Comparison of h-scaled Htickel theory and Hartree-Fock theory of boranes and carboranes

The h-scaled Hiickel method is used to calculate the electronic energy surfaces of the four boranes B,Hz(12 =8-l 1) and the carborane C,BsH:, . These electronic energy surfaces and their minimum energy geometries are directly compared to both the single crystal x-ray determined structures and to Hartree-Fock optimized geometries. Bond distances differ on the average by 0.04 A between alternate methods. It is shown that h-scaled Hiickel results may be directly interpreted by analysis of the highest occupied and lowest unoccupied molecular orbitals. Also studied by the h-scaled Hiickel and Hartree-Fock methods are the isomerization pathways of BsHi-, B,,H:;, and C,BsHfR . Reaction barriers and transition state geometries found by the two different calculational methods are in fair agreement with each other and known literature values. Using the h-scaled Hiickel method one can readily deduce that the BsHiand B,,H:; isomerizations are WoodwardHoffmann allowed reactions. In the case of B,Hithis allowed mechanism is contrasted to an alternate Woodward-Hoffmann forbidden pathway. Hartree-Fock calculations on the C,BsHT, confirm earlier h-scaled Hiickel based findings, that a second less stable isomer of C,B,H:, exists which, in contradiction to Wade’s rules of electron deficient clusters, has a pair of open square faces in the cluster.