Nucleophilic Degradation of Fenitrothion Insecticide and Performance of Nucleophiles: A Computational Study

Abstract

Ab initio and density functional theory (DFT) calculations have been performed to understand the destruction chemistry of an important organophosphorus insecticide <i>O</i>,<i>O</i>-dimethyl <i>O</i>-(3-methyl-4-nitrophenyl) phosphorothioate, fenitrothion (<b>FN</b>), toward nucleophilic attack. Breaking of the P–OAr linkages through nucleophilic attack is considered to be the major degradation pathway for <b>FN</b>. One simple nucleophile, hydroxide (OH^–), and two different α-nucleophiles, hydroperoxide (OOH^–) and hydroxylamine anion (NH₂O^–), have been considered for this study. Nucleophilic attack at the two different centers, S_N2@P and S_N2@C, has been monitored, and the computed reaction energetics confirms that the S_N2@P reactions are favorable over the S_N2@C reactions for all the nucleophiles. All electronic structure calculations for the reaction are performed at DFT-B3LYP/6-31+G­(d) level of theory followed by a refinement of energy at ab initio MP2/6-311++G­(2d,2p) level. The effect of aqueous polarization on both the S_N2 reactions is taken into account employing the conductor-like screening model (COSMO) as well as polarization continuum model (PCM) at B3LYP/6-31+G­(d) level of theory. Relative performance of the two α-nucleophiles, OOH^– and NH₂O^–, at the P center has further been clarified using natural bond orbital (NBO), conceptual DFT, and atoms in molecules (AIM) approaches. The strength of the intermolecular hydrogen bonding in the transition states and topological properties of the electron density distribution for −X–H···S (X = O, N) intermolecular hydrogen bonds are the subject of NBO and AIM analysis, respectively. Our calculated reaction energetics and electronic properties suggest that the relative order of nucleophilicity for the nucleophiles is OOH^– > NH₂O^– > OH^– for the S_N2@P, whereas for the S_N2@C the order, which gets little altered, is NH₂O^– > OOH^– > OH^–