can dissolve easily in a non-polar solvent of low dielectric constant. In this case it also helps that the surface of the cation is [[hydrophobic]].
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| In [[SN1|S<sub>N</sub>1 reaction]]s the carbocation intermediate may form an ion-pair with an anion, particularly in solvents of low dielectric constant, such as [[diethylether]].<ref>{{cite journal| last=Winstein| first=S.| coauthors=Clippinger, E.; Fainberg, A. H.; Heck, R.; RobinsonG. C.| year=1956| title=Salt Effects and Ion Pairs in Solvolysis and Related Reactions. III.1 Common Ion Rate Depression and Exchange of Anions during Acetolysis| volume=78|issue=2|pages=328–335|issn=| doi=10.1021/ja01583a022| journal=Journal of the American Chemical Society}}</ref> This can affect both the kinetics of the reaction and the stereochemistry of the reaction products.
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| == Characterization ==
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| [[Vibrational spectroscopy]] provides the most widely used means for characterizing ion-associates. Both [[infrared spectroscopy]] and [[Raman spectroscopy]] have been used. Anions containing a CN group, such as [[cyanide]], [[cyanate]] and [[thiocyanide]] have a vibration frequency a little above 2000 cm<sup>−1</sup> which can be easily observed as the spectra of most solvents (other than [[nitrile]]s) are weak in this region. The anion vibration frequency is "shifted" on formation of ion-pairs and other associates and the extent of the shift gives information about the nature of the species. Other [[Valence (chemistry)|monovalent]] anions which have been studied include [[nitrate]], [[nitrite]] and [[azide]]. Ion-pairs of monatomic anions, such as [[halide]] ions, cannot be studied by this technique. [[NMR spectroscopy]] is not very useful as association/dissociation reactions tend to be fast on the NMR time scale, giving time-averaged signals of the cation and/or anion.
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| Nearly the same shift of vibration frequency is observed for solvent-shared ion-pairs of LiCN, Be(CN)<sub>2</sub> and Al(CN)<sub>3</sub> in liquid ammonia. The extent of this type of ion-pairing decreases as the size of the cation increases. Thus, solvent-shared ion-pairs are characterized by a rather small shift of vibration frequency with respect to the "free" solvated anion and the value of the shift is not strongly dependent on the nature of the cation. The shift for contact ion-pairs is, by contrast, strongly dependent on the nature of the cation and decreases linearly with cations's charge:radius squared ratio in the two series Cs<sup>+</sup> > Rb<sup>+</sup> > K<sup>+</sup> > Na<sup>+</sup> > Li<sup>+</sup>, and Ba<sup>2+</sup> > Sr<sup>2+</sup> > Ca<sup>2+</sup>.<ref name=JBG/>
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| The extent of contact ion-pairing can be estimated from the relative intensities of the bands due to the ion-pair and free ion. It is greater with the larger cations .<ref name=JBG/> This is counter to the trend expected if coulombic energy were the determining factor. Instead, the formation of a contact ion-pair is seen to depend more on the energy needed to displace a solvent molecule from the [[metal ions in aqueous solution|primary solvation sphere]] of the cation. This energy decreases with the size of the cation, making ion-pairing occur to a greater extent with the larger cations. The trend may be different in other solvents.<ref name=JBG/>
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|
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| Higher ion-aggregates, sometimes triplets M<sup>+</sup>X<sup>-</sup>M<sup>+</sup>, sometimes dimers
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| of ion pairs (M<sup>+</sup>X<sup>-</sup>)<sub>2</sub>, or even larger species can be identified in
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| the Raman spectra of some liquid ammonia solutions of Na<sup>+</sup> salts by the presence of bands which cannot be attributed to either contact- or solvent-shared ion-pairs.<ref name=JBG/> | |
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| Evidence for the existence of fully solvated ion-pairs in solution is mostly indirect as the spectroscopic properties of such ion-pairs are indistinguishable from those of the individual ions. Much of the evidence is based on the interpretation of [[conductivity (electrolytic)|conductivity]] measurements.<ref>{{cite book|last=Davies|first=C.W.|title=Ion Association| publisher = Butterworths|location=London|year=1962}}</ref><ref>{{cite journal| last=Miyoshi| first=K.| year=1973|title=Comparison of the Conductance Equations of Fuoss - Onsager, Fuoss-Hsia and Pitts with the Data of Bis(2,9-dimethyl-1,10-phenanthroline)Cu(I) Perchlorate |journal=Bull. Chem. Soc. Japan |volume=46|issue=2|pages=426–430| doi=10.1246/bcsj.46.426}}</ref>
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| == See also ==
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| *[[Intimate ion pair]]
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| == References ==
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| {{Reflist|30em}}
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| == External links ==
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| *[http://scitation.aip.org/content/aip/journal/jcp/25/6/10.1063/1.1743177 JCP 25, 1199]
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| *[http://www.stanford.edu/~cgong/cee373/documents/CEE373Lecture03.pdf Stanford edu]
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| [[Category:Equilibrium chemistry]]
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