Thermodynamic electronic effect

Besides electronic effects, structure sensitivity phenomena can be understood on the basis of geometric effects. The shape of (metal) nanoparticles is determined by the minimization of the particles free surface energy. According to Wulffs law, this requirement is met if (on condition of thermodynamic equilibrium) for all surfaces that delimit the (crystalline) particle, the ratio between their corresponding energies cr, and their distance to the particle center hi is constant [153]. In (non-model) catalysts, the particles real structure however is furthermore determined by the interaction with the support [154] and by the formation of defects for which Figure 14 shows an example. 

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

Among the factors which govern the thermodynamic localization of the redox potential, in addition to the previously mentioned electronic effects, there are also the steric influences.134... 

La Mar (150) and Walker (156) have found a thermodynamic cis effect in the formation of hemichrome salts [Fe(TRP)L2]Cl (->-[27]) according to equilibrium (77) which was studied by 1H-NMR and optical spectroscopy for L = l-Melm (Table 22). As the electron-donating power of the para-phenyl-substituent of the porphyrin increases, the total formation constant, /J2, increases. This is because the product of the reaction contains a positively charged center which is stabilized by electron-donating groups. As a Hammet relation exists, the mesomeric part of the electronic transmission is also operative, and hence dative porphyrin-to-metal tr-bonding seems to be involved. 

The study of lactonization via an intermediate phenonium ion has been further pursued for several methyl 4-aryl-5-tosylhexanoates (55) as substrates. The intermediate phenonium ion (56) has two possibilities for ring closure, yielding products (57) or (58). In all the substrates, Ar contained one or two methoxy groups and sometimes also a methyl group. The effects of reaction medium, temperature, and time on the product ratios were examined. It was concluded that substrates (55) give y-lactone (57) selectively under thermodynamic conditions, but 5-lactone (58) under kinetic conditions. Substituents in Ar influence the selectivity through their electronic effects. 

Wood B. I (1980). Crystal field electronic effects on the thermodynamic properties of Fe minerals. In Advances in Physical Geochemistry, vol. 1, S. K. Saxena (series ed.). New York-Heidelberg-Berlin Springer-Verlag. 

The system aminophosphorane (116)-phosphazene (116a) (Scheme 29) studied by Sanchez et al,191 was found to behave differently only 116a can be detected by NMR, i.e. there is no equilibrium. Exactly the opposite situation was found with the system 117-118, in which the only observable species was the aminophosphorane 117. Evidently, the increase in thermodynamic stability results from the formation of a spirophosphorane structure. Similar conclusions were reached by Gololobov et al.191 in the course of a study of structures similar to that of 117. More recently, Stegmann et al.193 extended the scope of their research to substituted 1,2-aryldiamines in a study of the equilibria 119 119a and 120 120a. The thermodynamic parameters AH, AG and AS were determined by NMR. Here too, the position of the equilibrium was found to depend on the substituents (steric and electronic effects), on the solvent and on the temperature. 

A consideration of these relationships reveals8 that because E° is a thermodynamic parameter and represents an energy difference between two oxidation states and in many cases the spectroscopic or other parameter refers to only one half of the couple, then some special conditions must exist in order for these relationships to work. The special conditions under which these relationships work are that (a) steric effects are either unimportant or approximately the same in both halves of the redox couple and (b) changes in E° and the spectroscopic or other parameters arise mainly through electronic effects. The existence of many examples of these relationships for series of closely related complexes is perhaps not too unexpected as it is likely that, for such a series, the solvational contribution to the enthalpy change, and the total entropy change, for the redox reaction will remain constant, thus giving rise to the above necessary conditions. 

In order to distinguish the different Me-H interactions (such as size effects and electronic effects) in transition metal hydrides, the thermodynamics of H solutions have been carefully studied. Hydrogen activities can be established electrochemically at metal surfaces by using the metal as a hydrogen electrode (cathode). If the proton activity (pH) has been predetermined in an appropriate aqueous solution, the equilibrium hydrogen activity is determined through the electrochemical reaction H+(aq) + e (Me) = H. However, when we study the kinetics of the hydrogen electrode, various reaction steps such as... 

It can be seen from the examples in Table II that both (+1) and (—I) substituents in the 4-position, as exemplified by methyl and trifluoro-methyl groups, respectively, can sterically hinder the attack of water molecules on C(4), so that attack occurs in the 5,6-7,8 positions. After the lapse of some hours, however, the hydration of 4-trifluoromethyl-pteridine cation has shifted to the 3,4-position, which is thermodynamically favored by the (—1) effect of the substituent.42 When a (— I) group is not merely hindering but, from its bulk, actually preventive, the water molecules are permanently excluded from the 3,4-position even when the electronic effects are favorable the 4-ethoxycarbonyl-... 

The arrangement of atoms in a molecule is based on attractive and repulsive forces (see Fig. 2.2), as well as the directionality of the bonds, which is determined by the orientation of the bonding orbitals and their desire for maximum overlap. At first glance, the simple mechanical model (see Fig. 2.1) does not explicitly include specific electronic interactions. However, in developing a model that reproduces experimentally derived structural and thermodynamic data, it is inevitable that electronic factors are included implicitly to account for electronic effects responsible for some of the structural and thermodynamic variation present in the data used in the parameterization. Depending on the model used, the electronic effects may not be directly attributable to specific parameters. 

A priori, a 1-OH substituent will have a rather small electronic effect. A donor group placed on the negatively charged part of the transition state is unfavorable, but it populates the <7co orbital, which is favorable. According to Yoo and Houk,70 the electronic effect lowers the activation energy only by 0.4 kcal mol-1. The remaining 2.3 kcal mol-1 are due to the thermodynamic factor, an a-hydroxyketone being more stable than an enol ether ... 

Wood, B. J. (1981) Crystal field electronic effects on the thermodynamic properties of Fe2+ minerals. In Thermodynamics of Minerals and Melts. (R. C. Newton, A. Navrotsky B. J. Wood, eds Springer-Verlag, New York), Adv. Phys. Geochem., 1,63-84. 

The additions of H20 or alcohols to the C=0 double bond of carbonyl compounds as well as the oligomerizations or polymerizations of aldehydes are reversible reactions. Therefore, the extent of product formation is subject to thermodynamic control. The equilibrium constant of the formation of the respective addition product is influenced by steric and electronic effects. 

---