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Hydrogen bond enthalpies (AHAs) for the interaction of p-fluorophenol with 65 bases have been determined calorimetrically, using two independent methods wherever feasible. Heats of protonation (AH's) in fluorosulfuric acid for these bases have been measured also. AH¡ and AH, are used to compare the energetics of hydrogen bonding and proton transfer in solution, and it has been found that no single relationship exists to correlate protonation and hydrogen bonding, but that separate lines are necessary for different functional groups. If AH, is plotted vs. AH:, points representing data for basic types such as amides, phosphoroxy compounds, pyridines, sulfides, and sulfoxides fall on separate parallel lines. Solvent effects on AH¡ are discussed especially with regard to recent attempts to correct for them. AH¡ is correlated with various acid-base solvation parameters and we find that Gutmann's donicity numbers, Drago's E and C parameters, or Av values (the Badger-Bauer relationship) can be used to estimate reasonable AH; values, often within about 0.5 kcal/mol of our experimental results. AH; values and independently measured equilibrium constants for hydrogen bond formation (KAs) are used to consider the extrathermodynamic relations between AG A, AHA, and ASA Neither AG A nor ASA showed any general correlation with AHA, but some of the data could be resolved into separate trends for different groups of bases. Moreover, large changes in AGf°and AHA for pyridines, sulfoxides, amides, and phosphoroxy compounds are found to be nearly independent of entropy changes. The relation of current theories of the hydrogen bond is examined and attention is drawn to conceptual fuzziness in the definition of hydrogen-bonded systems. In conclusion, the advantages of using proton affinities in the gas phase as a primary reference point for discussing "basicity" are cited.

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