The principal components of the C-13 Shielding tensors in two antibiotics,
ampicillin and penicillin-V, are calculated using the coupled Hartree-Fock
gauge-including atomic orbital (CHF-GIAO) method as well as using a hybrid
density functional scheme. Calculated results are compared with solid state
experimental nuclear magnetic resonance data. Using the known X-ray struct
ures of these antibiotics, it is demonstrated that the computed shieldings
compare favorably with experiment such that, in some cases, calculations ca
n now be utilized in assigning shielding tensor data.