The origin of barriers to internal rotation. An energy decomposition study for CH3CH3, CH3NH2 and CH3OH

Abstract The energy decomposition analysis of Morokuma and Kitaura-Morokuma is used to elucidate the origin of the rotational barrier. The barrier of ethane, defined as the difference between the methyl radical-methyl radical interaction energy in a staggered and an eclipsed conformation, is found to be dominated by the exchange interaction, in particular, the orthogonality requirement term. Similar results are obtained for methyl amine and methyl alcohol. The results and conclusions of the present method are compared with those of previous workers.