17O NMR spectroscopic investigation of intramolecular hydrogen bonding for ortho-nitrophenols

Abstract

Natural abundance 17O NMR chemical shift data for seventeen nitrophenols and four nitrobenzene control compounds in acetonitrile at 75°C are reported. The phenolic OH 17O NMR chemical shift for ten 4-substituted-2-nitrophenols is correlated with Hammett σ − values ( r = 0.982); however, the magnitude of the rho value in this correlation is reduced by 25% compared to the value for 4-substituted phenols. For the ortho-nitrophenols, after correction for torsion angle and electronic effects, the nitramolecular hydrogen bonding component (Δδ HB) of the 17O NMR chemical shift of the nitro group was determined to be 19 ± 2 ppm (shielding); two cases with strong electron withdrawing groups present gave a value of 14 ppm. The Δδ HB) value for the nitro group hydrogen bond acceptor and literature values for other hydrogen bond acceptor functional groups qualitatively parallel basicity of the functional groups. 17O NMR data in toluene at 75°C for six ortho-nitrophenols and two phenolic control compounds are presented. The Δδ ref value (the change in chemical shift of internal 2-butanone due to inter- molecular hydrogen bonding) in toluene solution for the highly hindered 3-methyl-2-nitrophenol suggests both intermolecular and intramolecular hydrogen bonding contributions to the chemical shift. The Δδ ref value for 2,6-dihydroxynitrobenzene in toluene provides clear evidence that both oxygens of the nitro group are fully involved in intramolecular hydrogen bonding.