Cycloproparenyl anions: From models to real systems

An overview of our recent work on cycloproparenyl anions is given. Preparation, the electronic structure, and the properties of the progenitor of the series, cyclopropabenzenyl anion, are discussed. It is shown that the cyclopropabenzenyl anion is by ca. 145 kJ mol-1 more stable than the parent cyclopropenyl anion according to results of the MP2/6-31+G(d) calculations. This finding was attributed to a delicate balance of two opposing effects: (a) propensity of the aromatic ring to alleviate unfavorable 4π electron interaction within the three-membered ring by the anionic resonance effect and (b) a pyramidalization of the anionic center, which tends to maximize the s-character of the lone pair. We have also shown that stability of the cyclopropabenzenyl anion could be considerably enhanced by substitution of the aromatic ring with fluorine and cyano groups, as well as by a linear extension of the aromatic backbone. Finally, the impact of the fusion of additional cyclopropenyl ring to the benzene moiety to acidity of the benzylic position in cyclopropabenzene is discussed.

The Effect of Structural Changes on the Polarizability and Second Hyperpolarizability in Some Benzene Isomers and Their Anions

The polarizability and second hyperpolarizability of fulvene, dimethylene-cyclobutene, trimethylene- cyclopropane, bicyclopentadiene-propane, bicyclopentadiene-propene, Dewar benzene, benzvalene and several of their anions are computed, employing a CNDO coupled Hartree-Fock procedure. The interaction of the lone pair on the anionic center with double and single bonds, the HOMO-LUMO gap, and substituent effects are employed to illuminate the structurepolarization relationship. An approximate, computationally found formula for the second hyperpolarizability is proposed.