Photochemistry of Thin Solid Films of the Neonicotinoid Imidacloprid on Surfaces

Imidacloprid (IMD) is the most widely used neonicotinoid insecticide found on environmental surfaces and in water. Analysis of surface-bound IMD photolysis products was performed using attenuated total reflectance Fourier transfer infrared (ATR-FTIR) analysis, electrospray ionization (ESI-MS), direct analysis in real time mass spectrometry (DART-MS), and transmission FTIR for gas-phase products. Photolysis quantum yields (ϕ) for loss of IMD were determined to be (1.6 ± 0.6) × 10^-3 (1s) at 305 nm and (8.5 ± 2.1) × 10^-3 (1s) at 254 nm. The major product is the imidacloprid urea derivative (IMD-UR, 84% yield), with smaller amounts of the desnitro-imidacloprid (DN-IMD, 16% yield) product, and gaseous nitrous oxide (N₂O). Theoretical calculations show that the first step of the main mechanism is the photodissociation of NO₂, which then recombines with the ground electronic state of IMD radical to form IMD-UR and N₂O in a thermally driven process. The photolytic lifetime of IMD at a solar zenith angle of 35° is calculated to be 16 h, indicating the significant reaction of IMD over the course of a day. Desnitro-imidacloprid has been identified by others as having increased binding to target receptors compared to IMD, suggesting that photolysis on environmental surfaces increases toxicity.