COMPUTATIONAL MODELLING OF THE ACTIVATION PARAMETERS OF DIELS ALDER REACTION OF QUINOFLAVONOID AND 2, 3 DIMETHYLBUTA-1, 3-DIENE

Abstract

Kinetics and thermodynamics of Diels alder reaction of quinoflavoniod and 2, 3 dimethylbuta-1,3-diene was studied using ab-initio method with Hartree Fock at 6-311G* basis set in spartan, Conformational search was carried out on the molecules to obtain the structures that has the lowest energy, an indication of the stability of the structure. Reaction path calculation was done on the most stable structure using C26 – C15 and C27 – C14 as reaction coordinate in 20 iterations, this was done to ascertain the reaction mechanism. Geometric calculation was carried out on the structures of the reactant, transition state and products to obtain the bond length, bond angle, dihedral angle and atomic charge distributions. Kinetics and thermodynamic parameters was calculated at 3480C. The reaction proceeded through a concerted asynchronous six-membered cyclic transition state. True saddle point was characterized and verified by the presence of only one imaginary vibrational frequency which smoothly connect the reactant, the transition state and the product. Negative values obtained for ?H* (-66.65kJ/mol shows that the reaction is exothermic and according to literature this may be as a result of conversion of two of the ?-bonds to two new ?-bonds). Values obtained for energy of formation (-1329.974 hatrees, -1329.911 hatrees, and 1330.023 hatrees for GS, TS and PRD respectively) according to Hammond’s postulate, confirms the exothermicity of the reaction. ?Greaction (-60.16kJ/mol, shows that the reaction is favourable, Negative ?S* (-11.29J/mol.K) suggests the spontaneity of the reaction and Ea (-63.76kJ/mol) suggests that the reaction involves transfer of electrons. It was also observed that rate constants decrease with increase in temperature, although very uncommon but suggested a complex reaction mechanism.