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

Barriers to internal rotation have been calculated for an extensive se ries of molecules using the molecular mechanics (MM) and quantum mecha nical semiempirical methods (MNDO, AM1 and PM3). Molecular geometries were fully optimized in the semiempirical study and partially optimize d by the MM treatment. Both rigid and non-rigid rotor approaches were used by the MM method. Semiempirical harmonic frequency calculations w ere performed in order to characterize the stationary points located o n the potential energy surface, as minimum or maximum. Comparison betw een theoretical and experimental results revealed a satisfactory agree ment for the MM barriers. The semiempirical rotational barriers are co nsistently underestimated when compared with experiment. This may be a ttributed to a possible inadequacy of the MNDO/AM1/PM3 empirical nucle ar repulsion functions for describing transition state structures. An attempt to scale the semiempirical barriers, using a simple procedure, was made.