AmericanChemicalSociety.com

Hydrocarbon ions and neutrals are known to be important in combustion, supersonic ram jets, in molecular synthesis in interstellar gas clouds, and in the chemistry of planetary atmospheres (such as that of Titan, a planetary satellite of Saturn).  Single ring hydrocarbons have been detected in some of these media and they are particularly interesting chemically because of the possibility of aromaticity and anti-aromaticity with and without heteroatoms in the ring (pyridine, C₅H₅N; pyrimidine, C₄H₄N₂; piperidine, C₅H₁₁N; 1-4 dioxane, C₄H₈O₂; benzene, C₆H₆; cyclohexane, C₆H₁₂ ; toluene, C₇H₈, all six-membered rings have been studied), and the five-membered rings, pyrrole, C₄H₅N; pyrrolidine, C₄H₉N; furan, C₄H₄O and tetrahydrafuran, C₄H₈O.  Important in hydrocarbon plasmas are the stable ion species CH₃⁺ and C₃H₃⁺ and the reactivity of these with the rings listed above have been investigated in our Selected Ion Flow Tube.

Figure 1.  University of Georgia Selected Ion Flow Tube (SIFT).

Almost none of these reactions had been studied previously.  The reactions with CH₃⁺ have shown reactivity by charge transfer (dissociative and non-dissociative), proton transfer, H-atom abstraction and surprisingly association.  Association was not expected to be competitive with the other processes and indeed does not compete when the ring is anti-aromatic with no p electrons in the ring.  However, with p electrons in the ring, association is a very significant channel.  From theoretical studies, the mechanism is believed to proceed rapidly initially by attraction of CH₃⁺ to the p electrons and then with an isomerization in which the CH₃⁺ attaches to a carbon in the ring, displacing the H-atom to the para position.  Following this, it was expected that C₃H₃⁺ might behave similarly, however this is not the case.  C₃H₃⁺ also has an additional complication that two isomers were detected, the cyclic cyclopropenyl and the acyclic propargyl cations. These ions were produced in a ~75% to ~25% cyclic to acyclic ratio by electron impact on propyne, C₃H₄, in a low pressure ion source.    Examples were seen where the two isomers reacted at the same rate and where the cyclic form reacts much more slowly. 

Figure 2.  The figure above shows the reaction of C₃H₃⁺ with two different neutrals, 1,4-dioxane and pyridine.  In the reaction with 1,4-dioxane the acyclic isomer reacts at a much faster rate than the cyclic isomer whereas with pyridine both isomers react at the same rate. 

Association is still an important product channel but was seen both with aromatic and anti-aromatic rings.  For the hydrocarbon six membered rings, (C₆H₆, C₆H₁₂, C₇H₈), the cyclic form of the ion reacts slowly.  For the acyclic form of the ion, the reactions are gas kinetic and association is an important channel for the aromatic rings.  No association was observed for the anti-aromatic C₆H₁₂.  For reactions of both cyclic and acyclic ions with nitrogen containing heterocycles, both the aromatic and anti-aromatics (C₅H₅N, C₄H₄N₂, C₅H₁₁N) react rapidly and in all cases association is a very important channel (76 to 89%) even for the anti-aromatic C₅H₁₁N.  For the anti-aromatic with oxygen in the ring (C₄H₄O₂), the acyclic C₃H₃⁺ reacts rapidly, and the cyclic slowly, but association is again seen.  For the five membered rings (C₄H₅N, C₄H₉N, C₄H₄O, C₄H₄O) the rates of the reactions are variable as are the amounts of association with no particular trends through the data. Reactions of other hydrocarbon ions, C₃H⁺, C₃H₂⁺ and C₃H₄⁺, were also studied.  For these ions, the anti-aromatic reactions are generally slow as is that of toluene in the aromatics.  Reactions with the five membered rings show no particular trends except that C₃H₄⁺ reactions seem to react somewhat slower than the other ions.

This award has enabled us to study the mechanisms of ion-molecules reactions of compounds, which are important in hydrocarbon plasmas of petroleum species.  The data obtained have permitted us to successfully write proposals relevant to applications in the interstellar medium and the Titan planetary satellite atmosphere.

The students funded from this award have learned a great deal about the kinetics of ionic reactions important in a variety of plasma environments.  They have learned how to operate complex apparatuses (Selected Ion Flow Tubes and Flowing Afterglows) for obtaining these kinetics and for understand reaction mechanisms.  One student has graduated with a Ph.D and one other has almost completed her PhD.  More than 57 reactions have been studied.  The students have written two papers and three more are in preparation.