Interaction enthalpies

NMR signals of the amino acid ligand that are induced by the ring current of the diamine ligand" ". From the temperature dependence of the stability constants of a number of ternary palladium complexes involving dipeptides and aromatic amines, the arene - arene interaction enthalpies and entropies have been determined" ". It turned out that the interaction is generally enthalpy-driven and counteracted by entropy. Yamauchi et al. hold a charge transfer interaction responsible for this effect. 

Kieninger and Suhai122 applied the DFT(SVWN), DFT(B88/P86), and DFT(PW86/P86) methods to study the NH3. . NH+ complex. The BSSE corrected interaction enthalpies obtained using nonlocal functionals were in a fair agreement with the results deduced from experiment and with the ones derived from the MP4 calculations. The corresponding values amounted to -32.0, -26.0, —27.6, —25.9, and —24.8 kcal/mol for DFT(SVWN), DFT(B88/P86), DFT(PW86/P86), MP4, and experiment, respectively. 

In order to compare the computed results with those obtained from the experiment, the interaction enthalpy at 0 K has to be evaluated. The interaction enthalpy, AH, differs from the AE by the change in the zero point energies, AZPE, between the complex and the isolated subsystems. 

Since CSj is by far the best solvent for S rings, a special interaction between the two kinds of molecules has long been suspected. From the temperature dependence of the CS2 solubility in liquid sulfur at 94-158 °C an interaction enthalpy of 27 kJ/mol has been calculated s. S,2 is the only sulfur molecule which crystallizes from CSj as an adduct its crystal structure shows, however, that there is no stronger than van-der-Waals interaction between the two components. The solvent molecules just occupy the empty spaces of the Sj2 layer lattice The conformation of sulfur rings can be described in terms of the so-called motif which is the order of the signs (-1- or —) of the torsional angles around the ring. If 0 stands for t = 0°, then the motifs are ... 

The principle of the liquid chromatography under critical conditions (LC CC) was elucidated in Section 16.3.3. The mutual compensation of the exclusion—entropy and the interaction—enthalpy-based retention of macromolecules can be attained when applying in the controlled way the interactions that lead to either adsorption or enthalpic partition. The resulting methods are called LC at the critical adsorption point (LC CAP) or LC at the critical partition point (LC CPP), respectively. The term LC at the point of exclusion-adsorption transition (LC PEAT) was also proposed for the procedures employing compensation of exclusion and adsorption [161]. It is anticipated that also other kinds of enthalpic interactions, for example the ion interactions between column packing and macromolecules can be utilized for the exclusion-interaction compensation. 

To rationalize the effect of polar groups on and Sj, we can imagine that polar interactions with the water molecules around the solute cavity replace some of the hydrogen bonds between the water molecules. As indicated by the experimental data, this loss of water water interaction enthalpy seems to be compensated by the enthalpy gained from the organic solute water polar interactions. At this point it should also be mentioned that additional polarization effects could enhance the interaction between the organic solute and the water molecules in the hydration shell... 

The number of binding sites on the protein, n, and the binding constant, K, were determined from the Y-intercept and the slope. Thermodynamic parameters of free energy of interaction, enthalpy and entropy were found from K. 

Fig. 4. (A) The general case of a dipole-dipole interaction. The angles 0, and 02 define the orientation of the two electronic dipole moments and with respect to the vector r, which passes from the center of one dipole to the next. The magnitude of the vector is the separation of the two dipoles. (B) The colinear dipole-dipole interaction, the enthalpically optimal case. (C) The antiparallel dipole-dipole interaction, which is often seen between macroscopic helix dipoles in proteins. (D) The parallel dipole-dipole interaction, which is enthalpically unfavorable. (E) The behavior of dipole-dipole interaction enthalpy as a function of the parameter 02 with 0, = 0°.

Table 12-1. Interaction enthalpies and Gibbs free interaction energies for gas phase and liquid phases using the BLYP/SVP+sp approach. Energies are in kcal/mol...

The Flory-Huggins parameter Xi was initially derived as an experimentally determined interaction enthalpy parameter which was supposed to be independent of poljnner concentration. However, experiments have shown that it is dependent on pol5mier concentration. Evans and Napper [40] have shown that the polymer volume fraction dependence of X can be given by... 

Yanson et al. [41] using field-ionization mass spectrometry studied the formation of gas-phase GC, CC, AT and TT pairs. From measurements of temperature dependence of equilibrium constants, an interaction enthalpy for the base pair formation was derived. This technique was sometimes questioned because the determination of enthalpy from the slopes of appropriate van t Hoff curves might not be unambiguous. From Table 6 it is evident that the agreement with the present theoretical values is good, and concerns not only the relative interaction enthalpies but even the absolute values the average absolute error is less than 1.5 kcal/mol. 

Agreement between experimental and theoretical interaction enthalpies at 380 K is very good what gives confidence to both, theoretical calculations and experiment. 

In this equation, Aads f corresponds to the enthalpy difference between occupied and unoccupied adsorption sites and contains Meads-s be difference of the solvation enthalpies of Meads and S. Asub is the sublimation enthalpy, which is related to the interaction enthalpy per Me bond, Hle-Me, approximated as y/i in the case of first nearest neighbors (cf. eqs. 2.2 and 2.3). The terms and Vads represent the mean vibrational volumes of an atom in a kink site position or in an adatom position, respectively [3.269]. They are related to the mean atomic vibration frequencies in the 3D Me bulk lattice and in the Meads overlayer, respectively. 

Molecular mechanics (AMBER, Singh et al., 1986) was employed by the authors to study the effect of the whole system (protein + solvent) on the scissile bond in the Michaelis complex and in the TI. They find that interaction enthalpy is more favorable in the TI due to stabilization by the catalytic triad, by the oxyanion hole and by the environment, a total of 26 kcal/mol. The breakdown to interaction components demonstrates the importance of electrostatic stabilization of the TI. More reliable estimates of these interactions require free energy perturbations or related free energy methods. 

In some instances, the precise estimation of the interaction enthalpy AH or the stoichiometry n rather than the affinity constant fCassoc is desired. Then, raising the c-value well over 1000 by an increase of the initial concentration is a beneficial option. The titration curve will then appear as a step (jump) function as in Fig. 3.4 because the titrant added in aliquots from the syringe will be totally converted to the complex in each addition until the reaction partner in the cell is consumed completely. The subsequent injections will only show the spurious heats of dilution and mixing and thus will end in parallel to the molar ratio axis. The jump... 

Skerjanc J, Kogej K, Vesnaver G. Poly electrolyte-surfactant interactions. Enthalpy of binding of dodecyl- and cetylpyridinium cations to polyfstyrene-sulfonate) anion. J Phys Chem 1988 92 6382—6385. 

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