Linear or Cyclic? Theoretical Investigation of Astrophysically Relevant Magnesium-Bearing MgCnH Carbon Chains and Related Isomers

Magnesium-containing molecules, including MgC₂H, MgC₄H, and MgC₆H, have been detected in the interstellar medium, largely facilitated by their high dipole moments. However, despite great efforts, MgC_2m + 1H species remain elusive. Given the challenges in obtaining experimental data for these molecules, theoretical studies play a crucial role in guiding their detection. In this work, we present a theoretical analysis of MgC_nH isomers (n = 4–7) using density functional theory and coupled-cluster methods to identify low-lying isomers and characterize their structural and electronic properties. Our findings reveal that across the entire series investigated, the global minimum geometry is linear for even values of n, whereas for odd values, a cyclic geometry is favored. Additionally, our calculations highlight the enhanced stability of anionic MgC_nH⁻ systems, providing insights that could aid future astronomical detections in the interstellar medium.