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A QUANTUM MECHANICAL STUDY OF THE PROTONATION AND COVALENT HYDRATION OF QUINAZOLINE IN THE PRESENCE OF METAL CATIONS
Citation:
Sawunyama, P. AND G W. Bailey. A QUANTUM MECHANICAL STUDY OF THE PROTONATION AND COVALENT HYDRATION OF QUINAZOLINE IN THE PRESENCE OF METAL CATIONS. Presented at 220th American Chemical Society National Meeting, Washington, DC, August 20-24, 2000.
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
We have investigated the protonation and reversible covalent hydration of quinazoline in the presence of Li+, Na+, and Ca2+ ions using ab initio quantum mechanical calculations at the MP2/6-31G**//HF/6-31G*level of theory. Proton affinities, enthalpies of hydration at 298.15K (DH298), and activation energies for several species that are considered to be present in acidic aqueous solutions of quinazoline as well as DH298 for the metal cations were obtained. Protonation of quinazoline results in three stable species whose relative stability is in the order; N1-N3 dication > N3 cation > N1 cation. The energy difference between the N3 and N1 species is ~0.57 kcal mol-1. DH298 values range from 0.40 kcal mol-1 for neutral quinazoline to -11.09 kcal mol-1 and -10.05 kcal mol-1 for N1 and N3 cations, respectively. DH298 values for the metal cations are approximately 3 - 6 fold larger than those for the quinazoline cation species. Thus, in the presence of metal cations a local competition for water occurs leading to the inhibition of hydration of quinazoline cation species. Overall, our findings are crucial to the understanding of the mechanisms of organic compound hydration at the clay-water interface, processes of major significance in the prediction of the fate and transport of organic pollutants in the ecosystems.