APPLICATION OF THE FENSKE-HALL MOLECULAR-ORBITAL METHOD TO THE CALCULATION OF B-11 NMR CHEMICAL-SHIFTS - ANTIPODAL SUBSTITUENT EFFECTS IN DELTAHEDRAL CLUSTERS

Utilizing Fenske-Hall wave functions and eigenvalues combined with the Ramsey sum over states (SOS) approximation, we demonstrate that the sign and magnitude of the “paramagnetic” contribution to the shielding correlates well with the observed 11B chemical shifts of a substantial variety of boron- and metal-containing compounds. Analysis of the molecular orbital (MO) contributions in the SOS approximation leads to an explanation of the large downfield shifts associated with metal-rich metallaboranes. A similar analysis demonstrates the importance of selected cluster occupied and unoccupied MO’s in explaining both exo-cage substituent effects in which the antipodal boron resonance is shifted upfield and endo-cage substituent effects (interchange of isolobal fragments within the cage framework) in which the antipodal boron resonance is shifted downfield. Exo- and endo-cage substitution perturbs these MO’s in an understandable fashion, leading to an internally consistent explanation of the observed chemical shift changes. © 1990, American Chemical Society. All rights reserved.