AN AM1 SEMIEMPIRICAL MOLECULAR-ORBITAL INVESTIGATION OF THE GROUP-14 [1.1.1]PROPELLANES AND BICYCLO[1.1.1]PENTANES

AM1 semiempirical calculations were performed on the group 14 [1.1.1]p ropellanes and bicyclo[1.1.1]pentanes. For the carbon and tin propella nes, the results were in agreement with several previous theoretical a nd experimental studies, predicting substantial bonding interaction be tween the bridgehead atoms in the case of carbon and none in the case of tin, Calculations on the silicon propellane were in sharp disagreem ent with ab initio calculations using a variety of basis sets. Both ge ometry and singlet-triplet splitting were incorrectly reproduced. The silicon propellane calculations were repeated using the HF/6-31G basi s set, and the singlet-triplet order was in agreement with the previou s work. The germanium results show very strong charge localization. We suggest a possible reason for this observation.