Forming NCO(-) in Dense Molecular Clouds: Possible Gas-Phase Chemical Paths From Quantum Calculations.

E Yurtsever,F A Gianturco,R Wester

doi:10.1021/acs.jpca.5b10472

Abstract

The existence of NCO– anions in the interstellar medium (ISM) has been suggested and searched for over the years but without any formal definitive sighting of that molecule. We discuss in this work the possible formation of either NCO– directly or of NCO neutral as a precursor to NCO– formation by electron attachment. We follow simple, gas-phase chemical reactions for which the general features are obtained from accurate quantum calculations. The results are shedding some additional light on the likely presence of this anion in the ISM environment, drawing further information from the specific features of the considered reactions on the additional chemical options that exist for its formation.

Highlights

The presence and role of molecules carrying a charge has been recognized for a long time as playing a significant role in the gas-phase chemistry of the interstellar space.[1,2] The existing chemicals, and especially H and H2, are ionized in the diffuse regions of the interstellar medium (ISM) by cosmic rays and can produce both ionic species and free electrons, which can in turn recombine with the neutrals present to further form anionic molecules
Gas-grain chemical model calculations, representing a wide range of conditions, have been quoted[12] as suggesting that the neutral radical NCO might be quite abundant in warm sources and in dark molecular clouds (DMC) environments, presenting another chemical path for a two-step formation of its anion: the radical reaction with oxygen, as given by eq 4, as a first step provided one further considers radiative electron attachment (REA) processes as a second step via the reaction
We have computationally analyzed in detail the energy landscapes for the radical reaction 4, as well as those for the ion−molecule reaction 5 that is directly producing the anionic molecule

Summary

INTRODUCTION

The presence and role of molecules carrying a charge has been recognized for a long time as playing a significant role in the gas-phase chemistry of the interstellar space.[1,2] The existing chemicals, and especially H and H2, are ionized in the diffuse regions of the interstellar medium (ISM) by cosmic rays and can produce both ionic species and free electrons, which can in turn recombine with the neutrals present to further form anionic molecules. Given the existence of many simple molecules with large, positive electron affinities (EA), it is possible that negatively charged molecular species can exist there and would survive due to the reduced influence of photodetachment channels It was pointed out a while ago[5] that those negative ions if formed would rapidly undergo electron loss by photodetachment processes induced by the cosmic radiation while the inner cores of the dark molecular clouds would be partially shielded from the outer photon sources and “protect” the anions from photodetachment. The present study intends to present a computational analysis of ion−molecule reactions that could lead to the formation of yet another molecular anion, the NCO−, a stable species exhibiting a closed-shell 1Σ+ ground electronic state, a large positive electron-affinity of 3.609 eV10 and a permanent dipole moment of 1.5 D,11 which is not far from what is called a “critical” value of ∼1.67 D.9 The meaning of this comment will become clearer in the discussions of Section 3. We shall demonstrate in the following that both reactions are strongly exothermic processes so that the possibility might exist for them to take part into the formation of NCO−, either directly via eq 2 or indirectly via eq 3, followed by the REA process of eq 1

THE AB INITIO STRUCTURAL ENERGIES

PRESENT CONCLUSIONS

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