View source for Dilithium

{{short description|Diatomic molecule}}
{{about|the real substance|other uses|Dilithium (disambiguation)}}
{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 458280126
| ImageFile1 = Dilithium-2D-dimensions.svg
| ImageFile1_Ref = {{chemboximage|correct|??}}
| ImageSize1 = 100
| ImageName1 = Wireframe model of dilithium
| ImageFile2 = Dilithium-3D-vdW.png
| ImageFile2_Ref = {{chemboximage|correct|??}}
| ImageSize2 = 100
| ImageName2 = Spacefill model of dilithium
| IUPACName = Dilithium(''Li—Li''){{cn|date=January 2024}}
|Section1={{Chembox Identifiers
| CASNo = 14452-59-6
| CASNo_Ref = {{cascite|changed|??}}
| PubChem = 139759
| ChemSpiderID = 123254
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| SMILES = [Li][Li]
| StdInChI = 1S/2Li
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = SMBQBQBNOXIFSF-UHFFFAOYSA-N
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
 }}
|Section2={{Chembox Properties
| Li=2
<!-- | Binding Energy=1.03 eV / unk parameter. what is intended? 12-06-2016 -->
 }}
}}
'''Dilithium''', Li<sub>2</sub>, is a strongly [[Nucleophile|nucleophilic]], [[diatomic]] molecule comprising two [[lithium]] [[atom]]s [[covalent bond|covalently bonded]] together.  Li<sub>2</sub> has been observed in the [[gas]] [[Phase (matter)|phase]]. 
It has a [[bond order]] of 1, an internuclear separation of 267.3&nbsp;[[picometre|pm]] and a [[bond energy]] of 102&nbsp;kJ/mol or 1.06&nbsp;eV in each bond.<ref>''Chemical Bonding'', Mark J. Winter, Oxford University Press, '''1994''', {{ISBN|0-19-855694-2}}</ref> 
The [[electron configuration]] of Li<sub>2</sub> may be written as σ<sup>2</sup>.{{fact|date=September 2024}}

Being the third-lightest stable{{fact|date=September 2024}} neutral [[Homonuclear molecule|homonuclear]] [[diatomic molecule]] (after [[dihydrogen]] and [[dihelium]]), dilithium is an extremely important model system for studying fundamentals of physics, chemistry, and [[Ab initio quantum chemistry methods|electronic structure theory]].

