During the 1921–22 school year, Carothers held a one-year appointment as a chemistry instructor at the University of South Dakota. It was at the University of South Dakota that he began his independent research that resulted in an article accepted by the "Journal of the American Chemical Society". In this paper he measured physical properties of phenyl isocyanate and of diazobenzene-imide (now known as phenyl azide). The properties have very similar values, which led him to the conclusion that the structure of the second compound is C6H5-N=N=N, with the three nitrogen atoms in a linear chain rather than a ring as previously thought.