Theoretical calculations of natural penicillins: structural and electronic properties

Abstract

The influence of the calculation method and small structural change in the molecule on the results of geometry and other properties of compounds was studied on the example of known antibiotics. The structural, electronic, and thermodynamic properties of penicillin and phenoxymethylpenicillin were calculated using molecular mechanics and quantum mechanics methods. A comparative analysis of penicillin structures based on experimental data and calculations was carried out. A molecular model of the experimental geometry was considered as the starting structure, which was then optimized. The geometric parameters were computed using the Ellinger MM2 force field method, semi-empirical PM6 one, and ab initio Hartree-Fock (HF) method with the Dunning’s correlation consistent basis set cc-pVDZ. Although theoretical calculations were carried out in gaseous phase, cc-pVDZ-optimized geometry of the molecules is close to the crystal structure. Some theoretical parameters for optimized structures of the title compounds, such as total electronic energy, zeropoint energy, rotational constants and dipole moments were defined by HF method. The electronic properties as HOMO and LUMO energies for both penicillins were calculated. Thermodynamic properties (heat capacity, entropy) of ones were computed by an ab initio method that took into account the correlation effects.