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, COON, COCH3, COCl, CN, NO2, NO) and of their conformers, where the nitro group rotates by 10 from phi = 0 degrees to phi = 90 degrees, 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 pi-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 sigma(-)(p) constants. Based on this dependence the sigma(-)(p) constants for the nitro group as a function of the out-of-plane dihedral angle 0 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.