Density Functional Theory (DFT) studies for the ring opening mechanism of tetraethyl 2-aryl-1,2-epoxygembisphosphonates

Abstract: The study reported in this thesis was aimed at investigating the ring opening mechanism(s) associated with the breaking of the carbon-oxygen oxirane bond of tetraethyl 2-aryl-1,2-epoxygembisphosphonate. Density Functional Theory (DFT) was employed to theoretically determine the mechanism(s) of the oxirane ring opening for tetraethyl 2-aryl-1,2-epoxy-2-phenylbisphosphonate. Simulations were carried out using the Gaussian09 version D01 software whereas visualisations were performed in GaussView 5.0.8, a graphic user interface compatible and part of the Gaussian software packages. The generalized gradient approximation functional BP86 in combination with the 6-31+G(d,p) and aug-cc-pVTZ basis sets was used for investigating the proposed reaction pathways. The method was selected after comparing energies and geometries obtained at both the BP86 and B3LYP functionals on selected local minima. Two transition states were obtained and found to be associated with opening on carbon 1 and 2 of the oxirane ring. These transition states were verified by running a frequency calculation and an intrinsic reaction coordinate (IRC) calculation at the same level of theory. Solvent effects were incorporated implicitly using the conductor-like polarised continuum model (CPCM) to investigate the effects on the energy and structures involved in the proposed mechanistic pathways. The effect of both electron donating and electron withdrawing groups on the transition state energies was also investigated. The ring opening at C2 was found to be the most favoured pathway both in vacuum and in the selected solvents.M.Sc. (Chemistry