Hydrogenation of HOCO and formation of interstellar CO2: a not so straightforward relation

ABSTRACT Carbon dioxide (CO$_2$) is one of the most important interstellar molecules. While it is considered that it forms on the surface of interstellar dust grains, the exact contribution of different chemical mechanisms is still poorly constrained. Traditionally it is deemed that the CO + OH reaction occurring on top of ices is the main reaction path for its formation. Recent investigations showed that in reality the reaction presents a more complex mechanism, requiring an additional H-abstraction step. Building on our previous works, we carried out a detailed investigation of such H abstraction reactions with the hydrogen atom as a reactant for the abstraction reaction. We found an unconventional chemistry for this reaction, markedly depending on the isomeric form of the HOCO radical prior to reaction. The favoured reactions are t-HOCO + H $\longrightarrow$CO + H$_2$O, c-HOCO + H $\longrightarrow$CO$_2$ + H$_2$, and t/c-HOCO + H $\longrightarrow$c/t-HCOOH. We estimate bounds for the rate constants of the less favoured reaction channels, t-HOCO + H $\longrightarrow$CO$_2$ + H and c-HOCO + H $\longrightarrow$CO + H$_2$O, to be approximately 10$^{4-6}$ s$^{-1}$. However, these estimates should be interpreted cautiously due to the significant role of quantum tunnelling in these reactions and the complex electronic structure of the involved molecules, which complicates their study. Our findings underscore the need for detailed investigation into the chemistry of interstellar CO$_2$ and pave the way for a re-evaluation of its primary formation mechanisms in the interstellar medium.