The reaction of ozone with phenol, thiophenol, catechol and resorcinol: A computational study of the thermodynamic stability of the primary ozonides and reaction barriers in the gas phase and in water
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Date
2019
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University of Namibia
Abstract
Ozone is a highly reactive substance that can react with a vast number of chemical species. In the reaction of ozone with molecules containing carbon-carbon multiple bonds, the initial intermediate is a primary ozonide (POZ) which rearranges to a more stable secondary ozonide. The primary ozonides are transient species and very difficult to isolate experimentally. Thus, computational chemistry provides an alternative route to study the POZs. This study used quantum chemical techniques to investigate the effects of solvent (water) on the structures and thermodynamic stability of POZs derived from phenol, thiophenol, catechol and resorcinol. The reaction barriers leading to the formation of the elusive POZs were also studied.
Geometry optimization of the POZs was done at the B3LYP/6-31G(d,p) level in the gas phase and in water using the polarized continuum model. After optimizing the structures in the gas phase and in water, single point CCSD(T)/6-31G(d,p) calculations were performed to obtain more accurate energies. The effect of the solvent was determined by comparing the geometry, thermodynamic stability and reaction barriers of the POZs in water with those obtained in the gas phase.
The results of this study show that all the POZs are thermodynamically stable relative to the reactants, both in gas phase and in water. Specifically, the B3LYP/6-31G(d,p) model predicts that POZs of phenol, thiophenol, catechol and resorcinol are 24.9, 22.5, 28.7 and 26.5 kcal/mol, respectively, below the corresponding reactants in the gas phase. At the CCSD(T)/6-31G(d,p) level, the stability of the POZs increases to 28.6, 24.2, 34.4 and 30.7 kcal/mol for phenol, thiophenol, catechol and resorcinol, respectively. The most favorable pathways for the formation of the POZs go through energy barriers of 5.1, 7.4, 3.1 and 2.6 kcal/mol, for phenol, thiophenol, catechol and resorcinol, respectively, using the B3LYP/6-31G(d,p) model. Corresponding values at the CCSD(T)/6-31G(d,p) level are 9.5, 11.2, 7.6 and 8.0 kcal/mol. The B3LYP/6-31G(d,p) results indicate that planarity is retained for some POZs of phenol, thiophenol, catechol and resorcinol in the gas phase and in water. The POZs also appear to be thermodynamically more stable in water than in the gas phase. B3LYP/6-31G(d,p) reaction barriers of 4.1, 5.7, 2.7 and 2.0 kcal/mol were computed for phenol, thiophenol, catechol and resorcinol respectively, in water. The latter results suggest that the formation of the POZs would be faster in an aqueous medium relative to the gas phase.
Description
A thesis submitted in fulfillment of the requirements for the Degree of Master of Science (Chemistry)
Keywords
Ozone, Phenol, Ozonides, Thiophenol, Resorcinol