CBe₅H_n^n-4 (n = 2-5): Hydrogen-Stabilized CBe₅ Pentagons Containing Planar or Quasi-Planar Pentacoordinate Carbons

The diagonal relationship between beryllium and aluminum and the isoelectronic relationship between BeH unit and Al atom were utilized to design a new series ppC- or quasi-ppC-containing species C_5v CBe₅H₅⁺, C_s CBe₅H₄, C_2v CBe₅H₃^-, and C_2v CBe₅H₂^2- by replacing the Al atoms in previously reported global minima planar pentacoordinate carbon (ppC) species D_5h CAl₅⁺, C_2v CAl₄Be, C_2v CAl₃Be₂^-, and C_2v CAl₂Be₃^2- with BeH units. The three-center two-electron (3c-2e) bonds formed between Be and bridging H atoms were crucial for the stabilization of these ppC species. The natural bond orbital (NBO) and adaptive natural density partitioning (AdNDP) analyses revealed that the central ppCs or quasi-ppCs possess the stable eight electron-shell structures. The AdNDP analyses also disclosed that these species are all 6σ+2π double-aromatic in nature. The aromaticity was proved by the calculated negative nucleus-independent chemical shifts (NICS) values. DFT and high-level CCSD(T) calculations revealed that these ppC- or quasi-ppC species are the global minimum or competitive low-lying local minimum (C_s CBe₅H₄) on their potential energy surfaces. The Born-Oppenheimer molecular dynamic (BOMD) simulations revealed that the H atoms in C_2v CBe₅H₃^- and C_2v CBe₅H₂^2- can easily rotate around the CBe₅ cores and the structure of quasi-planar C_5v CBe₅H₅⁺ will become the planar structure at room temperature; however, these interesting dynamic behaviors did not indicate the kinetic instability as the basic ppC structures were maintained during the simulations. Therefore, it would be potentially possible to realize these interesting ppC- or quasi-ppc-species in future experiments.