DOUBLE-BONDS AND BRIDGED STRUCTURES IN THE HEAVIER ANALOGS OF ETHYLENE

The singlet potential surfaces for all group 14 X2H4 systems are explored through ab initio SCF+CI calculations. Effective core potentials are used, which include relativistic effects for tin and lead atoms. In all cases but carbon, the bridged structures are found to be true minima with the trans isomer being favored over the cis by 2 kcal/mol. For carbon, a trans-bridged form is found to be a saddle point. The HX2=XH2 doubly bonded forms, planar or trans bent, are found to be true minima in all cases except for lead where it is only a saddle point. The most stable structures of Si2H4 and Ge2H4 are the trans-bent doubly bonded isomers, while the most stable structures of Sn2H4 and Pb2H4 are the trans-bridged forms. The H3X-XH isomers lie between these two symmetrical forms and are never found to be at the absolute minimum of the surfaces. The trans-bridged structures have a rather constant binding energy with respect to 2XH2(1A1) whatever the X atom (30 ± 3 kcal/mol). The stability of the bridged structures may be due to the significant ionicity of the bridges X+-H--X+ in the planar four-membered rings. © 1990, American Chemical Society. All rights reserved.