Some Routes in Forming C3Hn+ Ions and Deuterated Variants under Interstellar Conditions

Laboratory experiments on hydrogenation and deuteration of C3+, C3H+ and C3H2+ in collisions with H2 and HD have been performed from room temperature down to 15 K using a 22-pole ion-trap. At room temperature C3+ reacts slowly with H2 but the reactivity increases with decreasing temperature. It has been shown that the association reaction C3+ + H2 → C3H2+ + hν can compete with the exothermic reaction C3+ + H2 → C3H+ + H. In collisions of C3+ with HD, formation of C3D+ is slightly favored over C3H+ formation. A pronounced competition between various channels has been detected for deuterated variants of the C3H+ + H2 system. Most surprising is that formation of C3HD+ is over one hundred times faster then formation of C3H2+ in collisions of C3H+ and HD. An tentative explanation is that the H-HD exchange takes place via an open-chain H2CCCH+ intermediate. Reactions of C3H2+ and C3H3+ with H2 are very slow. The formation of C3H2D+ or C3HD2+ and finally C3HD via dissociative recombination has been discussed. The reaction C3H3+ + HD → C3H2D+ + H2 can be ignored in astrochemical models since the reaction rate at 15 K is very small; however, quite efficient routes have been found starting from C3+ and proceeding via deuterated C3H+ to C3H2+ and C3H3+. The new reaction rate coefficients are recommended to be included in astrophysical databases. Nonetheless it is still unclear how to explain the large abundance of C3H2 and larger hydrocarbons and their deuterated variants observed in cold interstellar clouds