# Is Nuclear Spin Isomer a kind of allotrope?

In my book there's a question:

Does $\ce{H2}$ shows allotropy ? Enlist its allotropes and their application?

I pondered over different way Hydrogen can be arranged(I don't know but I have weak intuition that hydrogen bonding can play a role) but to no resolute

After awhile I googled to find different answers :Some are enlisting NSIs of hydrogen as allotrope

So can NSI(s) of an element can be considered allotrope ?

If yes then how Nuclear Spin Isomer can affect their physical form/structures?

*Wikipedia also doesn't enlist any allotrope of $\ce{H2}$ as such. https://en.wikipedia.org/wiki/Allotropy

## 1 Answer

Some peer review journal articles and an India high school textbook and various study guides do refer to the spin isomers as allotropes.

There are physical differences such as different heat capacities. See Orthohydrogen, Parahydrogen and Heavy Hydrogen for comprehensive information.

However, aside from the spin isomers, there are genuine allotropes of hydrogen. Solid hydrogen has two normal pressure allotropes, a face centered cubic and a hexagonal closest packed allotrope.

Also, at high pressure, there is metallic hydrogen.

• So is this a general consensus among chemistry community to consider NSI to be allotrope? Are physical difference other than structural composition sufficient to define an element as "Allotrope"(The definition of allotrope i've heard is based only on structure and rest of the physical properties as a consequences of it)? As a follow-up question:Is there any example of element in which NSIs have different physical structures/effect on structure? – Xasel Jan 26 '17 at 15:40
• @Xasel The IUPAC definition of "allotropes" is "Different structural modifications of an element" goldbook.iupac.org/A00243.html so I would say, no, it is not a consensus. – DavePhD Jan 26 '17 at 15:45
• @Xasel one article says "In H2, the crystal structure as a function of temperature and pressure depends upon the ortho-para (o-p) concentration. At low pressure, single molecules are in almost free-rotor states. Parahydrogen molecules are in the spherically symmetric state with rotational quantum number J=0 and cannot orientationally order, whereas o-H2 molecules are in the J=1 states and the molecules can order along crystallographic directions. Orthohydrogen has an hcp (hexagonal-closepacked) to fcc (face-centered-cubic) transition at zero pressure and 2.8 K..." – DavePhD Jan 26 '17 at 15:53
• – DavePhD Jan 26 '17 at 15:53
• Thankyou sir for taking out your precious time to help me out and giving hunting an d paraphrasing the relevant stuff whic proved quite helpful to me because Unfortunately ,I don't have means to access those JOurnals(I wonder why public-funded research is not available to us but that's question for other day).Have a nice day:) – Xasel Jan 26 '17 at 16:00