 By measuring the effects of aromatic substituents and applying Hammond's postulate it was concluded that the rate-determining step involves formation of a transition state that should resemble the intermediate complex.During the 1940s and 1950s, chemists had trouble explaining why even slight changes in the reactants caused significant differences in the rate and product distributions of a reaction.In 1955 George Hammond, a young professor at Iowa State University, postulated that transition-state theory could be used to qualitatively explain the observed structure-reactivity relationships.