{{short description|Attractive interactions between aromatic rings}}

In chemistry, '''stacking''' refers to superposition of molecules or atomic sheets owing to attractive interactions between these molecules or sheets.

==Metal dichalcogenide compounds== thumb|MoS<sub>2</sub>, the most common metal dichalcogenide, adopts a layered structure. Metal dichalcogenides have the formula ME<sub>2</sub>, where M = a transition metal and E = S, Se, Te.<ref>Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. {{ISBN|0-19-855370-6}}.</ref> In terms of their electronic structures, these compounds are usually viewed as derivatives of M<sup>4+</sup>. They adopt stacked structures, which is relevant to their ability to undergo intercalation, e.g. by lithium, and their lubricating properties. The corresponding diselenides and even ditellurides are known, e.g., TiSe<sub>2</sub>, MoSe<sub>2</sub>, and WSe<sub>2</sub>.

==Charge transfer salts== {{Anchor|TTF-TCNQ}} [[File:SegStackEdgeOnHMTFCQ.jpg|thumb|Edge-on view of portion of crystal structure of hexamethyleneTTF/TCNQ charge transfer salt, highlighting the segregated stacking.<ref>{{cite journal|author1=D. Chasseau|author2=G. Comberton|author3=J. Gaultier|author4=C. Hauw|journal=Acta Crystallographica Section B|title=Réexamen de la structure du complexe hexaméthylène-tétrathiafulvalène-tétracyanoquinodiméthane|year=1978| volume=34|issue=2|page=689|doi=10.1107/S0567740878003830|doi-access=|bibcode=1978AcCrB..34..689C }}</ref>]] A combination of tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF) forms a strong charge-transfer complex referred to as ''TTF-TCNQ''.<ref>{{cite journal |author1=P. W. Anderson |author2=P. A. Lee |author3=M. Saitoh | journal = Solid State Communications | volume = 13 | year = 1973 | pages = 595–598 | doi = 10.1016/S0038-1098(73)80020-1 | title = Remarks on giant conductivity in TTF-TCNQ |issue=5 | bibcode=1973SSCom..13..595A}}</ref> The solid shows almost metallic electrical conductance. In a TTF-TCNQ crystal, TTF and TCNQ molecules are arranged independently in separate parallel-aligned stacks, and an electron transfer occurs from donor (TTF) to acceptor (TCNQ) stacks.<ref>{{cite journal |doi=10.1021/acs.jchemed.5b00340|title=Opposites Attract: Organic Charge Transfer Salts |year=2015 |last1=Van De Wouw |first1=Heidi L. |last2=Chamorro |first2=Juan |last3=Quintero |first3=Michael |last4=Klausen |first4=Rebekka S. |journal=Journal of Chemical Education |volume=92 |issue=12 |pages=2134–2139 |bibcode=2015JChEd..92.2134V }}</ref>

==Graphite== Side view of ABA layer stacking in graphite|thumb|left|220px

Graphite consists of stacked sheets of covalently bonded carbon.<ref>{{cite book |last1=Delhaes |first1=Pierre |chapter=Polymorphism of carbon |editor-last1=Delhaes |editor-first1=Pierre |title=Graphite and precursors |date=2000 |publisher=Gordon & Breach |isbn=9789056992286|pages=1–24}}</ref><ref>{{cite book |last1=Pierson |first1=Hugh O. |title=Handbook of carbon, graphite, diamond, and fullerenes : properties, processing, and applications |date=2012 |publisher=Noyes Publications |isbn=9780815517399 |pages=40–41}}</ref> The individual layers are called graphene. In each layer, each carbon atom is bonded to three other atoms forming a continuous layer of sp<sup>2</sup> bonded carbon hexagons, like a honeycomb lattice with a bond length of 0.142&nbsp;nm, and the distance between planes is 0.335&nbsp;nm.<ref>{{cite book |title= Graphite and Precursors |author= Delhaes, P. |publisher= CRC Press |year= 2001 |url= https://books.google.com/books?id=7p2pgNOWPbEC&pg=PA146 |isbn= 978-90-5699-228-6}}</ref> Bonding between layers is relatively weak van der Waals bonds, which allows the graphene-like layers to be easily separated and to glide past each other.<ref>{{cite journal |last1=Chung |first1=D. D. L. |title=Review Graphite |journal=Journal of Materials Science |date=2002 |volume=37 |issue=8 |pages=1475–1489 |doi=10.1023/A:1014915307738 |s2cid=189839788 }}</ref> Electrical conductivity perpendicular to the layers is consequently about 1000 times lower.<ref>{{Cite book |last=Pierson |first=Hugh O. |title=Handbook of carbon, graphite, diamond, and fullerenes : properties, processing, and applications |date=1993 |publisher=Noyes Publications |isbn=0-8155-1739-4 |location=Park Ridge, N.J. |oclc=49708274}}</ref>

