Abstract
MINDO/3, MNDO and AM1 calculations have been carried out in order to calculate the geometry of some penicillins. Theoretical bond lengths, bond angles and dihedral angles have been checked with crystallographic data. Results show that MINDO/3 is the best method for predicting the geometry of the bicyclic system. However, it is not very good in predicting the conformations and pyramidality of the β-lactam nitrogen. In spite of its poor accuracy in predicting the geometry of the antibiotic, the MNDO method reproduces very well the pyramidality of the β-lactam nitrogen. The AM1 method predicts the geometry of the system quite well, except for the S-C and C7-N4 bond lengths which are underestimated and overestimated respectively. In addition, AM1 is very good at treating conformations of the thiazolidine ring.
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