Orbital interactions and solvent effects determining the stability of condensed cyclopentadienides in solution

The structure and the stability of various cyclopentadienides, which involv e 6 pi, 10 pi, 14 pi, and 22 pi electrons, are investigated from computatio ns at various levels of theory as well as from orbital interaction analyses . The reason that some of the cyclopentadienides are stabilized and others are destabilized by the introduction of aromatic rings is discussed in term s of absolute hardness and orbital interaction. Cyclopentadienide, a specia l Bit-electron system, has the largest value of absolute hardness among the condensed cyclopentadienides investigated; thus this carbanion resists bot h oxidation and reduction most strongly. The absolute hardness decreases wh en aromatic rings are introduced to cyclopentadienide to form condensed cyc lopentadienides, depending on the way they are connected. Computed values o f the ionization potential and oxidation potentials measured in solution ha ve a linear correlation within isomers of the same size, but are not in agr eement for different sets of isomers. Solvent effects on the ionization pot ential are assessed by performing self-consistent reaction field calculatio ns, the results being in excellent agreement with experiments. It is demons trated that the solvent effects are significant in small cyclopentadienides of 6 pi- and 10 pi-electron systems, compared to larger ones and that addi tion of condensed aromatic rings intrinsically stabilizes the formed conden sed cyclopentadienides with respect to ionization potential.