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Substituent Constants (σ_p−) 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

Michał A. Dobrowolski ; Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
Tadeusz M. Krygowski ; Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
Michał K. Cyrański ; Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland

Sažetak

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 σ_p− constants. Based on this dependence the σ_p− 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.

Ključne riječi

nitrophenols; DFT calculations; aromaticity; hydrogen bonding; Hammet parameters

Hrčak ID:

38631

URI

https://hrcak.srce.hr/38631

Datum izdavanja:

30.6.2009.

Posjeta: 1.879 *