C-13 chemical shielding tensors in ampicillin and penicillin-V: A theoretical study

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.