Free radicals in electron donor‐acceptor reactions between a soil humic acid and photosynthesis inhibitor herbicides

Abstract Products obtained by interaction of a soil humic acid with a number of s‐triazines and substituted ureas, well known to act as photosynthesis inhibitor herbicides, were studied by infrared (IR) and electron spin resonance (ESR) spectrometries. Analysis of spectroscopic results showed that single‐electron transfer processes take place reasonably between ring or chain donor groups of the herbicide molecule and acceptor quinone units of the humic acid, thus producing additional unpairing of electrons in the resulting charge‐transfer complexes. Experimental and theoretical considerations suggested the hypothesis that the investigated herbicides may react chemically with humic compounds by simulating analogous processes that take place on biological scale in chloroplasts, on the basis of similar electron donor‐acceptor reactions involving free radicals.