Abstract
Equilibrium twist angles and rotational barriers for the three isomeric phenylpyrroles were calculated by means of MNDO and AM1. In each case MNDO incorrectly predicts the perpendicular conformation as the most stable one. In agreement with experimental evidence AM1 predicts only slight deviations from planarity and very low 0°-barriers (1-phenylpyrrole: 0 = 28°, E = 1.6 kJ mol-1; 2-phenylpyrrole: θ = 27°, E = 0.8 kJ mol-1; 3-phenylpyrrole: θ = 19°, E = 0.2 kJ mol-1). The reasons for the complete failure of MNDO are analyzed by partitioning the total energy into one- and two-center terms. The most significant improvement of AM1 over MNDO is found to be the much better description of repulsive interactions between nonbonded atoms. Possible further improvements of AM1 are briefly discussed.
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