Basicity. Comparison of hydrogen bonding and proton transfer to some Lewis bases

Abstract

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.

Revisiting Catalysis by Chymotrypsin Family Serine Proteases Using Peptide Substrates and Inhibitors with Unnatural Main Chains

“…In contrast to their structural similarity, however, the hydrogen bonding properties of ester linkages within a peptide main chain are altered in two major ways compared with natural peptide bonds. First, the ester carbonyl has a pK a that is 4 units lower than that of an amide carbonyl; consequently, the ester carbonyl is a much weaker hydrogen bond acceptor than an amide carbonyl (35,38). Second, the hydrogen bond donating amide-NH is replaced with the electronegative oxygen atom of the ester.…”

Amide hydrogen bonding in organic medium

“…The pure base calorimetric method was used for all calorimetrically determined enthalpies reported in this work. This method was developed for systems of the type studied here, i.e., systems with low formation constants and small enthalpy changes (17). According to the pure base method, if a small quantity of hydrogen bonding acid is injected into the pure base as solvent, two contributions to the heat observed are involved, the heat due to hydrogen bonding and that heat term which might occur if there were no hydrogen bonding.…”

Studies of Densities, Viscosities and Ultrasonic Speeds of Binary Mixtures Containing Isopropyl Alcohol and Ketones at Different Temperatures

“…The intermolecular free length decreases with decrease of temperature and hence the close packing of molecules which in effect decreases the sound velocity [5,6]. The compressibility decreases with increase of velocity that gives insight into the structure making and structure breaking of components in binary mixtures [7,[18][19][20]. In this work we report the experimental values of the density, viscosity and ultrasonic velocity of the above binary mixtures at the temperatures 298.15 and 308.15 K. These data have been used to calculate various acoustical parameters like adiabatic compressibility (β), free volume (V f ) and free length (L f ).…”