The Use of D Index in the Estimation of Aromaticity: The Case of Cycloparaphenylene

Cycloparaphenylenes are important compounds for their photophysical and electronic properties. The curved structure of these compounds induces an alteration of the aromatic character of these compounds. D values have been used to describe the aromatic character of compounds from [5]- to [10]cycloparaphenylene. D values are compared with HOMED and with the strain energy are obtained by using an homodesmotic reaction.

Substituent effects on stability of oxiranes, oxirenes, and dioxiranes

Homodesmotic reactions were designed to study substituent effects on stability of oxiranes, oxirenes, and dioxiranes. Good or fair correlation between their homodesmotic stabilization energies and Taft's dual-substituent-parameters has been found. Oxiranes are stabilized by σ-donating and π-donating substituents but destabilized by σ-accepting and π-accepting substituents. The π-effects on oxiranes are comparable to σ-effects for π-acceptor substituents and are much stronger than σ-effects for π-donor substituents. Oxirenes are stabilized by σ-donating, π-donating, and π-accepting substituents but destabilized by σ-accepting substituents. The π-effects on oxirenes are stronger than σ-effects. Oxirenes with strong π-donor substituents such as F, OH, and NH2, or strong σ-donor substituents such as Li and Na are neither real molecules nor transition states. Dioxiranes are stabilized by σ-donating and π-donating substituents but destabilized by σ-accepting and π-accepting substituents. The π-effects on dioxiranes are stronger than σ-effects for π-donor substituents, but weaker than σ-effects for π-acceptor substituentsKey words: oxirane, oxirene, dioxirane, homodesmotic reaction, substituent effect.