Tetrahedranes. A theoretical study of singlet E₄H₄ molecules (E = C–Pb and B–Tl)

Abstract

Quantum chemical calculations using DFT (B3LYP) and ab initio (CCSD(T)) methods have been carried out to elucidate the E₄H₄ singlet potential energy hypersurface for the group-13 and group-14 elements. The results show that there are numerous E₄H₄ energy minima for the compounds of group-13 and group-14 elements. This holds particularly for the heavier atoms. The classical T_d tetrahedron structure is energetically more favourable for the group-13 compounds E₄H₄ where the tetrahedron isomers are energy minima for all elements E except for thallium. The tetrahedranes Al₄H₄ 1Al and Ga₄H₄ 1Ga are the global energy minima on the singlet PES while 1B is slightly less stable than the singly hydrogen-bridged planar form 12B which has a rhombic B₄ core. The lowest-lying energy minima for In₄H₄ and Tl₄H₄ are tetrahedranes where the E₄ core is face-bridged by the hydrogen atoms. The classical T_d tetrahedron structure of the group-14 elements is only for carbon and silicon an energy minimum at B3LYP/def2-TZVPP. The latter form of Si₄H₄ is a second-order saddle point at BP86/def2-TZVPP and MP2/def2-TZVPP. The global energy minimum form for C₄H₄ is the vinylideneacetylene isomer 27C which is −63.7 kcal/mol lower in energy than the tetrahedrane. The most stable isomers of Si₄H₄ and Ge₄H₄ at CCSD(T)/def2-QZVPP//B3LYP/def2-TZVPP//B3LYP/def2-TZVPP are the singly hydrogen edge-bridged tetrahedrane structures 18E, which are more than 40 kcal/mol lower in energy than the classical tetrahedrane. The B3LYP/def2-TZVPP calculations predict that tetrahedrane structure with edge-bridging hydrogen atoms 3Ge is slightly more stable than 18Ge. The former isomer is calculated as the global energy minimum on the singlet PES of Sn₄H₄ and Pb₄H₄.

Keywords:

• tetrahedranes
• DFT
• ab initio calculations

Acknowledgement

The generous allotment of computer time by the HRZ Marburg, the CSC Frankfurt, the HHLR Darmstadt and the HLR Stuttgart, as well as their excellent service is gratefully acknowledged. This work was supported by the Deutsche Forschungsgemeinschaft.