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

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 Td tetrahedron structure is energetically more favourable for the group-13 compounds E₄H ₄ 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 Td 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.