Enthalpy of hydrogen bonding

Depending on amide and hydroperoxide, the equilibrium constants at room temperature vary from 0.1 to 20 L mol-1. The enthalpy of hydrogen bond hydroperoxide and amide is around 20 to 24 kJ mol-1. Three types of hydrogen bonds are formed ROOH 0 = C<, ROOH N<, and NH 0(H)0R. 

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. 

B3LYP and MP2 calculations of the enthalpies of hydrogen-bonded complexes of methanol with neutral bases and anions Comparison with experimental data ... 

The enthalpy of hydrogen bond in cyclic dimers (Fig. 20a) in the ABA and AOBA is equal to —35.3 0.8 kJ/mol per hydrogen bond at T 300 K. For the quantitative estimation of the enthalpy the Iogansen method [296] has been applied, which is based on the analysis of the shift of the frequency of twisting vibration pOH and that of the gravity center of the Voh band of associates with relation to the corresponding shifts for monomers. The determination of frequencies pQ on and of ABA and AOBA monomers, which are needed... 

Cyclic dimers, open associates, and monomers exist also in the liquid state however, with temperature the dynamic equilibrium is shifted to monomers. Note that a more simple pattern of the molecular structure is set with dissolving of the acids studied in CCLp Only cyclic dimers and monomers are found at equilibrium, and the quantity of monomers increases with dilution of the solution or with its heating open associates do not arise. The enthalpy of hydrogen bond of dimer molecules in the solution is approximately the same as in the crystal at the same temperature. 

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 ... 

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. 

Electron excitation causes only a very small change in the enthalpy of hydrogen bonding a value of about 0.00 eV is reported. ... 

For certain polymers Rider has drawn solubility maps. Thus the area of solubility was represented by a pair of symmetric quarters of a plane lying in coordinates b,C. Values of parameters were defined from data for enthalpies of hydrogen bonds available from the earlier works. The model is a logical development of the Hansen method. A shortcoming of this model is in neglecting all other factors influencing solubility, namely dispersion and polar interactions, change of entropy, molecular mass of polymer and its phase condition. The model was developed as a three-dimensional dualistic model (see Section 4.1.5). 4.1.4 HANSEN S SOLUBILITY... 

Rider proposed a model of solubility for liquids in which enthalpy limits the miscibility of polymers and solvents. For substances capable of hydrogen bonding. Rider proposed a new factor that relates their miscibility to the enthalpy of mixing that, in turn, depends on the enthalpy of hydrogen bond formation. He introduced the new parameter -the hydrogen bond potential, HBR If HBP is positive the system is more miscible and vice versa. 

Rider has drawn solubility maps for some polymers. In these, the solubihty region is represented as a pair of symmetric quarters of a plane lying in co-ordinates b and Parameters are determined from enthalpies of hydrogen bonding. The Rider model further... 

Connelly, P.R., Aldape, RA., Bruzzese, F.J., et al (1994) Enthalpy of hydrogen bond formation in a protein-ligand binding reaction. Proceeding of the National Academy of Sciences of the United States of America, 91,1964-1968. 

Coleman et al. uses Fourier transform infrared (ETIR) spectroscopy to obtain the association constants, K, for all of the hydrogen-bonding equilibria occurring in the pure polymers, and in the blends. Using a van t Hoff-type plot, the enthalpies of hydrogen bonding can be calculated. 

Kone, M., lUien, B., Graton, J. and Laurence, C. (2005) B3LYP and MP2 calculations of the enthalpies of hydrogen-bonded complexes of methanol with neutral bases and anions comparison with experimental data. J. Phys. Chem. A, 109, 11907-11913. 

The earlier one was made by Arnett and co-workers [10,11], using two different methods. The first is referred to as the high-dilution calorimetric method . In this case, a small quantity of base is injected into a dilute CCU solution of 4-fluorophenol. The observed heat of reaction is corrected for the heat due to the solution of the base in pure CCU. It is related to the enthalpy of hydrogen-bond formation (AH°) by the expression... 

The last term is constant and has the value 8.78 kJ mol In this method, no measurement in a reference inert solvent is required and the enthalpies of hydrogen bonding are anchored... 

Unlike Arnett and Catalan, Laurence el al. chose to measure the enthalpy of hydrogen bonding from the temperature dependence of the equilibrium constant. The variation of an equilibrium constant with temperature is given by Equation 4.22 ... 

Table 4.27 Determination of the enthalpy of hydrogen bonding of trimethyl phosphate (initial mole fraction 6.5 x to 4-fluorophenol (initial mole fraction 4 x W ) in CCI4.

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