Nguyên Trong Anh

SOME DIFFICULTIES ENCOUNTERED WITH AM1 AND PM3 CALCULATIONS

Abstract AM1 and PM3 underestimate frontier interactions with respect to steric repulsions. Therefore, if two structures differ by ∼1 kcal/mol, their calculated ordering is unreliable. Activation energies tend to increase with substitution, regardless of electronic effects. Atomic charges are sometimes unrealistic (in enolates, the negative charge is larger on C than on O). At van der Waals distances, acid-base and coulombic interactions can prevail over steric repulsions. At all distances, basicities are overestimated and nucleophilicities underestimated. This may lead to anomalous ion-molecule and transition structures in gas phase reactions. Transition structures are tighter than in ab initio calculations. Optimisations may give chemically unreasonable structures. Minimum energy paths are then difficult to obtain. Usually, but not systematically, PM3 gives more reliable structures and AM1 more realistic energies.

The difficult marriage of theory and French organic chemistry in the 20th century

Abstract In the first half of this century, the postivism of many French organic chemists, who were reluctant to speculate about phenomena not directly observable, generated a certain scepticism for theories, with many unfortunate consequences. Thus, reaction mechanisms were not taught until the late 1950s and organic synthesis of complex molecules was consequently hampered. MO theory was practically ignored until 1965, when Woodward and Hoffmann published their epoch-making papers. The situation was restored in an astonishlingly short time, but the marriage between theory and French organic chemistry was, and still is, a difficult one.