Spectroscopic investigation of electron donor‐acceptor processes involving organic free radicals in the adsorption of substituted urea herbicides by humic acidsa

Products of the interaction between soil humic acids and substituted‐urea herbicides were prepared in the laboratory and subjected to elemental, functional group, differential thermal, infrared spectroscopic and electron‐spin resonance analysis, in order to examine the various chemical mechanisms that may take place upon adsorption. The results indicate that, besides the involvement of hydrogen bonding, a prominent role is played by electron donor‐acceptor processes involving organic free radicals, which leads to the possible formation of charge‐transfer complexes. The phytotoxicity of the urea herbicides, expressed as the inhibition potency towards the Hill reaction (pl50 values), appeared to be correlated with their capacity to interact with soil humic components, and it is suggested that these two processes depend upon the same molecular parameters.