Solvent dependent ESI-collisionally induced dissociation of protonated nitenpyram

Abstract

Electrospray ionization-collisionally induced dissociation (ESI-CID) pathways of nitenpyram (NP), a neonicotinoid insecticide, have been investigated. An extensive radical initiated-mechanism was observed which varied according to the nature of the solvent. Two pathways- the NO2● and ●OH radical eliminations- were observed and rationalized using DFT calculations. Elimination of ●OH was found to be a charge site mediated process while NO2● loss was a charge remote process. The radical eliminations were rationalized by the extensive analysis of the energy and geometry optimized NP and its protonated forms (protomers). Among the five possible protomers (NP-1 to NP-5), two (NP-3 and NP-5) exhibited a C–N bond elongation corresponding to NO2● loss, for which the bond dissociation energies were calculated. The stabilization of the resulting radical cation was proposed to be the driving force for the elimination of NO2●. It was observed that the nature of solvent affects the spectra considerably. In 2:1 mixture of water and methanol, the spectrum predominantly consists of peaks arising from the ●OH radical elimination and subsequent reactions. However, when the spectrum was recorded in acetonitrile, the NO2● elimination was found to be the major reaction. Intricate mechanisms of formation of the various ions were probed with the help of multistage MS and the analysis of fragments formed from NP in different solvents. The results were compared with three structurally similar molecules: thiamethoxam (TMX), ranitidine (RNTD) and the reported case of imidacloprid. It was interesting that neither ●OH radical elimination nor solvent effect was observed in the case of TMX, RNTD or imidacloprid. It is proposed that preferential N- and O- protonation in NP is the reason for the solvent dependence of its ESI-CID pathway.