Editing Charge-transfer complex (section)

===Dihalogen/interhalogen CT complexes===
Early studies on donor-acceptor complexes focused on the [[solvatochromism]] exhibited by iodine, which often results from I<sub>2</sub> forming adducts with electron donors such as amines and [[ether]]s.<ref>{{cite journal |doi=10.1021/cr60255a003 |title=Structural chemistry of donor-acceptor interactions |date=1968 |last1=Bent |first1=Henry A. |journal=Chemical Reviews |volume=68 |issue=5 |pages=587–648 }}</ref> Dihalogens X<sub>2</sub> (X = Cl, Br, I) and interhalogens XY(X = I; Y = Cl, Br) are Lewis acid species capable of forming a variety of products when reacted with donor species. Among these species (including oxidation or protonated products), CT adducts D·XY have been largely investigated. The CT interaction has been quantified and is the basis of many schemes for parameterizing donor and acceptor properties, such as those devised by Gutmann, Childs,<ref>{{cite journal |vauthors=Childs RF, Mulholland DL, Nixon A |year=1982 |title=Lewis acid adducts of α,β-unsaturated carbonyl and nitrile compounds. A nuclear magnetic resonance study |journal=Can. J. Chem. |volume=60 |issue=6| pages=801–808 |doi=10.1139/v82-117| doi-access=free}}</ref> [[Gutmann–Beckett method|Beckett]], and the [[ECW model]].<ref>{{cite journal|vauthors=Vogel GC, Drago RS |year=1996 |journal=Journal of Chemical Education |volume=73 |pages=701–707 |title=The ECW Model |issue=8 |bibcode=1996JChEd..73..701V |doi=10.1021/ed073p701}}</ref>

Many organic species featuring chalcogen or pnictogen donor atoms form CT salts. The nature of the resulting adducts can be investigated both in solution and in the solid state.

In solution, the intensity of charge-transfer bands in the UV-Vis absorbance spectrum is strongly dependent upon the degree (equilibrium constant) of this association reaction. Methods have been developed to determine the equilibrium constant for these complexes in solution by measuring the intensity of absorption bands as a function of the concentration of donor and acceptor components in solution. The [[Benesi-Hildebrand method]], named for its developers, was first described for the association of iodine dissolved in aromatic hydrocarbons.<ref>H. Benesi, J. Hildebrand, ''A Spectrophotometric Investigation of the Interaction of Iodine with Aromatic Hydrocarbons'', J. Am. Chem. Soc. 71(8), 2703-07 (1949) doi:10.1021/ja01176a030.</ref>

In the solid state a valuable parameter is the elongation of the X–X or X–Y bond length, resulting from the antibonding nature of the σ* LUMO.<ref>{{Cite journal|last1=Aragoni|first1=M. Carla|last2=Arca|first2=Massimiliano|last3=Demartin|first3=Francesco|last4=Devillanova|first4=Francesco A.|last5=Garau|first5=Alessandra|last6=Isaia|first6=Francesco|last7=Lippolis|first7=Vito|last8=Verani|first8=Gaetano|date=2005-06-16|title=DFT calculations, structural and spectroscopic studies on the products formed between IBr and N,N′-dimethylbenzoimidazole-2(3H)-thione and -2(3H)-selone|url=https://pubs.rsc.org/en/content/articlelanding/2005/dt/b503883a|journal=Dalton Transactions|language=en|issue=13|pages=2252–2258|doi=10.1039/B503883A|pmid=15962045|issn=1477-9234}}</ref> The elongation can be evaluated by means of structural determinations (XRD)<ref>{{Cite journal|last1=Barnes|first1=Nicholas A.|last2=Godfrey|first2=Stephen M.|last3=Hughes|first3=Jill|last4=Khan|first4=Rana Z.|last5=Mushtaq|first5=Imrana|last6=Ollerenshaw|first6=Ruth T. A.|last7=Pritchard|first7=Robin G.|last8=Sarwar|first8=Shamsa|date=2013-01-30|title=The reactions of para-halo diaryl diselenides with halogens. A structural investigation of the CT compound (p-FC6H4)2Se2I2, and the first reported "RSeI3" compound, (p-ClC6H4)SeI·I2, which contains a covalent Se–I bond|url=https://pubs.rsc.org/en/content/articlelanding/2013/dt/c2dt31921g|journal=Dalton Transactions|language=en|volume=42|issue=8|pages=2735–2744|doi=10.1039/C2DT31921G|pmid=23229685|issn=1477-9234}}</ref> and FT-Raman spectroscopy.<ref>{{Cite journal|last1=Arca|first1=Massimiliano|last2=Aragoni|first2=M. Carla|last3=Devillanova|first3=Francesco A.|last4=Garau|first4=Alessandra|last5=Isaia|first5=Francesco|last6=Lippolis|first6=Vito|last7=Mancini|first7=Annalisa|last8=Verani|first8=Gaetano|date=2006-12-28|title=Reactions Between Chalcogen Donors and Dihalogens/Interalogens: Typology of Products and Their Characterization by FT-Raman Spectroscopy|journal=Bioinorganic Chemistry and Applications|volume=2006 |page=58937 |doi=10.1155/BCA/2006/58937 |pmid=17497008 |pmc=1800915 |language=en|doi-access=free }}</ref>

A well-known example is the complex formed by [[iodine]] when combined with [[starch]], which exhibits an intense purple [[charge-transfer band]].  This has widespread use as a rough screen for counterfeit currency. Unlike most paper, the paper used in US currency is not [[Sizing#Papermaking|sized]] with starch. Thus, formation of this purple color on application of an iodine solution indicates a counterfeit.