Recent experimental and theoretical studies reporting remarkable monobridged structures for Si2H2, A12H2, and Ga2H2 have motivated us to re-investigate the singlet potential energy surface of Ge2H2 using the self-consistentfield (SCF), single and double excitation configuration interaction (CISD), and single and double excitation coupled cluster (CCSD) methods in conjunction with a double-£ plus polarization basis set (DZP). In addition to the dibridged (or butterfly) ground state and the low-lying vinylidene-like minimum reported earlier by Grev and DeLeeuw, our study predicts the existence of a monobridged isomer, which is characterized as a minimum by means of harmonic vibrational frequency analyses. Relative energies of the above structures were also predicted with use of the coupled cluster single, double, and (perturbative) triple excitation method (CCSD(T)), which employed a triple-£ plus polarization basis set augmented with a set of f functions on the germanium atoms (TZP+f). Final energetic predictions suggest that the monobridged structure is the second most stable isomer of Ge2H2, lying 8.9 kcal mol-1 above the butterfly ground state and 2.1 kcal mol-1 below the branched (vinylidene-like) minimum.

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Ab initio quantum mechanical methods have been used to study the singlet potential energy surface of A12H2. Optimum geometries and harmonic vibrational frequencies were obtained for four geometrical isomers using the self-consistent-field (SCF), configuration interaction (Cl), and coupled cluster (CC) methods. Both correlation methods including single and double excitations (CISD, CCSD) were employed, and all wave functions were determined with both DZP and TZ2P basis sets. Final energy predictions are obtained using large atomic natural orbital basis sets, and including the effects of triple excitations perturbatively using CCSD(T) methods. We found the planar dibridged structure to be the global minimum, as predicted earlier by Baird. However, our analysis also predicts the existence of a remarkable monobridged minimum, which has recently been observed experimentally for Si2H2. Two additional low-lying minima are found, corresponding to trans-bent and vinylidene-like structures. The dibridged, monobridged, and trans-bent structures can be understood as resulting from the three possible ways of coordinating the two electron-rich sites of diatomic A1H to the electron-deficient aluminum centers. The energy of these structures with respect to dissociation to two A1H monomers is quite low and is related to the large difference between the first and second Al-H bond dissociation energies of the parent A1H3 compound.

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