It is the most thoroughly characterized compound in terms of the accuracy and completeness of the empirical [[Morse/Long-range potential|potential energy curves]] of its electronic states. Analytic empirical potential energy curves have been constructed for the X-state,<ref name=LeRoy(A-X)>{{cite journal|last=Le Roy|first=Robert J.|author2=N. S. Dattani |author3=J. A. Coxon |author4=A. J. Ross |author5=Patrick Crozet |author6=C. Linton |title=Accurate analytic potentials for Li<sub>2</sub>(X) and Li<sub>2</sub>(A) from 2 to 90 Angstroms, and the radiative lifetime of Li(2p)|journal=Journal of Chemical Physics|date=25 November 2009|volume=131|issue=20|page=204309|doi=10.1063/1.3264688|pmid=19947682|bibcode=2009JChPh.131t4309L}}</ref> a-state,<ref name=Dattani(c-a)>{{cite journal|last=Dattani|first=N. S.|author2=R. J. Le Roy|title=A DPF data analysis yields accurate analytic potentials for Li<sub>2</sub>(a)and Li<sub>2</sub>(c) that incorporate 3-state mixing near the c-state asymptote|journal=Journal of Molecular Spectroscopy |date=8 May 2013|volume=268|issue=1–2|pages=199–210|doi=10.1016/j.jms.2011.03.030|bibcode=  2011JMoSp.268..199.|arxiv = 1101.1361 |s2cid=119266866 }}</ref> A-state,<ref name=Gunton(A-X)>W. Gunton, M. Semczuk, N. S. Dattani, K. W. Madison, ''High resolution photoassociation spectroscopy of the <sup>6</sup>Li<sub>2</sub> A-state'', https://arxiv.org/abs/1309.5870</ref> c-state,<ref name=Semczuk(c-a)>{{cite journal|first1=M.|last1= Semczuk|first2=X.|last2= Li|first3=W.|last3= Gunton|first4=M.|last4= Haw|first5=N. S.|last5= Dattani|first6=J.|last6= Witz|first7=A. K.|last7= Mills|first8=D. J.|last8= Jones|first9=K. W.|last9= Madison|title=High-resolution photoassociation spectroscopy of the <sup>6</sup>Li<sub>2</sub> c-state|journal=Phys. Rev. A|year=2013|volume=87|issue= 5|article-number=052505|doi=10.1103/PhysRevA.87.052505|arxiv = 1309.6662 |bibcode = 2013PhRvA..87e2505S |s2cid= 119263860}}</ref> B-state,<ref name=Huang(B)>{{cite journal|last=Huang|first=Yiye|author2=R. J. Le Roy|title=Potential energy Lambda double and Born-Oppenheimer breakdown functions for the B<sup>1</sup>Pi<sub>u</sub> "barrier" state of Li<sub>2</sub>|journal=Journal of Chemical Physics|date=8 October 2003|volume=119|issue=14|pages=7398–7416|doi=10.1063/1.1607313|bibcode=2003JChPh.119.7398H}}</ref> 2d-state,<ref name=Li(l,d)>{{cite journal|last=Li|first=Dan|author2=F. Xie |author3=L. Li |author4=A. Lazoudis |author5=A. M. Lyyra|author5-link=Marjatta Lyyra |title=New observation of the, 13Δg, and 23Πg states and molecular constants with all <sup>6</sup>Li<sub>2</sub>, <sup>7</sup>Li<sub>2</sub>, and <sup>6</sup>Li<sup>7</sup>Li data|journal=Journal of Molecular Spectroscopy|date=29 September 2007|volume=246|issue=2|pages=180–186|doi=10.1016/j.jms.2007.09.008|bibcode=2007JMoSp.246..180L}}</ref> l-state,<ref name=Li(l,d) /> E-state,<ref name=Jastrzebski(E)>{{cite journal|last=Jastrzebski|first=W|author2=A. Pashov |author3=P. Kowalczyk |title=The E-state of lithium dimer revised|journal=Journal of Chemical Physics|date=22 June 2001|volume=114|issue=24|pages=10725–10727|doi=10.1063/1.1374927|bibcode=2001JChPh.11410725J}}</ref> and the F-state.{{huh|date=September 2024}}<ref name=Pashov(F)>{{cite journal|last=Pashov|first=A|author2=W. Jastzebski |author3=P. Kowalczyk |title=The Li<sub>2</sub> F "shelf" state: Accurate potential energy curve based on the inverted perturbation approach|journal=Journal of Chemical Physics|date=22 October 2000|volume=113|issue=16|pages=6624–6628|doi=10.1063/1.1311297|bibcode=2000JChPh.113.6624P}}</ref><!-- [Removing names of people who did the work / wrote the cited papers, because I've added their names in too many other places. If someone thinks it's helpful to include the names, they can put them back.] all by [[Robert J. LeRoy|Robert J. Le Roy]]<ref name=LeRoy(A-X) /><ref name=Dattani(c-a) /><ref name=Huang(B) />  of [[University of Waterloo]] and Nike Dattani<ref name=LeRoy(A-X) /><ref name=Dattani(c-a) /><ref name=Gunton(A-X) /><ref name=Semczuk(c-a) />  of [[University of Oxford]] --> The most reliable of these potential energy curves are of the [[Morse/Long-range potential|Morse/Long-range]] variety (see entries in the table below).<ref name=LeRoy(A-X) /><ref name=Dattani(c-a) /><ref name=Huang(B) /><ref name=Gunton(A-X) /><ref name=Semczuk(c-a) />

