Hydrogen bond formation enthalpy

The fact that water is a liquid at room temperature with high enthalpies of fusion and vaporisation can be attributed to hydrogen bond formation. The water molecule is shown in Figure 10.3. 

Some values for the enthalpy of hydrogen-bond formation are given in Tkble 2.5. The sequence of energies is in qualitative agreement with the bond lengths observed in crystals, OH 0=C < OwH- -Ow < NH- 0=Cquantitative relationships could be proposed. 

S. Itoh, D. J. Livingston, M. A. Navia, J. A. Thomson, and K. P. Wilson, Proc. Natl. Acad. Sci. U.S.A., 91, 1964 (1994). Enthalpy of Hydrogen-Bond Formation in a Protein-Ligand Binding Reaction. 

FIGURE 15. Solvatochromic hydrogen-bond shifts for 4-nitrophenol (longest wavelength ii n transition) in HBA solvents plotted against the enthalpy of hydrogen-bond formation of 4-fluorophenol in pure HBA solvents ... 

To emphasize the variation in reported experimental quantification of hydrogen bonds, we list several studies of the energetics (enthalpy or free energy) of hydrogen bond formation in biological interactions and related model systems. Early... 

Cf SF is responsible for the broad maximum at higher T. Cp captures the enthalpy fluctuations due to the hydrogen bond formation given by the terms proportional to the hydrogen bond number ApB. This term is present also in the SF model . To show that this maximum is due to the fluctuations of hydrogen bond formation, we calculate the locus of maximum fluctuation of Nhb, related to the maximum of dNHB/dT p [Fig. 8(a)], and find that the temperatures of these maxima correlate very well with the locus of maxima of [Fig. 9].We... 

The b and parameters can be used to estimate the enthalpy of hydrogen bond formation between donor and an acceptor by using Equation 22. This equation yields values in units of kcal/mol. 

Figure 1.101 Illustration of hydrophobic effect, (a) water forms an imperfect ordered solvent cage around two hydrophobic entities (blue). Each water of solvation is prevented from forming four hydrogen bonds with neighbouring water molecules for steric reasons. Waters of solvation are excluded upon association of two hydrophobic entities (b). These increase system entropy by entering bulk solution and release enthalpy through enhanced hydrogen bond formation. Short range Van der Waals interactions between hydrophobic entities may also contribute to system enthalpy.

The negligible effect of methyl groups on silicon-hydrogen bond dissociation enthalpies in methylsilanes is supported by simple correlations which only involve enthalpy of formation data. For example, when AH°(Me SiH, g) are plotted against AH°(Me SiH, g) (n = 0-3), the excelient linear fitting. Equation 10, implies " that the enthalpy of Reaction 11 is constant, which in turn... 

---