Minimum structural change principle

The preceding discussion has shown that the major course of the reduction of multiply unsaturated compounds can be understood in terms of a relatively small number of elementary reactions. Other reactions have been postulated for various reasons and it is obviously desirable to find criteria for judging the probable importance of the many conceivable changes. Perhaps the most important criterion is an experimental one which is coupled with the principle of minimum structural change. Thus the demonstration that 2-butyne yields, almost exclusively, cis-2-butene-2,3-du implies that the structure (A), a logical... 

The possibility that reacting species prefer to react along those paths in which they undergo the least modification has always been intuitively attractive. At one time or another, so-called principles of minimum structural change or deformation, configurational change, and minimum atomic and electronic motion have been invoked (Wheland, 1960 Hine, 1966). To account for Michael s rule of favored anti 1,2-addition, Pfeiffer formulated acetylenes as tram-heat structures in 1904 Frankland (1912) suggested that anti elimination is favored by an inherent tendency to centric symmetry. The more conscious applications of PLM by Muller after 1886, are probably misapplications of the principle, since they were usually concerned with complex pyrolytic reactions above 1000° (Muller and Peytral, 1924). 

Jochum C, Gasteiger J, Ugi I, Dugundji J (1982) The principle of minimum chemical distance and the principle of minimum structure change, Z Naturforsch 37B 1205... 

Other macroscopic properties that in principle can be measured are the excess density and the excess compressibility of the interfacial liquid. These excess quantities can be positive or negative and follow from a comparison of the corresponding quantities in systems with the liquid and solid separated. Alternatively, liquid behaviour in pores can be studied. An example of this kind has been given by Derjaguin ) who claims that water in narrow pores of silica gel or Aerosil does not exhibit the typical thermal expansion minimum at 4 C because of structural changes near the surface. Ldring and Findenegg ) studied surface excesses dilatometrically. 

Only minima are shown, because they always interested us most, as they correspond to structures which in principle can exist in solution and in crystals. As argued above, a point is not unique a change in, say, a CCOH torsion might cause a slight change in 0 or Y or both. Therefore it would be correct to say that a minimum in conformational space is represented by a small but unspecified area around a point in the conformational map, which was earlier (7) termed a manifold. Therefore, a difference in (0,y) of (10,10) is really no difference at all. 

The principle of maximum symmetry can be justified by recognizing that the free energy of any symmetric system must necessarily be either a maximum or a minimum with respect to small shifts that break that symmetry, since shifts in opposite directions will produce identical changes in the energy. Thus equilibrium structures will tend to adopt the most symmetric configuration that corresponds to a minimum in the free energy. 

The question of methanol protonation was revisited by Shah et al. (237, 238), who used first-principles calculations to study the adsorption of methanol in chabazite and sodalite. The computational demands of this technique are such that only the most symmetrical zeolite lattices are accessible at present, but this limitation is sure to change in the future. Pseudopotentials were used to model the core electrons, verified by reproduction of the lattice parameter of a-quartz and the gas-phase geometry of methanol. In chabazite, methanol was found to be adsorbed in the 8-ring channel of the structure. The optimized structure corresponds to the ion-paired complex, previously designated as a saddle point on the basis of cluster calculations. No stable minimum was found corresponding to the neutral complex. Shah et al. (237) concluded that any barrier to protonation is more than compensated for by the electrostatic potential within the 8-ring. 

As can be seen, this index attains its maximal value of unity for two identical structures (9 = 9,

monotonously decreases. The use of this index for the formulation of the least motion principle arises from the following simple idea. Let us assume that we are on a reaction path at point characterised by the wave function P(, q> ) and we are looking for such an infinitesimally close structure (9, (p) for which the transformation (9,

requires minimal change in electronic configuration. This condition is equivalent to a search of the direction along which the derivative of K at the point = 9 and q> = q> attains its minimum. This directional derivative can be mathematically described as (27),... 

The Structure chapters in Handbook-Ill follow the same general format as those in the Handbook-II with a few relatively minor variations. Within this format, some sections have been largely rewritten whereas others have new material added with mostly minimum changes. New material has been selected to illustrate principles and trends, or to introduce new developments in the subject. Some material from Handbook-II has been deleted and replaced by examples of more recent work. CHEC-III has been the major source of new material and, in addition to references to the primary literature, relevant sections of CHEC-III are widely cited throughout the chapters. 

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