In this paper, we have disussed the internal rotation in sulfonylurea molecules,which belong to high active and low toxic herbicides and take the form of conformational iso-mers in their crystal structures. In order to understand the formation of their enantimers, weobtained a series of data of molecular energy of different conformation by using the molecularmechanics method and the stepwise rotation single-bond method. We found out the lowestinternal rotation passage and obtained the energy barrier of hindered rotation which is less 60kJ/mol. This result shows that there is a lower energy passage to exchange the conforma-tional isomers of these molecules, so exchanging the different conformational isomers is notvery difficult.
