Complexation reaction enthalpies

All of the complexation reaction enthalpies are endothermic for all X investigated. This seems unlikely in that this complexation reaction most probably has AS < 0. Accordingly, the reaction would have a net AG > 0 and so it is not obvious how these complexes would be formed. 

Benzene addition to Ir(P Pr3)2Cl is an exothermic reaction (22kcal mol-1), while addition to Ir(P Pr3)2(CO)Cl is endothermic (—5kcal mol-1).501 The reaction enthalpies of substitution reactions to complexes containing the Ir(P1Pr3)2Cl fragment are supplied. Reaction of Ir(P1Pr3)2Cl with 2-pyridyl esters gives a p2 (C,0)-bound ketene, (307), where R2 = R1 = aryl or R2 = aryl,... 

Calorimetry investigations of zinc ions with functionalized pyridines have been carried out in both dimethylformamide and acetonitrile. The pyridines used were pyridine, 3-methylpyridine, and 4-methylpyridine. In DMF, for all three pyridines, four- and six-coordinate species formed and their formation constants, reaction enthalpies and entropies were determined. The stability increases linearly with increasing basicity of the pyridine derivative. The formation of the 3-methylpyridine complex is enthalpically less favorable and entropically more favorable than... 

Moreover, ab initio calculations indicate that additions of chlorocarbenes to ethylene or TME lead to cyclopropanes either without the intermediacy of CACs,39 or via broad shallow wells for complexes that might occur in the reaction enthalpy profile but are not minima on the free energy surface.40 More recent calculations confirm these conclusions CACs are not predicted to be im-... 

It may prove possible to apply titration calorimetry data in one further direction. If AG can be estimated for SAL-goethite complexation and reaction enthalpies can be obtained under equilibrium conditions, then an entropy change for this reaction can also be derived. This can only be done, however, if the adsorption reaction can be shown to be reversible. Since this has not been proven as yet in our systems, such thermodynamic extensions of titration calorimetry can only be speculative at this time. 

This treatment yields the time course of the relaxation which is of most concern to us, but ignores the relative magnitudes of the relaxations (contained in the X and Y terms in (1.163)). These latter are complex functions of reaction enthalpies and absorbance coefficients, but can yield equilibrium constants for the two steps.However, relaxation data are much less used for thermodynamic than kinetic information. 

The majority of reported studies of formation of cyclodextrin inclusion complexes in solution have been mainly concerned with determination of the stability constants by using equilibrium spectroscopic techniques, and the measurement of the enthalpy and entropy changes characterizing the complexation reaction. The aim of much of this work has been to determine the driving force of complex-formation. Despite the amount of research in this area, however, no general agreement has been reached, and... 

The enthalpy and entropy changes of the total complexation reaction, AH° and AS°, reflect, primarily, step 1 [Eq. (3.18a)], the dehydration snbreaction. 

In accord with calculations performed by Cavell et al. [110], the oxidative addition of C2-X functionalized azolium cations (X = halogen) to metal centers proceeds faster and with a more favorable reaction enthalpy than the oxidative addition of the C2-H substimted imidazolium cations [118, 119]. The former reaction was applied successfully for the preparation of nickel and palladium complexes bearing a variety of different ylidene ligands [119]. 

The second water molecule to be dissociated is Wat2, with a reaction enthalpy for 2a — + H2O amounting to 9.2 kcal mol-1. The pentacoordinate complex 3a (Fig. 9), with Wat2 removed, is... 

Almost 40 years ago150, the enthalpy of hydride transfer to a collection of N-substituted aldimines R1CH=NR2 was reported and compared with the reaction enthalpy of the same (complex metal) hydride with the corresponding amine R CH2NHR2 reactions 65 and 66, respectively. 

More recently, a quantitative scale for Lewis acidity based on fluoride ion affinities was calculated using ab initio calculations at the MP2/B2 level of theory.26 Due to its high basicity and small size, the fluoride ion reacts essentially with all Lewis acids thus the fluoride affinity (or reaction enthalpy) may be considered as a good measure for the strength of a Lewis acid. An abbreviated pF scale is given in Table 1.3. This scale was used recently by Christe and Dixon112 for estimating the stability of salts of complex fluoro anions and cations. The pF value represents the fluoride affinity in kcal mol 1 divided by 10. 

Titration calorimetry has been successfully employed in the determination of thermodynamic parameters for complexation (Siimer et al., 1987 Tong et al., 1991a). The technique has the advantage of employing direct calorimetric measurements and has been proposed as the most reliable method (Szejtli, 1982). It should be noted that the information derived from multistep series reactions is macroscopic in nature. In contrast to spectrophotometric methods that provide information concerning only the equilibrium constant(s), titration calorimetry also provides information about the reaction enthalpy that is important in explaining the mechanism involved in the inclusion process. 

Many complex reactions consisting of several elementary steps feature a strongly temperature-sensitive overall selectivity as well as an inversion point with a maximum or minimum selectivity parameter. However, the empirical rule that stereoselective reactions should be performed at the lowest possible temperatures to achieve the highest selectivities is not always followed. Instead, the competition of enthalpy and entropy determines the overall selectivity, depending on the temperature range. 

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