Kinetic Study and Theoretical Analysis of Hydroxyl Radical Trapping and Spin Adduct Decay of Alkoxycarbonyl and Dialkoxyphosphoryl Nitrones in Aqueous Media

Abstract

Spin-trap development is important because of limitations that still exist among the currently used nitrone spin traps. This study correlates the experimental kinetic data with theoretical calculations, a novel approach that could be helpful in the future design of new spin traps. The kinetics of hydroxyl radical (•OH) trapping and spin adduct decay of the alkoxycarbonyl-nitrones 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO) and 5-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BocMPO) as well as the dialkoxyphosphoryl-nitrones 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) and 5-diisopropyloxyphosphoryl-5-methyl-1-pyrroline N-oxide (DIPPMPO) have been investigated and compared with those of unsubstituted 5,5-dimethyl-1-pyrroline N-oxide (DMPO). Kinetic investigation was performed by the steady-state generation of •OH from H2O2 by UV photolysis in the presence of a nitrone. Apparent rate constants of •OH trapping by EMPO, BocMPO, DEPMPO, and DIPPMPO in competition with ethanol are all comparable, with kapp values ranging from 4.99 ± 0.36 to 4.48 ± 0.32 M-1 s-1 and the commonly used spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) having a lower kapp of 1.93 ± 0.05 M-1 s-1. Half-lives of the •OH adducts of EMPO, DEPMPO, and DIPPMPO are much longer (t1/2 = 127−158 min) than those of DMPO and BocMPO with half-lives of only 55 and 37 min, respectively. Geometry optimizations, frequency analyses, and single-point energies of the nitrones and their corresponding spin adducts were determined at the B3LYP/6-31G*//HF/6-31G* level to rationalize the experimental results.