MNDO/AM1/PM3 QUANTUM-MECHANICAL SEMIEMPIRICAL AND MOLECULAR MECHANICS BARRIERS TO INTERNAL-ROTATION - A COMPARATIVE-STUDY

Barriers to internal rotation have been calculated for an extensive series of molecules using the molecular mechanics (MM) and quantum mechanical semiempirical methods (MNDO, AM1 and PM3). Molecular geometries were fully optimized in the semiempirical study and partially optimized by the MM treatment. Both rigid and non-rigid rotor approaches were used by the MM method. Semiempirical harmonic frequency calculations were performed in order to characterize the stationary points located on the potential energy surface, as minimum or maximum. Comparison between theoretical and experimental results revealed a satisfactory agreement for the MM barriers. The semiempirical rotational barriers are consistently underestimated when compared with experiment. This may be attributed to a possible inadequacy of the MNDO/AM1/PM3 empirical nuclear repulsion functions for describing transition state structures. An attempt to scale the semiempirical barriers, using a simple procedure, was made.