Dissertation (MSc. Chemistry)The geometries of various tautomers of different ring size lactams have been studied by ab initio Hatree–Fock (HF) and Density Functional Theory (DFT) calculations at Becke, three-parameter, Lee-Yang-Parr functional (B3LYP) level by using different basis sets in both gas and solvent phase. Optimized geometries and total energies for 12 possible tautomers of studied lactams were determined. Thermodynamic properties and tautomeric equilibria between different tautomers were calculated by using the frequency calculations. The calculated bond length results of ß-lactam were consistent with experimental data. In the basis sets with polarized functions i.e. 6-311G(d,p) and 6-311G++(d,p); the polar bonds such as C=O were shorter while non polar bonds such as C–C were longer compared to the basis sets without polarized functions i.e. 6-311G and 6-311G++. Equilibrium constant results suggested that the lactam (amide) form is a more dominant tautomer for all cases and the lactim (iminol) forms are not present in a detectable amount. The second part of this research work was the calculation of global and local reactivity descriptors for different ring size lactam tautomers at different basis sets and different solvents by HF and DFT/B3LYP methods. The global reactivity descriptors were calculated. In the gaseous phase the calculated values led to the conclusion that global reactivity trends based on electrophilicity (ω), were given by K1B > E2B > K1G > K1D > E1G > K1E > E1D > E1E > E1B > E2D > E2G > E2E and E2B > E1G > E1E > K1B > K1D > E1 > E1B > K1E > K1G > E2G > E2D > E2E for HF and DFT methods at 6-311G++(d,p) respectively. From the Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) results the tautomers were categorized into two main groups (clusters) according to the similarities in their HOMO-LUMO energy gaps.