Chemistry during star formation

In a broad way, stars form in large clouds which inhabit the ISM, and which are cold enough (10~30 K) to present most of its gas in molecular form. In diffuse clouds, where star formation is not happening yet, the hydrogen chemistry can evolve in the gas to form other molecules and ions. Once H₂ is formed, it can collide with a CR to produce H₂⁺, similar to Eq. 15:

H₂ + CR → H₂⁺ + e⁻ + CR

H₂⁺ then will mainly react with other H2 molecules to give H₃⁺ :

H₂⁺ + H₂ → H₃⁺ + H

H₃⁺ is very important species, because it can easily donate a proton to other species to form more complex molecules, for example:

H₃⁺ + CO → H₂ + HCO⁺

This ion is also important because it will set the deuteration degree ofother species. It reacts with HD to give H₂D⁺, following:

H₃⁺ + HD ⇌ H₂D⁺ + H₂

At the temperatures of molecular clouds, only the forward channel is available, which means that the initial  deuteration fraction of any species (the relative concentration of a deuterated species versus the non-deuterated one) is set during the stages previous to the start of the star formation process, and will only decrease as time evolves, because deuterated molecules get destroyed by reactions with neutral species and radiation. H₂D⁺ will later react to form other deuterated species, such as:

H₂D⁺ + H₂CO → H₂DCO⁺ + H₂

H₂DCO⁺ + e⁻ → HDCO + H

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