Dilithium

{{short description|Diatomic molecule}}

{{about|the real substance|other uses|Dilithium (disambiguation)}}

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| ImageFile1 = Dilithium-2D-dimensions.svg

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| ImageName1 = Wireframe model of dilithium

| ImageFile2 = Dilithium-3D-vdW.png

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| IUPACName = Dilithium(Li—Li){{cn|date=January 2024}}

|Section1={{Chembox Identifiers

| CASNo = 14452-59-6

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| PubChem = 139759

| ChemSpiderID = 123254

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| SMILES = [Li][Li]

| StdInChI = 1S/2Li

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| StdInChIKey = SMBQBQBNOXIFSF-UHFFFAOYSA-N

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|Section2={{Chembox Properties

| Li=2

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Dilithium, Li₂, is a strongly electrophilic, diatomic molecule comprising two lithium atoms covalently bonded together. Li₂ has been observed in the gas phase.

It has a bond order of 1, an internuclear separation of 267.3 pm and a bond energy of 102 kJ/mol or 1.06 eV in each bond.Chemical Bonding, Mark J. Winter, Oxford University Press, 1994, {{ISBN|0-19-855694-2}}

The electron configuration of Li₂ may be written as σ².{{fact|date=September 2024}}

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

It is the most thoroughly characterized compound in terms of the accuracy and completeness of the empirical potential energy curves of its electronic states. Analytic empirical potential energy curves have been constructed for the X-state,{{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 Li₂(X) and Li₂(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}} a-state,{{cite journal|last=Dattani|first=N. S.|author2=R. J. Le Roy|title=A DPF data analysis yields accurate analytic potentials for Li₂(a)and Li₂(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 }} A-state,W. Gunton, M. Semczuk, N. S. Dattani, K. W. Madison, High resolution photoassociation spectroscopy of the ⁶Li₂ A-state, https://arxiv.org/abs/1309.5870 c-state,{{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 ⁶Li₂ c-state|journal=Phys. Rev. A|year=2013|volume=87|issue= 5|pages=052505|doi=10.1103/PhysRevA.87.052505|arxiv = 1309.6662 |bibcode = 2013PhRvA..87e2505S |s2cid= 119263860}} B-state,{{cite journal|last=Huang|first=Yiye|author2=R. J. Le Roy|title=Potential energy Lambda double and Born-Oppenheimer breakdown functions for the B¹Pi_u "barrier" state of Li₂|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}} 2d-state,{{cite journal|last=Li|first=Dan|author2=F. Xie |author3=L. Li |author4=A. Lazoudis |author5=A. M. Lyyra |title=New observation of the, 13Δg, and 23Πg states and molecular constants with all ⁶Li₂, ⁷Li₂, and ⁶Li⁷Li 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}} l-state, E-state,{{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}} and the F-state.{{huh|date=September 2024}}{{cite journal|last=Pashov|first=A|author2=W. Jastzebski |author3=P. Kowalczyk |title=The Li₂ 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}} The most reliable of these potential energy curves are of the Morse/Long-range variety (see entries in the table below).

Li₂ potentials are often used to extract atomic properties. For example, the C₃ value for atomic lithium extracted from the A-state potential of Li₂ by Le Roy et al. in is more precise than any previously measured atomic oscillator strength.{{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|pages=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|url-status=dead|archive-date=2020-06-25}}

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}}	Term symbol	colspan=2 | Bond length (pm)	colspan=2 | Dissociation energy (cm−1)	Bound vibrational levels	References
1 (Ground)	X	11Σg+	{{decimal cell|267.298 74(19)}}	{{decimal cell|8 516.780 0(23)}}	39
2	a	13Σu+	{{decimal cell|417.000 6(32)}}	{{decimal cell|333.779 5(62)}}	11
3	b	13Πu	colspan=2 |	colspan=2 |
4	A	11Σg+	{{decimal cell|310.792 88(36)}}	{{decimal cell|9 353.179 5 (28)}}	118
5	c	13Σg+	{{decimal cell|306.543 6(16)}}	{{decimal cell|7 093.492 6(86)}}	104
6	B	11Πu	{{decimal cell|293.617 142(310)}}	{{decimal cell|2 984.444}}	118
7	E	3(?)1Σg+	colspan=2 |	colspan=2 |