THE MOLECULAR-STRUCTURE OF TRIASTERANE - AN EXPERIMENTAL AND THEORETICAL-STUDY

The molecular structure of triasterane (tetracyclo-[3.3.1.0(2,3).0(2,6)]nonane) has been investigated by using the gas-phase electron-diffraction method and by theoretical calculations. The experimental data are in accordance with the molecule having D3h symmetry. The C-C bonds in the three-membered rings are observed to be slightly shorter than those in the methylene bridges [r(a) 1.508(5) versus 1.520(4) angstrom; 2-sigma in parentheses]. The C-C-C angle at the methylene bridges is observed to be 118.9(3)-degrees, while the M-C-CH2 angle is determined to be 123.9(2)-degrees (C-M is bisecting the cyclopropyl ring). Theoretical force-field, semiempirical, and ab initio calculations of the triasterane molecule have also been carried out. The semiempirical results deviate substantially from the experimentally determined parameters, while the ab initio results, using a 6-31G* basis set, are in excellent agreement with the latter. The steric strain in triasterane has its origin mainly in nonbonded repulsion between the cyclopropyl groups and in torsion at the C-C bonds involving the methylene groups,