COMPUTATIONAL STUDY OF THE REACTION MECHANISMS OF IODINE MOLECULE WITH SOME HYDRAZINES AND BIOMOLECULES
COMPUTATIONAL STUDY OF THE REACTION MECHANISMS OF IODINE MOLECULE WITH SOME HYDRAZINES AND BIOMOLECULES
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Date
2015-09
Authors
SHALLANGWA, GIDEON ADAMU
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Abstract
The oxidation of hydrazine, hydrazinium ion,1,2-diphenylhydrazine, L-tyrosine, D-ascorbic acid and D-fructose, by molecular iodine was studied using semi-empirical and density functional theory (DFT) methods, respectively. The computational method used for calculating geometries in this work is termed a ―cascade method‖ because of its use of molecular mechanics to remove strain energies followed by semi-empirical methods as precursors for more accurate DFT methods.The semi-empirical studies were carried out at either themodified neglect of diatomic overlap(MNDO) or parameterization method 3 (PM3) levels while all the DFT studies were carried out using 6-311+G** basis set of the density functional theory (DFT) method at the Becke 3 term, Lee Yang, Par (B3LYP) level of computation. For each system studied, molecular information such as net charges, values of frontier orbital energies, composition, proportions and bonding contribution were determined and analysed. Thus, through these means, possible reactive sites of molecules or reacting species were searched and obtained. Postulated transition states, intermediates and products were also searched and computed using the PM3 and DFT methods. Based on the results of the computation for the different systems studied, different possible reaction mechanisms for the various systems were proposed. For the 1,2-diphenylhydrazine – iodine reaction system, two possible reaction mechanisms were proposed, for the L-tyrosine – iodine reaction system, previously published reaction mechanism was modified based on the most favourable energetics of the transition states, for the L-ascorbic acid– iodine reaction system, two possible reaction mechanisms were proposed, for the D-fructose – iodine reaction system, one reaction mechanism was proposed, while for the hydrazine / hydrazinium ion– iodine reaction system, four reaction mechanisms were proposed. For each system investigated, out of the various plausible mechanisms postulated, comparisons of the enthalpies of reactions of the various pathways as well as the activation barriers of the respective rate determining steps were made. The computed enthalpies of the oxidation reactions
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were carried out at standard conditions of 298.15K and 1 atmosphere for the PM3 and DFT calculations. For each system, the pathway with the least activation barriers and enthalpy of reactions were chosen as the most probable mechanism of reaction for the system. Other thermodynamic parameters such as ΔGo andΔSo for reacting species, especially transition states and intermediates which were used to support the validity of the postulated mechanisms were also computed. The activation parameters for the most energetically favourable pathway for the respective reaction systems were calculated and given. The data obtained for the limiting step of the 1,2-diphenylhydrazine – iodine reaction system are: Δ‡G = -7.11 x103 kJ/mol, Δ‡S = 2.71 x101 kJ/mol andEa = 44.40kJ/mol. For theL-tyrosine – iodine reaction system, Δ‡G = -1.10 x105kJ/mol,Δ‡S = 1.28 x105 kJ/mol and Ea = 3.52 x103kJ/mol.For the limiting step for the D-ascorbic acid – iodine reaction system, Δ‡G = -2.93x104kJ/mol, Δ‡S = 1.28 x105kJ/mol and Ea = 3.14 x103kJ/mol.For the limiting step for the D- fructose – iodine reaction system, Δ‡G = -6.78 x104kJ/mol, Δ‡S = 6.74 x104kJ/mol and Ea = 3.82 x103kJ/mol. While for the hydrazine / hydrazinium ion – iodine reaction system, activation parameters for the limiting step of the most energetically favourable pathway were determined as, Δ‡G = -8.01 x104 kJ/mol, Δ‡S = -1.24 x106 kJ/mol and Ea = 6.86 x103kJ/mol.
Description
A THESIS SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES, AHMADU BELLO UNIVERSITY, ZARIA,
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD
OF A
DOCTOR OF PHILOSOPHY DEGREE IN PHYSICAL CHEMISTRY
DEPARTMENT OF CHEMISTRY,
AHMADU BELLO UNIVERSITY,
ZARIA, NIGERIA
Keywords
COMPUTATIONAL STUDY,, REACTION MECHANISMS,, IODINE MOLECULE,, HYDRAZINES,, BIOMOLECULES,