Ionic Reactions between Formaldehyde and Hydrocarbons. H2 Transfer as a Hydrogenation Mechanism in Astrochemistry

Hydrogen transfer is a major channel in the ionized C2H2 +/H2CO and H2CO+/C2H2 reaction systems, producing C2H4 + + CO. Similarly, H2 transfer from H2CO is observed in reactions of the ions C3H3 + (linear), CH3CCH+, H2CCCH2 +, C3H5 +, and HCCCCH+ with formaldehyde, resulting in hydrogenated hydrocarbon ions. The energetics of these reactions allow a two-step mechanism of H+ or H transfer forming a distonic (BH+·HCO) complex, followed by H transfer to form BH2 +. Ab initio calculations on the acetylene/formaldehyde system show that the reaction can proceed through a stable cyclic adduct, dissociating to yield C2H4 + that contains carbon atoms from both acetylene and formaldehyde. Both experiment and theory show that H2CO+ does not react with hydrogen, making it a stable reactant ion in interstellar chemistry. The reactions where either H2 + or H2 is transferred can provide significant pathways for the hydrogenation of unsaturated hydrocarbons in interstellar clouds. The reactions of three C2H x + (x = 0, 1, or 2) species with acetaldehyde are also reported.