Photochemistry of Solid Films of the Neonicotinoid Nitenpyram

The environmental fates of nitenpyram (NPM), a widely used neonicotinoid insecticide, are not well-known. A thin solid film of NPM deposited on a germanium attenuated total reflectance (ATR) crystal was exposed to radiation from a low-pressure mercury lamp at 254 nm, or from broadband low pressure mercury photolysis lamps centered at 350 or 313 nm. The loss during photolysis was followed in time using FTIR. The photolysis quantum yields (ϕ), defined as the number of NPM molecules lost per photon absorbed, were determined to be (9.4 ± 1.5) × 10^-4 at 350 nm, (1.0 ± 0.3) × 10^-3 at 313 nm, and (1.2 ± 0.4) × 10^-2 at 254 nm (±2σ). Imines, one with a carbonyl group, were detected as surface-bound products and gaseous N₂O was generated in low (11%) yield. The UV-vis absorption spectra of NPM in water was different from that in acetonitrile, dichloromethane, and methanol, or in a thin solid film. The photolytic lifetime of solid NPM at a solar zenith angle at 35° is calculated to be 36 min, while that for NPM in water is 269 min, assuming that the quantum yield is the same as in the solid. Thus, there may be a significant sensitivity to the medium for photolytic degradation and the lifetime of NPM in the environment.