Structures and fragmentation reactions of protonated neonicotinoid insecticides by infrared multiphoton dissociation spectroscopy and computational methods

Abstract

Imidacloprid and thiamethoxam belong to the nitroguanidine subclass of neonicotinoid insecticides. Nitroguanidine neonicotinoids induce higher acute and chronic toxicity in bees, therefore, analytical methods have been developed to evaluate the biological and ecological effects of neonicotinoids. The structures and unimolecular chemistries of protonated imidacloprid and protonated thiamethoxam have been studied in the gas phase by infrared multiphoton dissociation (IRMPD) spectroscopy and computational methods. Electronic structure calculations employing density functional theory (DFT) were conducted to determine thermodynamic results, relative enthalpies and Gibbs energies, and to compute the infrared frequencies of stretching and bending vibrations. Comparison of the experimental IRMPD and computed IR spectra was done to identify the vibrational features of the protonated neonicotinoids, imidacloprid and thiamethoxam. The lowest energy isomer of protonated imidacloprid, protonated at the pyridine nitrogen, generates an IR spectrum that is most consistent with the IRMPD spectrum. The spectroscopic IRMPD data for protonated imidacloprid urea (loss of N₂O), protonated desnitroimidacloprid (loss of NO2*), protonated desnitrosoimidacloprid (loss of NO*), and the loss NO* and OH from protonated imidacloprid are also presented. A DFT calculated potential energy diagram of protonated desnitroimidacloprid reveals the energetic barriers for intramolecular proton transfer versus bond dissociation of NO₂. A possible mechanism for the loss of NO* and OH from protonated imidacloprid describes isomerization of the NO loss product ion and tautomerization of the OH product ion. The structures, energetics, and infrared spectra of protonated thiamethoxam were computed using the B3LYP and M06-2X methods. There is a discrepancy in the computed energies for the lowest energy isomers of protonated thiamethoxam, therefore, a comparison of the calculated IR spectra and the IRMPD spectrum reveals that the oxygen protonated isomer is the best match. IRMPD spectroscopy experiments in conjunction with DFT computations are also performed for protonated desnitro-thiamethoxam (loss of NO₂), and protonated dechlorinated desnitro-thiamethoxam (loss of NO₂* and Cl*).