RELATIONSHIPS IN THE ROTATIONAL BARRIERS OF ALL GROUP 14 ETHANE CONGENERS H3X-YH3 (X, Y = C, SI, GE, SN, PB) - COMPARISONS OF ABINITIO PSEUDOPOTENTIAL AND ALL-ELECTRON RESULTS

The equilibrium structures and energies of the 15 H3X-YH3 hydrides, comprised of all possible combinations of group 14 elements, have been calculated in eclipsed and staggered conformations. The relationships between the central bond lengths and the rotational barriers are of particular interest. The barriers decrease but do not vanish at large X-Y separations (e.g. Pb2H6). Each of the tighter elements (X = C, Si, Ge) gives a correlation between the barrier heights and the X-Y distances, but the slope decreases down the group. The data for X = Sn, Pb are not as regular in this respect, but all these barriers are small. Most of the X-Y, X-H, and Y-H distances obtained from nonrelativistic all-electron and from quasirelativistic pseudopotential calculations are in good agreement. Only relativistic bond contractions for X or Y = Pb lead to significantly shorter bond distances in the pseudopotential calculations. No systematic effect of relativity on the rotational barriers is found. Stabilizing vinical sigma-XH-->sigma*-YH and sigma-YH-->sigma*-XH interactions in the staggered conformation are responsible for the rotational barriers of the complete series, in agreement with previous studies of ethane. The lower barriers for the heavier species appear to be due to poorer orbital overlap and to increasingly smaller differences between antiperiplanar, synperiplanar, and 120-degrees vicinal overlap. While relativistic bond contraction increases the individual interactions for diplumbane and methylplumbane, it does not appear to affect the rotational barriers. Previous 3-21G(*) results, indicating the rotational barrier in hexamethyldisilane to be very close to that in dislane (ca. 1 kcal/mol), despite the presence of six methyl groups, are confirmed by calculations at higher levels of theory (MP2/6-31G*//6-31G*). When the X-Y distances are even larger, as in most of the group 14 combinations, methyl and similar substituents also should have no significant effect on the rotational barriers.