Substituent effects in mono- and disubstituted 1,3,5,7-cyclooctatetraene derivatives in natural and planar conformations

Abstract

Substituent effect in cyclooctatetraene (COT) has been investigated by means of quantum-chemical (DFT-B3LYP) calculations. Two substituents, nitroso (π-electron-withdrawing) and hydroxy (π-electron-donating), have been examined and compared with monosubstituted and unsubstituted systems. Additionally, both the natural (tub-shaped) and the planar geometries of the COT have been taken into account. The behavior of COT ring, being the antiaromatic 4nπ-electron system, has been compared with that of the benzene ring, being the aromatic 4n + 2 system. The 1,4-substitution differs significantly from the 1,3- and 1,5-substitution, which corresponds to an analogous situation found for benzene derivatives (para vs. meta substitution). Substituents influence COT more effectively in its planar geometry as compared with the tub-shaped geometry. This is due to the better overlapping of the p_z orbitals of the carbon atoms within the ring in its planar conformation. As far as the π-electron communication between the substituents is concerned, the behavior of both 4n and 4n + 2 systems is in some way similar. Both systems tend to keep their π-electronic structures. Nevertheless, benzene ring, i.e.4n + 2 system seems to be a clearly worse medium for transmission of the substituents communication than its 4n counterpart.