Editing Hydrogen (section)

==== Spin isomers ====
{{Main|Spin isomers of hydrogen}}
Molecular {{chem2|H2}} exists as two [[nuclear isomer]]s that differ in the [[Spin (physics)|spin states]] of their nuclei.<ref name="uigi">{{cite web|author=Staff|date=2003|url=http://www.uigi.com/hydrogen.html|title=Hydrogen (H<sub>2</sub>) Properties, Uses, Applications: Hydrogen Gas and Liquid Hydrogen|publisher=Universal Industrial Gases, Inc.|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080219073329/http://www.uigi.com/hydrogen.html|archive-date=19 February 2008|url-status=live}}</ref> In the '''orthohydrogen''' form, the spins of the two nuclei are parallel, forming a spin [[triplet state]] having a [[Spin quantum number#Total spin of an atom or molecule|total molecular spin]] <math>S = 1</math>; in the '''parahydrogen''' form the spins are antiparallel and form a spin [[singlet state]] having spin <math>S = 0</math>. The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form.<ref name="Green2012">{{cite journal |last1=Green |first1=Richard A. |display-authors=etal |title=The theory and practice of hyperpolarization in magnetic resonance using ''para''hydrogen |journal=Prog. Nucl. Magn. Reson. Spectrosc. |date=2012 |volume=67 |pages=1–48 |doi=10.1016/j.pnmrs.2012.03.001 |pmid=23101588 |bibcode=2012PNMRS..67....1G |url=https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |access-date=28 August 2021 |archive-date=28 August 2021 |archive-url=https://web.archive.org/web/20210828222611/https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |url-status=live }}</ref> The ortho form is an [[excited state]], having higher energy than the para form by 1.455&nbsp;kJ/mol,<ref name="PlanckInstitut">{{cite web |url=https://www.mpibpc.mpg.de/146336/para-Wasserstoff |language=de |website=Max-Planck-Institut für Biophysikalische Chemie |title=Die Entdeckung des para-Wasserstoffs (The discovery of para-hydrogen) |access-date=9 November 2020 |archive-date=16 November 2020 |archive-url=https://web.archive.org/web/20201116064055/https://www.mpibpc.mpg.de/146336/para-Wasserstoff |url-status=live }}</ref> and it converts to the para form over the course of several minutes when cooled to low temperature.<ref>{{cite journal|last1=Milenko|first1=Yu. Ya.|last2=Sibileva|first2=R. M.|last3=Strzhemechny|first3=M. A.|title=Natural ortho-para conversion rate in liquid and gaseous hydrogen|journal=Journal of Low Temperature Physics|date=1997|volume=107|issue=1–2|pages=77–92
|doi=10.1007/BF02396837|bibcode = 1997JLTP..107...77M |s2cid=120832814}}</ref> The thermal properties of these isomers differ because each has distinct [[Rotational–vibrational spectroscopy|rotational quantum states]].<!-- This link is less direct than [[Rotational spectroscopy]] but presently the subject better (June 2021).--><ref name="NASA">{{cite web|last=Hritz|first=J.|date=March 2006|url=http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|title=CH. 6&nbsp;– Hydrogen|work=NASA Glenn Research Center Glenn Safety Manual, Document GRC-MQSA.001|publisher=NASA|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080216050326/http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|archive-date=16 February 2008}}</ref>

The ortho-to-para ratio in {{chem2|H2}} is an important consideration in the [[liquefaction]] and storage of [[liquid hydrogen]]: the conversion from ortho to para is [[exothermic]] and produces sufficient heat to evaporate most of the liquid if not converted first to parahydrogen during the cooling process.<ref name="Amos98">{{cite web|url=http://www.nrel.gov/docs/fy99osti/25106.pdf|title=Costs of Storing and Transporting Hydrogen|publisher=National Renewable Energy Laboratory|date=1 November 1998|first1=Wade A.|last1=Amos|pages=6–9|access-date=19 May 2015|archive-url=https://web.archive.org/web/20141226131234/http://www.nrel.gov/docs/fy99osti/25106.pdf|archive-date=26 December 2014|url-status=live}}</ref> [[Catalyst]]s for the ortho-para interconversion, such as [[ferric oxide]] and [[activated carbon]] compounds, are used during hydrogen cooling to avoid this loss of liquid.<ref name="Svadlenak">{{cite journal|last1=Svadlenak|first1=R. E.|last2=Scott|first2=A. B.|title=The Conversion of Ortho- to Parahydrogen on Iron Oxide-Zinc Oxide Catalysts|journal=Journal of the American Chemical Society|date=1957|volume=79|issue=20|pages=5385–5388|doi=10.1021/ja01577a013|bibcode=1957JAChS..79.5385S }}</ref>