Li<sub>2</sub> potentials are often used to extract atomic properties. For example, the C<sub>3</sub> value for atomic lithium extracted from the A-state potential of Li<sub>2</sub> by Le Roy et al. in <ref name=LeRoy(A-X) /> is more precise than any previously measured atomic oscillator strength.<ref name=Tang(C9,C11)>{{cite journal|last1=Tang|first1=Li-Yan|last2=Yan|first2=Zong-Chao|last3=Shi|first3=Ting-Yun|last4=Mitroy|first4=J.|s2cid=122544942|title=Third-order perturbation theory for van der Waals interaction coefficients|journal=Physical Review A|volume=84|issue=5|article-number=052502|year=2011|issn=1050-2947|doi=10.1103/PhysRevA.84.052502|bibcode = 2011PhRvA..84e2502T |url=http://pdfs.semanticscholar.org/ef51/860072b13b5ffdfe602fe5c8ea4d7d0928e6.pdf|archive-url=https://web.archive.org/web/20200625185843/http://pdfs.semanticscholar.org/ef51/860072b13b5ffdfe602fe5c8ea4d7d0928e6.pdf|archive-date=2020-06-25}}</ref> 
This lithium oscillator strength is related to the radiative lifetime of atomic lithium and is used as a benchmark for atomic clocks and measurements of fundamental constants.

{| class="wikitable sortable" style="text-align: center;"
|-
! [[Electronic state]] !! Spectroscopic symbol{{huh|date=September 2024}} !! [[Molecular term symbol|Term symbol]] !!colspan=2 | [[Bond length]] (pm) !!colspan=2 | Dissociation energy (cm<sup>−1</sup>) !! Bound vibrational levels !! References
|-
| 1 (Ground) || X || 1<sup>1</sup>Σ<sub>g</sub><sup>+</sup> || {{decimal cell|267.298 74(19)}}<ref name=LeRoy(A-X) />  || {{decimal cell|8 516.780 0(23)}}<ref name=LeRoy(A-X) />  || 39<ref name=LeRoy(A-X) />  || <ref name=LeRoy(A-X) /> 
|-
| 2 || a || 1<sup>3</sup>Σ<sub>u</sub><sup>+</sup> || {{decimal cell|417.000 6(32)}}<ref name=Dattani(c-a) />  || {{decimal cell|333.779 5(62)}}<ref name=Dattani(c-a) />  || 11<ref name=Dattani(c-a) /> ||<ref name=Dattani(c-a) /> 
|-
| 3 || b || 1<sup>3</sup>Π<sub>u</sub> ||colspan=2 |  ||colspan=2 |  || || <ref name=Li(l,d) /> 
|-
| 4 || A || 1<sup>1</sup>Σ<sub>g</sub><sup>+</sup> || {{decimal cell|310.792 88(36)}}<ref name=LeRoy(A-X) />  || {{decimal cell|9 353.179 5 (28)}}<ref name=LeRoy(A-X) />  || 118<ref name=LeRoy(A-X) />  || <ref name=LeRoy(A-X) /> 
|-
| 5 || c || 1<sup>3</sup>Σ<sub>g</sub><sup>+</sup> || {{decimal cell|306.543 6(16)}}<ref name=Dattani(c-a)/> || {{decimal cell|7 093.492 6(86)}}<ref name=Dattani(c-a)/> || 104<ref name=Dattani(c-a)/> ||
|-
| 6 || B || 1<sup>1</sup>Π<sub>u</sub> || {{decimal cell|293.617 142(310)}}<ref name=Huang(B) />  || {{decimal cell|2 984.444}}<ref name=Huang(B) />  || 118<ref name=Huang(B) />  ||
|-
| 7 || E || 3(?)<sup>1</sup>Σ<sub>g</sub><sup>+</sup> ||colspan=2 | ||colspan=2 | || || <ref name=Jastrzebski(E) /> 
|}

== See also ==

* [[Morse/Long-range potential]]
* {{section link|Molecular orbital diagram|Dilithium}}
* [[Dilithium (Star Trek)|Dilithium (''Star Trek'')]]

== References ==
{{reflist}}

== Further reading ==

* {{Greenwood&Earnshaw}}

{{Lithium compounds}}
{{Diatomic elements}}

[[Category:Lithium]]
[[Category:Homonuclear diatomic molecules]]
[[Category:Allotropes]]


[[it:Litio#Dilitio]]