Position-dependent rate thermodynamics

Changing the chirality at the Ca atom in proline-containing tetrapeptides showed position-dependent thermodynamic and kinetic effects on the CTI [70], Positions adjacent to proline were found to be critical for spontaneous bond rotation. Compared with L-amino acids, D-amino acids in positions preceding and following proline, lead to an increased cis population. Considering the stereospecificity of rate constants Ca chirality affects CTI mainly due to destabilization of the planar trans state relative to the twisted transition state of rotation. 

This reaction is thermodynamically spontaneous because the standard potential of the Pd couple is more positive than that of the ferrocene couple. The equilibrium position and rate of the heterogeneous ET reaction at the ITIES, however, depends on the phase boundary potential, which can be controlled externally for electrodeposition or by a common ion in the aqueous and organic phases for electroless deposition. Similarly, Pt NPs were deposited at nano-ITIES arrays. A more detailed discussion of particle deposition at the ITIES is given in the next section. 

While the theoretical potential of CO and methanol are positive to the hydrogen reference, for formic acid the data is negative. But experiments show (Sect. 5.2.3) that open-circuit potentials are much more positive than the thermodynamic values, especially in the case of formic acid. This is a consequence of the missing reaction equihbrium. In addition, the increase in rate positive to the open-circuit potential is much less than in the case of hydrogen oxidation. The rate of charge transfer at a given potential depends strongly on the catalyst and its surface structure. 

Finally, a consideration of equilibrium chemistry can only help us decide what reactions are favorable. Knowing that a reaction is favorable does not guarantee that the reaction will occur. How fast a reaction approaches its equilibrium position does not depend on the magnitude of the equilibrium constant. The rate of a chemical reaction is a kinetic, not a thermodynamic, phenomenon. Kinetic effects and their application in analytical chemistry are discussed in Chapter 13. 

The above method is unsatisfactory when hydration takes place at two alternative sites in the molecule, although one hydrate is usually present in only a very small proportion, at equilibrium. Which oxo compound is preferentially formed in such a case depends on the rates of oxidation at the different sites and on the rate of isomerization of the water molecule from one position to the other, hence this method does not indicate which is the thermodynamically more stable hydrate. 

Double bonds tend to migrate to more highly substituted positions within a substrate that is, terminal alkenes isomerize to disubstituted or trisubstituted alkenes, disubstituted alkenes tend to migrate to trisubstituted, and trisubstituted to tetrasubstituted alkenes. Of course, migration can go both ways, and adsorbed surface species may not exhibit the same thermodynamic stability as their desorbed relatives. (The rate of migration is strongly catalyst dependent for example, it frequently occurs rapidly on Pd and slowly on Pt.)... 

Quinolizine exists as a mixture of up to three species, namely the 2H-, 4H- and 9a/7-tautomers (17 and 18, respectively). This tautomeric equilibrium has been extensively studied in the case of tetramethyl quinolizine-1,2,3,4-tetracarboxylate, where, as previously mentioned, the 4/7-tautomer 17 has been shown to be thermodynamically more stable than the 9/7-species 18 through ab initio Hartree-Fock calculations. An interesting feature of these compounds is that the rate of the interconversion of the 9a//-tautomer 18 into the more stable 4//-cornpound 17 depends on the position of substituents in ring B <2003JST651>. 

Situations that depart from thermodynamic equilibrium in general do so in two ways the relative concentrations of different species that can interconvert are not equilibrated at a given position in space, and the various chemical potentials are spatially nonuniform. In this section we shall consider the first type of nonequilibrium by itself, and examine how the rates of the various possible reactions depend on the various concentrations and the lattice temperature. 

The dependence of rate constants for approach to equilibrium for reaction of the mixed oxide-sulfide complex [Mo3((i3-S)((i-0)3(H20)9] 1+ with thiocyanate has been analyzed into formation and aquation contributions. These reactions involve positions trans to p-oxo groups, mechanisms are dissociative (391). Kinetic and thermodynamic studies on reaction of [Mo3MS4(H20)io]4+ (M = Ni, Pd) with CO have yielded rate constants for reaction with CO. These were put into context with substitution by halide and thiocyanate for the nickel-containing cluster (392). A review of the chemistry of [Mo3S4(H20)9]4+ and related clusters contains some information on substitution in mixed metal derivatives [Mo3MS4(H20)re]4+ (M = Cr, Fe, Ni, Cu, Pd) (393). There are a few asides of mechanistic relevance in a review of synthetic Mo-Fe-S clusters and their relevance to nitrogenase (394). 

This interconversion can also be performed in solvents, and the rate of the isomerization is dependent on the solvent used. In the dipolar aprotic solvent DMSO the rate of the reaction is fast, but in methanol, acetone, or dioxane the rate is low. However, the value of the equilibrium constant is scarcely influenced by the solvent ( 134/133 = 6-10) (75JHC985).This is not too surprising, since the equilibrium position is controlled by the relative thermodynamic stability of the isomers, which is a function of their heats of formation and of solvation. Undoubtedly, the heat of formation is the more important factor to the thermodynamic stability (75JHC985). 

An internal electrochemical mechanism was proposed long ago for deposition on certain metal substrates, since the rate of deposition sometimes depended on the nature of the substrate [11].) The standard potential of Reaction (5.3) is -l- 0.08 V, considerably more positive than the rednction potential of S to (-0.45 V). Free sulphide, if formed, would be in a very low concentration, since it will be removed continually by precipitation of PbS this will move the S rednction potential strongly positive according to the Nemst equation [Eq. (1.32)]. This positive shift will be even greater than normal because of the non-Nemstian behavior of the S /S couple when [S] > [S ] (at least in alkaline solntion) [12]. In opposition to this, the solubility of S in the (slightly acidic) aqneons solntions is very low, which will move the potential in the opposite direction. Add to this the very small concentration of S in acid solution [Eq. (1.15)], and it becomes clear that it is not trivial to estimate the feasibility of the formation of PbS by free snlphide. The non-Nemstian behavior of the sulphur-rich S /S couple and the lack of knowledge of the solnbility of free S in the deposition solntion are the two factors that complicate what would have been a tractable thermodynamic calcnlation. 

A large number of molecules can react in this way and typically HX contains an H—S or H—O bond or else is a hydrohalic acid. There are both kinetic and thermodynamic considerations as to whether this type of reaction can take place. Firstly, the mechanism of the reaction rarely involves direct protonation of the M—OR bond. Instead, initial coordination of HX through lone pairs of electrons on X is necessary prior to transfer of die proton. Hence, the rate of the reaction will be dependent on the steric constraints of both HX and the metal coordination sphere as well as the electronic donor-acceptor properties of the two substrates. Thermodynamically the position of the equilibrium will depend on a number of variables, the relative strengths of the M—O and M—X bonds being important ones. 

---