Molecular structures of the 1,6-disubstituted triptycenes Sb2(C6F4)3 and Bi2(C6F4)3 using gas-phase electron diffraction and ab initio and DFT calculations.

The structures of the D(3h)-symmetric molecules dodecafluoro-1,6-distibatriptycene and dodecafluoro-1,6-dibismatriptycene \[Z2(C6F4)3 Z = Sb, Bi\] have been determined in the gas phase by electron diffraction, using the SARACEN method, with restraints obtained from quantum chemical calculations. Several methods of ab initio and density functional theory geometry calculations have been performed and recommendations made as to their relative suitabilities for determining the structures of such species. Calculations using the MP2 method with a small-core pseudopotential (aug-cc-pVQZ-PP) on the Sb and Bi atoms and the 6-311G* basis set on the light atoms were found to give the closest correlation with the experimental results for both molecules. Differences in structure were found depending on whether a large-core or small-core pseudopotential was used on the heavy atoms.