Substituent Constants (σp−) of the Rotated Nitro Group. The Interplay Between the Substituent Effect of a Rotated −NO2 Group and H-Bonds Affecting π-Electron Delocalization in 4-Nitrophenol and 4-Nitrophenolate Complexes: a B3LYP/6-311+G** Study

The geometries of a series of nine 4-substituted nitrophenols and 4-substituted nitrophenolates (X = H, CONH2, CHO, COOH, COCH3, COCl, CN, NO2, NO) and of their conformers, where the nitro group rotates by 10º from φ = 0° to φ = 90°, were optimized at the B3LYP/6-311+G** DFT level. These data were used to analyse the effect of rotating of the nitro group on π-electron delocalization in the ring. It has been shown that the substituent effect stabilization energy (SESE) estimated for p-substituted phenolates correlates very well with σp− constants. Based on this dependence the σp− constants for the nitro group as a function of the out-of-plane dihedral angle φ were obtained. Application of the model simulating varying strength of H-bond by approaching F− (HF) group to OH (O−) group of the 4-nitrophenol (4-nitrophenolate) with the rotating nitro group allowed to show interrelation between changes in aromaticity of the ring due to both rotation of the nitro group and changes in the strength of H-bonding. Two indices of aromaticity: Nucleus-Independent Chemical Shifts (NICS), and the Harmonic Oscillator Model of Aromaticity (HOMA) were used to quantify the aromatic character of the benzene fragment

On the aromatic stabilization of benzenoid hydrocarbons

A general scheme for estimation of aromatic stabilization energies of benzenoid hydrocarbons based on selected topological features has been presented. The reactions have been applied to benzene, naphthalene, anthracene, phenanthrene, pyrene, tetracene, benz[a]anthracene, chrysene, [4]-helicene, anthanthrene and coronene