FeC4H2: A Potential Astrophysical Molecule Featuring Planar Tetracoordinate Iron to Unveil the Mystery of Missing Iron in Interstellar Medium

Abstract

The exploration of iron-containing species represents a prominent area in contemporary astrochemical investigations. Iron in the interstellar medium exhibits predominantly low concentrations, hinting at the potential presence of missing iron in either a condensed or molecular state. The FeC₄H₂ neutral system has been investigated using computational calculations, emphasizing the hypothesis that missing iron may exist as iron–carbon hydride compounds or their higher order. A total of 61, 46, and 36 stationary points have been identified on the potential energy surface (PES) of the FeC₄H₂ in the singlet, triplet, and quintet electronic states, respectively, showcasing the existence of a planar tetracoordinate iron (ptFe). A linear geometry represents the global minimum on the PES of FeC₄H₂ as observed in the quintet ground electronic state. Meanwhile, the ptFe geometry is identified as the second most stable isomer in the quintet electronic state, which is the lowest energy solely in the singlet electronic state. Further, the observed ptFe geometry closely resembles the experimentally detected FeC₄ molecule in the gas phase. In light of its similarity to FeC₄ and the recent detection of FeC in the interstellar medium, the ptFe would be a potential astrophysical molecule that will also be identified in the gas phase. From that point of view, a comprehensive examination of the nature of chemical bonding of the ptFe geometry in the singlet, triplet, and quintet electronic states has been characterized, which dictates the stabilization of ptFe through multicenter bonding with surrounding carbon atoms. Ab initio molecular dynamics simulations revealed the dynamic stability of the system. Understanding the interstellar species with its structure and chemical bonding is an important aspect that would shed light on new insights into the experimental observations in the future.