Thermodynamic and Kinetic Study of Ibuprofen with Hydroxyl Radical: A Density Functional Theory Approach

Ibuprofen, a frequently detected pharmaceutical in natural and engineered waters, was studied in both neutral and anionic forms using density functional theory at the B3LYP/6-311++G**//B3LYP/6-31G* level of theory in its reaction with hydroxyl radical (center dot OH). The reaction pathways included center dot OH addition to aromatic ring, abstraction of a H-atom, and nucleophilic attack on the carbonyl group. The results showed that H-atom abstraction pathways are the most favorable. The free energy change for H-atom abstraction reaction ranges from -37.8 to -15.9 kcal/mol; for center dot OH addition ranges from -3.85 to -1.23 kcal/mol; and for nucleophilic attack on the carbonyl group is 13.9 kcal/mol. The calculated rate constant between neutral ibuprofen and center dot OH, 6.72 x 10(9) M(-1)s(-1), is consistent with the experimental value, 6.5 +/- 0.2 x 10(9) M(-1)s(-1). Our results provide direct evidence for byproduct formation and identification on the molecular level. (c) 2013 Wiley Periodicals, Inc.