Confirmation of pentavalent carbon in protonated methane (CH5+ ): Insights from Molecular Handycam Technique

The existence of pentavalent carbon has intrigued scientists since the discovery of CH${}_5^{+}$ in 1952 but remains elusive due to the lack of definitive evidence. The present study unveils the mystery through a comprehensive investigation of the structural and bonding nature of CH${}_5^{+}$ employing the newly developed Molecular Handycam Technique (MHT) by Bag and co-workers, focusing on its formation pathways and energetic favorability. Computational analysis at the coupled cluster theory (CCSD) level, we examine the formation of CH${}_5^{+}$ through the association of CH${}_3^{+}$ and H₂, compared to the protonation of methane. Our findings reveal a preference for the former pathway, highlighting distinct structural configurations, including a global minimum and two alternative geometries. We demonstrate the participation of higher orbitals of carbon (3d${}_{z^2}$) and its interaction with the bond pair of the approaching H${}_2$ molecule in the formation and stabilization of the fifth C-H bond. This analytical approach provides critical insights into the expanded valency of carbon, which could lead to a new class of carbon compounds.