Substitution order

Both electrophilic and nucleophilic reactions can generate halogenopur-ines with differences in regioselectivity dependent on substituents and on the nature of the substrate (anion, neutral molecule, or cation). In the neutral molecule nucleophilic displacements occur in the order 2 > 4 > 6 in the anion the imidazole ring may be sufficiently 7r-excessive for attack to occur at C-2, and the nucleophilic substitution order becomes 4 > 6 > 2. Strong electron donors are usually necessary to promote 2-halogenation by electrophilic halogen sources. 

A first group of superstructures, described in several paragraphs of this chapter and of Chapter 7, must be mentioned these include the types tP2-AuCu(I), cP4-AuCu3 and tP4-Ti3Cu which can be considered face-centred cubic-based substitutional ordered superstructures. 

Conversion of the halogenopurines into protonated forms should lead to a reversal of the substitution order and this is generally but not always the case. Thus reaction of 6,8-dichloropurine (58JA6671) or 2,6,8-trichloropurine <1897CB2220> with hot acid gave... 

Validate a quantitative problem solution by applying back-substitution, order-of-magnitude estimation, and the test of reasonableness. 

A small group of systems involving fee (100) surfaees, first investigated somewhat earlier than the (111) examples deseribed so far, inelude the e(2x2) surfaee phases formed by Mn on Cu(lOO), Ni(lOO) and Pd(lOO) and by Au and Pd on Cu(lOO). In all these eases the surfaee layer eomprises half a layer of the substrate speeies and half a layer of the adsorbate speeies to form a substitutional ordered surfaee alloy layer of 1 1 stoiehiometry (fig. 7). Cu/Au... 

Heixjeson, H. C. 1985, Activity/composition relations among silicates and aqueous solutions. I. Thermodynamics of intrasite mixing and substitutional order/disorder in minerals. Am. J. Sci. 285 769-844. 

DFT studies of binary hard-sphere mixtures predate the simulation studies by several years. The earliest work was that of Haymet and his coworkers [221,222] using the DFT based on the second-order functional Taylor expansion of the Agx[p]- Although this work has to some extent been superceded, it was a significant stimulus to much of the work that followed both with theory and computer simulations. For example, it was Smithline and Haymet [221] who first analyzed the Hume-Rothery rule in the context of hard sphere mixture behavior and who first investigated the stability of substitutionally ordered solid solutions. The most accurate DFT results for hard-sphere mixtures have come from the WDA-based theories. In particular the results of Denton and Ashcroft [223] and those of Zeng and Oxtoby [224] give qualitatively correct behavior for hard spheres forming substitutionally disordered solid solutions. 

In somewhat earlier work, Vlot et al. [229,230] made calculations of Lennard-Jones binary mixtures in which the pure components are identical but in which the unlike interactions have departures from the Lorentz-Berthelot combining rules. They use this as a model of mixtures of enantiomers. A variety of solid-fluid phase behavior can be obtained from the model. Both substitutionally ordered and substitutionally disordered solid solutions were found to occur. 

Although most of the studies of this model have focused on the fluid phase in connection with the theory of electrolyte solutions, its solid-fluid phase behavior has been the subject of two recent computer simulation studies in addition to theoretical studies. Smit et al. [272] and Vega et al. [142] have made MC simulation studies to determine the solid-fluid and solid-solid equilibria in this model. Two solid phases are encountered. At low temperature the substitutionally ordered CsCl structure is stable due to the influence of the coulombic interactions under these conditions. At high temperatures where packing of equal-sized hard spheres determines the stability a substitutionally disordered fee structure is stable. There is a triple point where the fluid and two solid phases coexist in addition to a vapor-liquid-solid triple point. This behavior can be qualitatively described by using the cell theory for the solid phase and perturbation theory for the fluid phase [142]. Predictions from density functional theory [273] are less accurate for this system. 

Order-disorder transitions are continuous transformations that are characterized by an order parameter that changes continuously from 1 at very low temperatures to 0 at the transition temperature. An ordered solution (in other words a compound) becomes a disordered alloy at temperatures above the transition point. Examples of materials having order-disorder transitions are ionic conductors and ferromagnetic and ferroelectric compounds. Substitutional order transitions involve diffusion and are sluggish those involving rotational disorder are rapid. 

For reaction 10, we find (28,29) that the series of substituents shown constitute a kind of substitution order in which... 

Instead, characteristic atomic chains formed by the same chemical species can be observed. This phenomenon is called short-range order (SRO) and may heavily influence the energetics of the alloy surface and consequently its stability. A quantitative description of metal alloy surfaces has to take such substitutional ordering effects into account One even has the possibility to quantify SRO by the so-called ordering parameters, which will be discussed in the next section. 

Such antisites have, for example, been detected at the (100) surface of B2-CoAl [114], which should be purely Al-terminated (as this is clearly favored over Co termination [116]). The existence of Co atoms (the B atoms in our case) in the top layer (Co antisites) is indeed due to a tiny surplus of Co in the real crystal [114,115]. Alternatively, such small deviations from the ideal 50 50 concentrations could be realized by A1 vacancies however, for CoAl(lOO), those clearly are energetically less favorable [117], wherefore vacancy segregation can be neglected. We will now take this example to demonstrate how it is possible to construct a surface phase diagram by coupling DFT with statistical physics. This will allow us to predict the preferred substitutional ordering at the surface as function of temperature and bulk concentration. 

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