AB-INITIO AND DENSITY-FUNCTIONAL THEORY STUDY OF THE GEOMETRY AND REACTIVITY OF BENZYNE, 3-FLUOROBENZYNE, 4-FLUOROBENZYNE, AND 4,5-DIDEHYDROPYRIMIDINE

The geometries and reactivity of three intermediates of the aromatic n ucleophilic substitution, 3-fluorobenzyne, 4-fluorobenzyne, and 4,5-di dehydropyrimidine, are studied using ab initio molecular orbital theor y and density functional theory (DFT). As compared to the traditional ab initio methods, all DFT methods clearly assign more triple-bond cha racter to the carbon-carbon bond in the ring on which the addition occ urs. Furthermore DFT methods are found to correctly describe reactive intermediates in which electron correlation effects are expected to be large. The reactivity of the three intermediates is studied using the chemical concepts of electronegativity and hardness. The Fukui functi on was calculated for a nucleophilic attack at different DFT levels an d found to be an adequate reactivity descriptor.