X-X DIRECT BONDS VERSUS BRIDGED STRUCTURES IN GROUP-13 X2H2 POTENTIAL-ENERGY SURFACES

The singlet potential energy surfaces for all group 13 X2H-2 systems h ave been explored through ab initio SCF + CI calculations. Effective c ore potentials including relativistic effects for the heaviest atoms o f the series were used. Geometries of the various isomers were determi ned at the Hartree-Fock level and confirmed to be minima by vibrationa l analysis. In all cases but boron, the global minimum is found to be the D2h di-H-bridged structure. For boron, the H-X=X-H linear form is found to be the global minimum, the 1DELTA(g) state being 14.7 kcal/mo l above the 3SIGMA(g)- state. For all other atoms, this linear form is only a transition state, and a trans-bent isomer appears as a minimum on the surfaces for Al, Ga, and In. However, our analysis reveals two other low-lying minima, namely, the asymmetric X-XH-2 isomer and the C(s) mono-H-bridged structure. A simple rule for the occurrence of tra ns-bent isomers is derived from a MO model treating sigma-pi mixing. I n HX=XH, a trans-bent distortion occurs when the singlet-triplet energ y separation of the XH fragments is larger than half of the bond energ y in the linear 1DELTA(g) state. Qualitative rules concerning the exis tence of the bridged structures are also established.