No abstract

NMR spectroscopy1 is an important method for the assessment of structural problems in organic chemistry,2 especially hydrogen-bonding phenomena.3 45Previous studies have shown that 170 NMR carbonyl data are sensitive to torsion-angle variations in aryl systems.2,4 In rigid, planar systems, 170 NMR methodology has been employed to detect lone-pair distortions (in-plane structural deformations).2,8 170 NMR data for intramolecular hydrogen-bonded aminoand amido-substituted acetophenones have been factored into electronic, torsional, and hydrogen-bonding components.6 170 NMR methodology has been shown7 8to be sensitive to the geometry as well as the relative p/Cas of intramolecular hydrogen-bonded N-H groups in rigid carbonyl systems. We report here an 170 NMR study of intraand intermolecular hydrogen bonding of phenols to ketone carbonyl groups.The 170 NMR chemical shift data for a series of intramolecular hydrogen-bonded phenols containing carbonyl groups, 1-10, in acetonitrile at 75 °C are listed in Table I. The signals for the intramolecular hydrogen-bonded carbonyl groups in 1-10 are shifted upfield.3,6,8 Data for several of the compounds listed in Table I have been reported previously.8,9 However, the 170 data were obtained under varied conditions such that quantitative analysis was difficult. In addition, the influence of electronic and torsional effects were not included in the determinations of the hydrogen-bonding components of the observed shielding effects.8,9 The contributions of intramolecular hydrogen bonding ( 6 ß) to the chemical shifts for the carbonyl groups in 1-10 ranged from 39 to 67 ppm after correction6 for electronic and torsional factors. The average 5 ß for one intramolecular hydrogen bond in these systems was determined to be 51 ± 7 ppm based upon the assumptions that the pKas of all the phenols are similar and that the basicities of the carbonyl groups are comparable.Proton coupling to the phenol oxygen is observed directly in six of the examples listed in Table I. Representative 170 NMR spectra for the OH signals ( -coupled and -decoupled) for 2,-hydroxyacetophenone are shown in Figure 1. This type of coupling can only occur when intramolecular proton transfer takes place without spin (1) (a)