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Abstract
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 (C 5H5N), 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+, NH 3+, NH 4+, H 3O + ,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. C 4H 4+ was reacted with hydrogen cyanide to
determine if this ion‐neutral reaction was a plausible formation route for pyridine in the
ionosphere of Titan. CH 3+ and C 3H 3+ were reacted with cyclohexane, piperidine,
pyrrolidine, 1,4‐dioxane, benzene, toluene, pyridine, pyrimidine, pyrrole, and furan. For
both CH 3+ and C 3H 3+ 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.