COMPARISON OF MNDO, AM1 AND PM3 ROTATIONAL BARRIERS IN BRANCHED ALKANES

The MNDO, AM1 and PM3 semi-empirical methods were used to calculate the equilibrium points and transition-state structures of the rotational barriers of ethane and all methyl-substituted propanes and butanes. Force constant methods were used to ensure proper geometries. In general, AM1 barriers tend to be lower than MNDO calculated ones, which are themselves generally lower than experiment. The PM3 values are closest to experiment, ranging from slightly above to slightly below the experimental values. The complexity of various possible rotational minima makes the development of empirical rules to predict barrier heights difficult.