2 research outputs found
Deprotonated Purine Dissociation: Experiments, Computations, and Astrobiological Implications
A central focus of astrobiology is
the determination of abiotic
formation routes to important biomolecules. The dissociation mechanisms
of these molecules lend valuable insights into their synthesis pathways.
Because of the detection of organic anions in the interstellar medium
(ISM), it is imperative to study their role in these syntheses. This
work aims to experimentally and computationally examine deprotonated
adenine and guanine dissociation in an effort to illuminate potential
anionic precursors to purine formation. Collision-induced dissociation
(CID) products and their branching fractions are experimentally measured
using an ion trap mass spectrometer. Deprotonated guanine dissociates
primarily by deammoniation (97%) with minor losses of carbodiimide
(HNCNH) and/or cyanamide (NH<sub>2</sub>CN), and isocyanic acid (HNCO).
Deprotonated adenine fragments by loss of hydrogen cyanide and/or
isocyanide (HCN/HNC; 90%) and carbodiimide (HNCNH) and/or cyanamide
(NH<sub>2</sub>CN; 10%). Tandem mass spectrometry (MS<sup><i>n</i></sup>) experiments reveal that deprotonated guanine fragments
lose additional HCN and CO, while deprotonated adenine fragments successively
lose HNC and HCN. Every neutral fragment observed in this study has
been detected in the ISM, highlighting the potential for nucleobases
such as these to form in such environments. Lastly, the acidity of
abundant fragment ions is experimentally bracketed. Theoretical calculations
at the B3LYP/6-311++GÂ(d,p) level of theory are performed to delineate
the mechanisms of dissociation and analyze the energies of reactants,
intermediates, transition states, and products of these CID processes
Reactions of Azine Anions with Nitrogen and Oxygen Atoms: Implications for Titan’s Upper Atmosphere and Interstellar Chemistry
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