==Linear chain compounds== Linear chain compounds are materials composed of stacked arrays of metal-metal bonded molecules or ions. Such materials exhibit anisotropic electrical conductivity.<ref name="Bera">{{cite journal |author1=Bera, J. K. |author2=Dunbar, K. R. |journal= Angew. Chem. Int. Ed. |year= 2002 |volume= 41 |issue= 23 |pages= 4453–4457 |title= Chain Compounds Based on Transition Metal Backbones: New Life for an Old Topic |doi= 10.1002/1521-3773(20021202)41:23<4453::AID-ANIE4453>3.0.CO;2-1 |pmid=12458505}}</ref> One example is {{chem2|Rh(acac)(CO)2}} (acac = acetylacetonate, which stack with {{chem2|Rh***Rh}} distances of about 326&nbsp;pm.<ref name=huq>{{cite journal|title=Refinement of the crystal structure of acetylacetonatodicarbonylrhodium(I)|last1=Huq|first1=Fazlul|last2=Skapski|first2=Andrzej C.|journal=J. Cryst. Mol. Struct.|year=1974|volume=4|issue=6|pages=411–418|doi=10.1007/BF01220097|s2cid=96977904}}</ref> Classic examples include Krogmann's salt and Magnus's green salt. [[File:ACABRH02.png|thumb|left|Portion of the lattice of Dicarbonyl(acetylacetonato)rhodium(I) ({{chem2|Rh(acac)(CO)2}}) showing the "stacking" of the individual planar units through {{chem2|Rh***Rh}} interactions.|140px]]

==Counterexample: benzene dimer and related species== {{main|Pi-stacking}} '''π–π stacking''' is a noncovalent interaction between the pi bonds of aromatic rings.<ref>{{March6th|page=114}}</ref> Such "sandwich interactions" are however generally electrostatically repulsive. What is more commonly observed are either a '''staggered stacking''' (parallel displaced) or '''pi-teeing''' (perpendicular T-shaped) interaction both of which are electrostatic attractive.<ref name = "Lewis_2016">{{cite book | vauthors = Lewis M, Bagwill C, Hardebeck L, Wireduaah S |chapter=Modern Computational Approaches to Understanding Interactions of Aromatics | veditors = Johnson DW, Hof F |title=Aromatic Interactions: Frontiers in Knowledge and Application |date=2016 |publisher=Royal Society of Chemistry |location=England |isbn=978-1-78262-662-6 |pages=1–17 | chapter-url = https://books.google.com/books?id=CmwoDwAAQBAJ&pg=PA1 }}</ref> For example, the most commonly observed interactions between aromatic rings of amino acid residues in proteins is a staggered stacked followed by a perpendicular orientation. Sandwiched orientations are relatively rare.<ref name="McGaughey_1998">{{cite journal | vauthors = McGaughey GB, Gagné M, Rappé AK | title = pi-Stacking interactions. Alive and well in proteins | journal = The Journal of Biological Chemistry | volume = 273 | issue = 25 | pages = 15458–63 | date = June 1998 | pmid = 9624131 | doi = 10.1074/jbc.273.25.15458 | doi-access = free }}</ref> Pi stacking is repulsive as it places carbon atoms with partial negative charges from one ring on top of other partial negatively charged carbon atoms from the second ring and hydrogen atoms with partial positive charges on top of other hydrogen atoms that likewise carry partial positive charges.<ref name = "Martinez_2012">{{Cite journal| vauthors = Martinez CR, Iverson BL |date=2012|title=Rethinking the term "pi-stacking"|url=http://xlink.rsc.org/?DOI=c2sc20045g|journal=Chemical Science|language=en|volume=3|issue=7|pages=2191|doi=10.1039/c2sc20045g|issn=2041-6520|hdl=2152/41033|s2cid=95789541 |hdl-access=free}}</ref>

thumb|right|The synthesis of catenane exploiting attractive pi-stacking interactions between electron-rich and electron-poor arenes. π–π interactions play a role in supramolecular chemistry, specifically the synthesis of catenane. The major challenge for the synthesis of catenane is to interlock molecules in a controlled fashion. Attractive π–π interactions exist between electron-rich benzene derivatives and electron-poor pyridinium rings.<ref>{{cite journal | vauthors = Ashton PR, Goodnow TT, Kaifer AE, Reddington MV, Slawin AM, Spencer N, Stoddart JF, Vicent C, Williams DJ |display-authors=6 |title=A [2] Catenane Made to Order |journal=J. Angew. Chem. Int. Ed. |year=1989 |volume=28 |issue=10 |pages=1396–1399 |doi=10.1002/anie.198913961 }}</ref> [2]Catanene was synthesized by treating bis(pyridinium) ('''A'''), bisparaphenylene-34-crown-10 ('''B'''), and 1, 4-bis(bromomethyl)benzene ('''C''') (Fig. 2). The π–π interaction between '''A''' and '''B''' directed the formation of an interlocked template intermediate that was further cyclized by substitution reaction with compound '''C''' to generate the [2]catenane product.

== See also == * Noncovalent interaction * Dispersion (chemistry) * Cation–pi interaction * Intercalation (biochemistry) * Intercalation (chemistry)

== References == {{reflist}}

== External links == * {{cite journal | vauthors = Luo R, Gilson HS, Potter MJ, Gilson MK | title = The physical basis of nucleic acid base stacking in water | journal = Biophysical Journal | volume = 80 | issue = 1 | pages = 140–148 | date = January 2001 | pmid = 11159389 | pmc = 1301220 | doi = 10.1016/S0006-3495(01)76001-8 | bibcode = 2001BpJ....80..140L }} * [http://www.scs.illinois.edu/denmark/wp-content/uploads/gp/2011/gm-2011-1_18.pdf Larry Wolf (2011): π-π (π-Stacking) interactions: origin and modulation]

{{Chemical bonds}}

Category:Organic chemistry Category:Chemical bonding Category:Supramolecular chemistry