Ortho- Para- Hydrogen Names and Benzene

Question

I read that the Hydrogen atoms with nuclear spin in same direction are called Ortho- Hydrogen whereas those in opposite nuclear spin are Para- Hydrogen. But is there any resemblance of such a nomenclature with Ortho- Meta- Para- of Benzene, cause for benzene, these denote the relative position of substituent. Or is it just named so?

Answer 1

There is no connection of ortho- para- organic nomenclature with the nuclear spin isomerism of hydrogen. However, that raises an interesting point, that which nuclear state should be called the ortho- vs. para- or what was the thought process among the early German discoverers?

[Subscription only access: Farkas, L. (1933). Über Para- und Orthowasserstoff. In: Ergebnisse der Exakten Naturwissenschaften, vol 12. Springer, Berlin, Heidelberg. https://doi.org/10.1007/BFb0111841]

"Die antiparallele Kernspinorientierung entspricht einem, "Kernspinsingulett" Zustand mit dem statistischen Gewicht I, die parallele Kernspinorientierung dem , Kerntriplett"-Zustand mit dem statistischen Gewicht 3."

Then a footnote reads on later pages:

"...Im allgemeinen werden immer die Zustände mit dem größeren statistischen Gewicht als Orthozustände bezeichnet."

which means that the antiparallel nuclear spin orientation corresponds to a nuclear singlet state with a statistical weight of 1, the parallel nuclear spin orientation corresponds to the nuclear triplet state with a statistical weight of 3. In general, the states with larger statistical weight are designated as ortho- states in nuclear spin isomers.

Perhaps this ortho- nomenclature is due to the other meaning of ortho which means, ‘right, correct, proper’, so the majority (=higher statistical weight) of the hydrogen's nuclear isomer is in the correct form/proper form, hence this is ortho- nuclear spin isomer of hydrogen.