Temperature variable ion trap studies of C3Hn+ with H2 and HD

Abstract

Hydrogenation and deuteration of C₃⁺, C₃H⁺, C₃H₂⁺ in collisions with H₂ and HD has been studied from room temperature down to 10 K using a 22-pole ion trap. Although exothermic, hydrogenation of C₃⁺ is rather slow at room temperature but becomes faster with decreasing temperature. In addition to the increasing lifetime of the collision complex this behavior may be caused by the floppy structure of C₃⁺ and the freezing of soft bending modes below 50 K. For C₃⁺ + HD it has been shown that production of C₃D⁺ is slightly favored over C₃H⁺ formation. The controversy over which products are really formed in C₃H⁺ + H₂ collisions and deuterated variants has a long history. Previous and new ion trap results prove that formation of C₃H₂⁺ + H is not endothermic but rather fast, in contradiction to erroneous conclusions from flow tube experiments and ab initio calculations. In addition the reaction shows a complicated isotope dependence, most probably caused by the influence of zero point energies in entrance and exit transition states. For example hydrogen abstraction with HD is faster than with H₂ while radiative association is slower. The most surprising result has been obtained for C₃H⁺ + HD. Here C₃HD⁺ formation is over one hundred times faster than C₃H₂⁺. In addition to the details of the potential energy surface it may be that in this case an H–HD exchange reaction takes place via an open-chain propargyl cation intermediate (H₂CCCH⁺). Reactions of C₃H₂⁺ and C₃H₃⁺ with H₂ are very slow but, due to the unique sensitivity of the trapping technique, significant rate coefficients have been determined. The presented results are of fundamental importance for understanding the energetics, structures and reaction dynamics of the deuterated variant of the C₃H_n⁺ collision system. They indicate that the previous quantum chemical calculations are not accurate enough for understanding the low energy behavior of the C_nH_m⁺ reaction systems. The laboratory experiments are of essential relevance for the carbon chemistry of dense interstellar clouds, both for formation of small hydrocarbons and deuterium fractionation.