Formation of hydroxy, cyano and ethynyl derivatives of C₄H₄ isomers in the interstellar medium

The study of cyclic hydrocarbons is of utmost relevance in current astrochemical research, as they are considered to be among the most significant reservoirs of carbon in the interstellar medium. However, while unsaturated cyclic hydrocarbons with three, five, and six carbon atoms have been widely investigated, the highly strained antiaromatic cyclobutadiene (c-C₄H₄) still remains uncharted. Here, we employed high-level CCSD(T)-F12/cc-pVTZ-F12//B2PLYPD3/aug-cc-pVTZ theoretical calculations to analyze whether the cyano (CN), ethynyl (CCH), and hydroxy (OH) derivatives of c-C₄H₄ and its structural isomers butatriene (H₂CCCCH₂) and vinylacetylene (H₂CCHCCH) can readily form via the gas-phase reaction: C₄H₄ + X → C₄H₃X + H (where X = CN, CCH, and OH). For each system, we thoroughly explored the corresponding potential energy surfaces, identifying their critical points to enable a detailed analysis of the thermochemistry. Hence, we found various exothermic pathways for the formation of CN and CCH derivatives of butatriene and vinylacetylene, with no net activation barriers, while the formation of the OH derivatives is in general less favorable. Prior to the mechanistic study, we also analyzed the complete conformational panorama and stability of all the derivatives at the CCSD(T)-F12/cc-pVTZ-F12 level. Overall, c-C₄H₃CN and c-C₄H₃CCH emerge as particularly promising candidates for interstellar detection, provided that the parental c-C₄H₄ is present in the gas phase. These findings highlight the potential for detecting polar derivatives of c-C₄H₄ as indirect evidence of its presence in the ISM, as it appears to be "invisible" to radioastronomical observations. Also, this study underscores the need for future laboratory and theoretical efforts to characterize the spectroscopic properties of the proposed derivatives, paving the way for their eventual identification in space.