Theoretical Study of Hydrogen-bridged Beryllium Compounds

Abstract

Ab initio closed-shell SCF method, combined with the energy gradient technique, was applied to study the molecular structures and the stability of (i) beryllium dihydride and its polymers (BeH₂)_n(n=1 to 5), and of (ii) monosubstituted beryllium hydrides HBeX (X=BH₂, CH₃, NH₂, OH, F and Cl). Basis set dependence on the geometries and the force constants of BeH₂ and (BeH₂)₂ was carefully examined. The minimal basis set gives us a qualitative picture for chemical bonding of beryllium, though at least the split-valence type basis set is needed to obtain quantitative results. The effect of the electron correlation on the dimerization energy of BeH₂ was studied with SDCI and MP 3 methods and was not so important as on the dimerization energy of Be atom. The dimer formation of BeH₂ results from the strong orbital interaction between a σ orbital (HOMO) of one of BeH₂ and a vacant 2p π orbital (LUMO) of the other. The energy gain from (BeH2)n (BeH₂)_n+1 was almost constant for n=-2, 3, and 4 (about 120 kJ/mol) and it is larger than that from BeH₂ to (BeH₂)₂ (about 80 kJ/mol). This result means that in the chemical bonding of Be atom the sp³ hybridization is more favorable than the sp² hybridization, and the sp² is more than the sp hybridization.
With STO-3 G and 3-21 G basis sets the molecular structures of a series of monosubstituted beryllium hydrides and their dimers were determined, and the vibrational frequencies were evaluated for them. Bond lengths between a Be atom and a neighboring atom become shorter as the electronegativity of the neighboring atom increases. In particular, the bonding with oxygen is found to be very strong. These hydrides tends to dimerize, and the dimerization energy is about 60-400 kJ/mol, when the bridged atoms are hydrogen atoms, i rrespective of the terminal substituents.