Reactions
of Azine Anions with Nitrogen and Oxygen
Atoms: Implications for Titan’s Upper Atmosphere and Interstellar
Chemistry
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Abstract
Azines are important in many extraterrestrial
environments, from
the atmosphere of Titan to the interstellar medium. They have been
implicated as possible carriers of the diffuse interstellar bands
in astronomy, indicating their persistence in interstellar space.
Most importantly, they constitute the basic building blocks of DNA
and RNA, so their chemical reactivity in these environments has significant
astrobiological implications. In addition, N and O atoms are widely
observed in the ISM and in the ionospheres of planets and moons. However,
the chemical reactions of molecular anions with abundant interstellar
and atmospheric atomic species are largely unexplored. In this paper,
gas-phase reactions of deprotonated anions of benzene, pyridine, pyridazine,
pyrimidine, pyrazine, and s-triazine with N and O atoms are studied
both experimentally and computationally. In all cases, the major reaction
channel is associative electron detachment; these reactions are particularly
important since they control the balance between negative ions and
free electron densities. The reactions of the azine anions with N
atoms exhibit larger rate constants than reactions of corresponding
chain anions. The reactions of azine anions with O atoms are even
more rapid, with complex product patterns for different reactants.
The mechanisms are studied theoretically by employing density functional
theory; spin conversion is found to be important in determining some
product distributions. The rich gas-phase chemistry observed in this
work provides a better understanding of ion-atom reactions and their
contributions to ionospheric chemistry as well as the chemical processing
that occurs in the boundary layers between diffuse and dense interstellar
clouds