Enthalpic factors

Cu (en) + xchange in ig. 4. For the case of Wyoming bentonite the Ag(en), - Cu(en) reaction is endothermic (12.5 kJ Eq ), while AG 9.665 kJ (see table IV), showing that enthalpic factors also rule the exchange among complex ions. 

Simple thermodynamic considerations state that the reduction process is favoured (i.e. more positive cu(ii)/cu(p potential values are obtained) if the electron transfer is exothermic (AH° negative) and if the molecular disorder increases (AS° positive). It is therefore evident that the positive potential value for the reduction of azurin (as well as that of the most blue copper proteins) is favoured by the enthalpic factor. This means that the metal-to-ligand interactions inside the first coordination sphere (which favour the stability of the reduced form over the oxidized form) prevail over the metal complex-to-solvent interactions inside the second... 

The basic structure of taxol is that of a tetracyclic compound -A, B, C and D rings-, in which the central B ring is an eight-membered carbocycle. In principle, the formation of this medium sized ring appears somewhat problematic because of both entropic and enthalpic factors. 

High-level computational methods are limited, for obvious reasons, to very simple systems. In the previous section we showed the contribution of the theory for a better imderstanding of the entropic and enthalpic factors that influence the reactions of hydrogen atom with the simplest series of silanes Me4 SiH , where n = 1-3. Calculated energy barriers for the forward and reverse hydrogen atom abstraction reactions of Me, Et, i-Pr and t-Bu radicals with Me4- SiH , where n = 0-3, and (H3Si)3SiH have been obtained at... 

In the classical chelate effect, the enthalpic changes are supposed not to be altered by linking X and Y as shown in Eq. 3, but the enthalpic factor also contributes to the enhanced ligation, giving much increased complex stabilities in the case of linked ligand X-Y ... 

These results clearly indicate that the chelate ligation is driven primarily by the enthalpic factor and the entropy plays merely a trivial role in determining the complex stability. This is quite reasonable since the structures of these chelate complexes are strictly defined by the number and direction of the coordination sites of given heavy/transition metal ions, and therefore there is little room for the entropic term to adjust flexibly the complex structure and stability. On the contrary, alkali and alkaline earth metal ions also have the formal coordination numbers, but the actual number and direction of ligand coordination are highly flexible in the weak interaction-driven ligation by hard donors like glyme and crown ether. 

The enthalpic factor, exp(—AH°/RT), clearly dictates the T dependence of Keq, whereas the entropic factor, exp(AS°/R), is T-independent. The exponential dependences in (8.29) remind us (cf. Sidebar 8.1) that even chemically modest changes in AH°, A5°, or T can have profound effects on Keq. 

The /1,/f-difluorinated radicals are more interesting in that RCH2CF2CH2- is ca. five times as reactive as RCH2CH2CH2- in both hydrogen atom abstractions from n-Bu3SnH and in additions to styrenes. Since the RCH2CF2CH2- radicals are effectively planar, their enhanced reactivities must derive from either polar or enthalpic factors. The latter is probably the more important. That is, the C-H... 

To explore the unusual display of reactivity of Mo(N[R]Ar)3 ([Mo]), the 0x0 complex Mo(0)(N[R]Ar)3 was synthesized by an independent route according to equation (25) and the nitrosyl compound Mo(NO)(N[R]Ar)3 is obtained according to equation (26). The reaction chemistry of equations (23), (25), and (26) were studied in detail to get kinetic data for these transformations and solution calorimetry studies were carried out in order to assess enthalpic factors... 

In contrast to Ann, Ann 0 for all mixtures investigated, and hence, the polymers or proteins are preferentially hydrated (solvated in the case of toluene + polystyrene mixture). Let us examine separately the contributions to Ann provided by the entropic and enthalpic factors. The contribution to Ann provided by the different sizes of the solvent and polymer (protein) molecules will be evaluated from the excess in an ideal... 

Details about ILs properties are covered in this book in the contributions by Seddon, Chiappe and Scott. However, two features deserve a comment for their possible consequences on reactivity and catalysis. First, depending on a delicate balance of entropie and enthalpic factors, including the polarity of the transition state structures with respect to regents, a reaction can be either speeded up or decelerated when carried out in an ionic liquid medium compared to a molecular solvent. An elegant study by Welton shows that in S-,2 reactions, primary, secondary and tertiary amines are more reactive as nucleophiles in ionic liquids, while halides react faster in conventional molecular solvents such as CH2CI2. In particular in a series of [Bmim] salts the order of nucleophilicity of halides is determined by the anion partner. To the same direction moves a kinetic study by Dyson on a cationic Ru(II) complex-catalysed hydrogenation of styrene in ILs, where it is clearly demonstrated that both the cation and the anion of the IL can inhibit or accelerate the formation of the active catalytic species. ... 

Notice that V is a (Gibbs) free energy. It can thus be due to enthalpic factors (as in a fat crystal network) or to entropic factors (as in a rubberlike material). Often, both enthalpy and entropy are involved. 

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