Molecular modeling of penicilloate anions: an RHF-SCF analysis

An ab initio restricted Hartree-Fock self-consistent field (RHF-SCF) analysis of penicilloate anions was performed at the TZV level with GAMESS. Geometry optimization was initialized by the semi-empirical AM1 method followed by optimization at the 6-31++G** level. The total energy obtained was -1116.0997 a.u. for the penicilloate amine, -1115.3164 a.u. for the imine, -1115.2969 a.u. for the enamine and -1115.2017 a.u. for the amine that was deprotonated at the thiazolidine nitrogen. Formation of the free thiolate in the imine and enamine anions by deprotonation of the penicilloate amine is associated with: (1) an increase in total energy (2) an increase in the energy of the highest occupied molecular orbital (HOMO) to that of anti-bonding (3) a decrease in chemical hardness (4) an increase in the chemical potential (5) a more negative Mulliken net charge on the sulfur atom and (6) an increase in the Mulliken atomic population on the former thiazolidine sulfur atom in the HOMO. The RHF-SCF analysis presented here suggests a potential role for the thiolate sulfur of penicilloate anions, especially of the imine, as a chemically reactive soft nucleophile.