Trihydrogen Cation - Ortho/Para-H₃+

Ortho/Para-H₃+

The most abundant molecule in dense interstellar clouds is H₂. When a H₃+ molecule collides with H₂, stoichometrically there is no net yield. However, a proton transfer still can take place, which can potentially change the total nuclear spin of the two molecules depending on the nuclear spins of the protons. Two different spin configurations for H₃+ are possible, called ortho and para. Ortho-H₃+ has all three proton spins parallel, yielding a total nuclear spin of 3/2. Para-H₃+ has two proton spins parallel while the other is anti-parallel, yielding a total nuclear spin of 1/2. Similarly, H₂ also has ortho and para states, with ortho-H₂ having a total nuclear spin 1 and para-H₂ having a total nuclear spin of 0. When an ortho-H₃+ and a para-H₂ collide, the transferred proton changes the total spins of the molecules, yielding instead a para-H₃+ and an ortho-H₂.