Ion-molecule reaction kinetics of a series of 5-membered and 6-membered cyclic molecules
Mathews, Lillian Dalila
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The importance of cyclic species in both the interstellar medium (ISM) and Titan’s atmosphere has been realized since benzene was positively identified in both environments. In the ISM, benzene is likely involved in the formation of multi-ring species such as polycyclic aromatic hydrocarbons (PAHs). These PAHs are considered to be responsible for the unidentified infrared bands (UIR) detected in the ISM. In addition, the haze that obscures the surface of Titan is thought to be composed partially of PAHs. PAHs are thought to be formed from the very abundant benzene present in the atmosphere. Nitrogen is also abundantly available in the atmosphere of Titan and is sure to be incorporated into the rings as they develop. Indeed, a mass corresponding to that of pyridine (C5H5N), a nitrogen analogue of benzene, has been detected by the Ion Neutral Mass Spectrometer aboard the Cassini Orbiter as it passed through Titan’s atmosphere. Because of the importance of cyclic species in this atmosphere and that of the ISM, and because of the lack of kinetic data available, a selected ion flow tube (SIFT) has been used to study the ion-neutral reactions of a series of ions with several five and six member cyclic molecules that are of interest to both environments. Rate coefficients and product distributions were determined for each reaction at 298 K. The ions N2+, N+, O+, O2+, NH3+, NH4+, H3O+,Ne+, Ar+, and Kr+ were reacted with piperidine, pyridine, and pyrimidine. The stability of each cyclic molecule was investigated as a function of the recombination energy of the various ions. C4H4+ was reacted with hydrogen cyanide to determine if this ion-neutral reaction was a plausible formation route for pyridine in the ionosphere of Titan. CH3+ and C3H3+ were reacted with cyclohexane, piperidine, pyrrolidine, 1,4-dioxane, benzene, toluene, pyridine, pyrimidine, pyrrole, and furan. For both CH3+ and C3H3+ association product channels were present. This provides a mechanism for increasing the complexity of cyclic species and possibly even conversion from a single ring to a multi-ring molecule. The relevance of these data to Titan and the ISM is discussed.