On the Structure, Magnetic Properties, and Infrared Spectra of Iron Pseudocarbynes in the Interstellar Medium
Carbon chains, especially polyynes, are the building blocks of complex molecules such as polycyclic aromatic hydrocarbons and fullerenes, and polyynes are observed in circumstellar and interstellar (CIS) environments. Yet these same CIS environments show only low levels of gaseous iron despite it being the fourth most abundant element in the solar abundance pattern. In this study we explore the structure, magnetic properties, and synthetic infrared (IR) spectra of iron bound to polyynes, yielding what we call iron pseudocarbynes. We find that polyynes of all lengths are characterized by an IR-active C-H stretching feature at λ ̃ 3 μm, and an IR-active CCH/CCC bending feature at λ ̃ 16 μm. The CCH bending feature exhibits a redshift in iron pseudocarbynes such as Fe12-C2H2, appearing at λ ̃ 15.8 μm with an IR intensity that is reduced by a factor of ̃5. Similarly, iron pseudocarbynes with different carbon-chain lengths such as Fe13-C2H2 and Fe13-(C2H2)6 also show IR features at nearly the same wavelengths with reduced IR intensities. Iron pseudocarbynes may have been overlooked because, based on calculations, their IR spectra are, within experimental uncertainties, identical to astronomically observed, iron-free species. The occurrence of iron pseudocarbynes in CIS environments would enhance Fe depletion, facilitate production of thermodynamically stable long-chain polyynes, provide a catalytic bridge over the composition gap between molecules containing nine or fewer carbon atoms and complex molecules, and supply a potential mechanism for the modulation and polarization of magnetic fields in CIS environments.