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 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 B-4 core. The lowest-lying energy minima for In4H4 and Tl4H4 are tetrahedranes where the E-4 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.