Ortho- and parahydrogen are two forms of the $\ce{H2}$ molecule that are distinguished by their pairing or antipairing of nuclear spins, giving rise to metastable singlet (ortho-) and triplet (para-) states. This constitutes an example of nuclear spin isomerism.

Whilst the thermal properties of these two species are different, my understanding is that their chemical properties are essentially identical. Are there any examples of this kind of isomerism having nontrivial implications for chemical reactivity?

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    $\begingroup$ This grant proposal gives some information about chemistry of nuclear spin isomers… from which it seems the answer could be yes (molecules being in different rovibrational states allowing or forbidding certain reactions depending on the isomer) $\endgroup$
    – F'x
    May 17, 2012 at 15:39
  • $\begingroup$ Interesting question... I was always fascinated by the fact that there are more than one H2s. Regarding the chemical nature, I don't think so--I've never seen magnetic properties affecting chemical ones. But I've not seen much. $\endgroup$ May 17, 2012 at 16:00
  • $\begingroup$ @Manishearth - magnetic properties (of electron configurations) affect chemical properties all the time. However nuclear magnetic properties are a whole other topic... $\endgroup$ May 19, 2012 at 8:26
  • $\begingroup$ @rich any examples? (I probably already know examples of this, just can't remember any. Stupid brain.) $\endgroup$ May 19, 2012 at 15:55

1 Answer 1



Papers in the field of preparation or selection of nuclear spin isomers have introductions that justify the work by potential applications and some of them include chemical reactions, either under that name or under the more descriptive terminology of “reactive collisions”. While no specific cases are cited as examples, the potential exist but only in limited situations such as bimolecular reaction in molecular beams.

From what I have seen, the canonical reference is “Detailed symmetry selection rules for reactive collisions”, Mol. Phys. 1977, 34, 477–504. This is the theoretical paper laying out the foundations of the effect of nuclear spin isomerism on reactions. I don't have access to the full text, but here is the abstract:

                   Paper abstract

It confirms what I suggested in my earlier comment: the different nuclear spin isomers have different rovibrational states, and accounting for effect changes the selection rules indicating which collisions are reactive or not.

One specific example which I found is “Observation of Ortho-Para H3+ Selection Rules in Plasma Chemistry”, Phys. Rev. Lett. 1997, 78, 3844–3847:

Large dependences of the intensity ratios on the parent gases have been observed, indicating the spin memory of protons even after chemical reactions. The results clearly demonstrate the existence of selection rules on nuclear spin modifications in chemical reactions theoretically anticipated by M. Quack.

Their results are obtained on the formation of H3+ ions from a H2 plasma.


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