Back to Table of Contents

46311-AC6
Kinetics, Reaction Mechanisms, and Relative Stabilities of Heterocyclic Ring Molecules in Ionized Environments

Nigel G. Adams, University of Georgia

Hydrocarbon ions and neutrals are known to be important in the chemistry of interstellar gas clouds, planetary atmospheres (such as the planetary satellite Titan) and in combustion in supersonic ram jets.  Some of the species detected are ring hydrocarbons implying that other rings could be present.  Possibilities are both homo-and hetero-cyclic and aromatic and non-aromatic molecules such as pyridine, C₅H₅N; pyrimidine, C₄H₄N₂; piperidine, C₅H₁₁N; dioxane, C₄H₈O₂; benzene, C₆H₆; cyclohexane, C₆H₁₂ (all six-membered rings) and the five-membered rings, pyrrole, C₄H₅N; pyrrolidine, C₄H₉N; furan, C₄H₄O.  A central ion in the chemistry of these media is CH₃⁺ which can react with the rings in very different ways (proton transfer, charge transfer (both dissociative and non-dissociative)), hydride abstraction and association.  The mechanisms of these reactions are expected to be variable depending on the availability of pi electrons in the rings and of lone pairs on the N and O heteroatoms with the breaking of the aromaticity in some cases.  In studying these reactions, it was a surprise to see that association is competitive with hydride abstraction and also with proton transfer and charge transfer when these are exothermic, since they are considered to proceed on every collision.  Association, except in the case of furan, occurs when there are pi electrons in the ring and does not occur when there are not.  From theoretical molecular dynamic calculations in the literature for the reaction of CH₃⁺ with benzene, the mechanism is considered to initially proceed by attraction of the ion to the plane of the pi electrons.  This fast forming complex then rapidly isomerizes to a stable sigma complex where the CH₃⁺ attaches to a carbon in the ring displacing the H atom. That proton transfer is only minor whereas charge transfer is competitive with the other channels could be because, for proton transfer, a localized charge site is required whereas charge transfer could be long range and delocalized.

            The reactions were studied experimentally using a Selected Ion Flow Tube, see Figure 1.

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

Here CH₃⁺ ions are created by low pressure electron impact on methane with the CH₃⁺ being selected in a quadrupole mass filter and injected into a flow tube containing helium at 0.5 Torr.  The helium then carries the ions to a downstream quadrupole mass filter/ion counting system, by the action of a Roots blower pump.  In the intervening region, neutral reactant ring compounds are introduced into the flow.  Ionized reaction products are also detected by the downstream counting system and rate constants and product ion distributions are determined in the standard way.  All of the ring compounds used in this study are liquids and were introduced into the gas phase by diluting the vapor in helium at a pressure less than the saturated vapor pressure.

             For the six-membered rings, when N atoms are introduced into the aromatic ring (C₅H₅N and C₄H₄N₂) the aromaticity is retained and the association and hydride abstraction channels are large.  When the ring is not aromatic (C₅H₁₁N and C₆H₁₂), there is no association, but hydride abstraction remains large, presumably because of the availability of a large number of H atoms.  In the five-membered aromatic rings containing N heteroatoms, association and hydride abstraction are consistently small or non-existent even though the aromaticity is maintained.  As expected, for O substitution where there are no pi electrons, these channels remain at zero.  Note that when O and N atoms are introduced into the rings, lone pairs become available introducing additional sites for reaction and other reaction channels.

            In these studies, the isomeric form of the products is not known, e.g., is the product of the CH₃⁺ + C₆H₆ reaction protonated toluene or some other isomeric form.  To investigate this sort of question, a crude SID (surface induced dissociation) detector has been constructed in the expectation that different isomeric forms will dissociate in different ways.  If this is successful, an upgraded form with be constructed.  A heated neutral source has also been constructed with which lower vapor pressure species from liquids and solids can be introduced into the flow tube.  This is waiting for a suitable time to be tested.

In an earlier study, it was established in reactions of O⁺ with C₅H₅N that C₄H₄⁺ + HCN was the only channel (and dominant with Ar⁺, N₂⁺, N⁺ and Kr⁺).  Similarly for C₄H₄N₂ only C₃H₃N⁺ + HCN resulted.  By microscopic reversibility, charged C₅H₅N⁺ and C₄H₄N₂⁺ should be produced by the reverse associations if there are no activation barriers.  Preliminary studies of the former reaction have shown rapid association and efforts are underway to determine whether ring closure occurs in the formation of the product.  If this occurs, then this will be an interesting mechanism and could be the source of the ionized pyridine in the Titan ionosphere.   

Back to top