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What is the mechanism by which spin isomers of hydrogen switch between the ortho and para forms?

If such a mechanism exists, why does ortho-hydrogen not increase in concentration on increasing temperature?

Edit: The question focuses on the how of the transition - in effect, asking about the mechanism, and if there isn't one, why, and not on why the transitions take place.

Edit(II) : Please keep in mind that I'm a high schooler. Please expain in a way that would be understandible, at least with effort, not something that requires me to know, say, grad-level chemistry.

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The first question is:

What is the mechanism by which spin isomers of hydrogen switch between the ortho and para forms?

There is explanation in Novel Insights into the Mechanism of the Ortho/Para Spin Conversion of Hydrogen Pairs: Implications for Catalysis and Interstellar Water:

One can define three situations. In the first, a magnetic conversion occurs without bond elongation or breaking. For example, in solid dihydrogen dipolar magnetic nuclear interactions are responsible for an extremely slow spin conversion.[4] In the presence of unpaired electrons the conversion is accelerated by the magnetic hyperfine interaction.[5] The second situation also involves a magnetic conversion mechanism, but is assisted by an intermediate HH bond elongation, for example, by binding to a transition metal center. Finally, in the third situation, a HH bond splitting and re-formation with other hydrogen atoms occurs, which corresponds to a chemical spin conversion

The second question is:

If such a mechanism exists, why does ortho-hydrogen not increase in concentration on increasing temperature?

The equilibrium fraction of ortho-hydrogen does increase from 0% at absolute zero to 75% at high temperature.

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    $\begingroup$ @ShivayVadhera Parahydrogen is the lowest energy, most stable form. There are 3 microstates corresponding to ortho-hydrogen (up-up, down-down, and up-down plus down-up) and only one microstate for parahydrogen (up-down minus down-up). So, according to Boltzmann distribution, the equilibrium fraction of orthohydrogen goes from 0% at 0K to 75% at infinite temperature. en.wikipedia.org/wiki/Boltzmann_distribution $\endgroup$ – DavePhD Aug 24 '17 at 20:29

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