STUDIES OF HYDROGEN-BONDED COMPLEX FORMATION WITH P-FLUOROPHENOL .4. FLUORINE NUCLEAR MAGNETIC RESONANCE METHOD

Using 0.01 M p-FC6H4OCH3 as an internal reference standard in CCl4, the shielding effects of hydrogen-bonded complex formation of 0.01 MP-FC6H4OH have been evaluated quantitatively. Using this method and reference acid, the formation constant, Kf, and limiting F nmr shifts, ∆, for formation of the 1:1 hydrogen-bonded complex have been obtained for 62 bases of widely different structures in CCl4 at 25 Some additional formation constants have been obtained in CCl4 at –20°. In general, the van't Hoff enthalpies of complex formation based upon Kf values at 25 and –20° are in satisfactory accord with the calorimetric values obtained in the companion collaborative study. A linear relationship between corresponding values of log Kf and A is shown to apply with generally good precision, except for bases of large steric requirements. This empirical relationship provides evidence that the potential energy change in the formation of the hydrogen-bonded complex is approximately proportional to the standard free energy change. The F nmr A values are essentially independent of temperature in CCl4 and are not increased in the polar solvent, ClCH2CH2Cl, indicating the formation of only hydrogen-bonded complexes (no appreciable contribution from the hydrogen-bonded ion pair, FC6H4O−—+HB). The ∆ values are interpreted as showing that a small extent of proton transfer (apparent proton transfer = 10-30%) occurs in the hydrogen-bonded complexes of p-FC6H4OH. It has been conclusively established that the aqueous pKA and the log Kf scales of base strengths for common functional groups are unrelated. © 1969, American Chemical Society. All rights reserved.