CH₂CHOH₂⁺ + PN: A proton-transfer triple play

Quantum chemical calculations are used to explore the proton-transfer reactivity of O-protonated vinyl alcohol, CH₂CHOH₂ ⁺, with phosphorus nitride, PN. This reaction is relevant to the chemical evolution of interstellar clouds, since O-protonated vinyl alcohol has been postulated (and tentatively identified) as a product of the association reaction between interstellar H₃O⁺ and C₂H ₂, while PN is the most widespread and abundant phosphorus-containing molecule seen in astrophysical environments. Furthermore, the reaction exhibits an unusual mechanistic feature, namely, an extended "proton-transport catalysis" mechanism, which we characterize here as a "proton-transfer triple play". The reaction proceeds initially by proton transfer from CH₂CHOH₂⁺ to PN, then from PMT to CH ₂CHOH, and finally from CH₃CHOH⁺ to PN, where the emphasized atom indicates the resultant site of protonation/deprotonation. Thus, the ultimate overall bimolecular proton-transfer reaction is expected to occur as CH₂CHOH₂⁺ + PN → CH ₃CHO + PNH⁺; that is, the apparent favored product channel exhibits not only proton transfer but also keto/enol tautomerization. The triple-play mechanism can be rationalized in terms of the proton affinities of vinyl alcohol, acetaldehyde, and phosphorus nitride, which here are satisfactorily reproduced by high-level ab initio calculations. Other neutrals with a proton affinity appropriate for the possible triple-play mechanism converting CH₂CHOH₂⁺ to CH₃CHO are also identified, with a view to encouraging experimental investigation of this mechanism.