Single TR Mechanism

We turn now to a detailed analysis of the motions of the atoms involved in the TR process. Let pe and pa represent the ligands of the pair, and te, t and ta the ligands of the trio. The equatorial t will remain equatorial as a result of this isomerization. 

Continuation of the TR process by an additional 30° rotation results in the new eclipsed 0°-TR model situation shown in the projection (12-11). This is analogous to the previous models, (10-1) and (10-11), with suitable changes in the nature of the ligands. The reversal of the tilt of the pair and the expansion of a diequatorial angle in the trio generates the new isomeric TBP (12). (In this illustration the pair, P Pe tilts away from te which is now apical, while angle ta—P—ta expands from the original 90° to 120°.) 

These three simultaneous ligand motions result in the exchange of apical and equatorial ligands pairwise and concertedly. The energy 

Point (G) is of particular interest. It corresponds to a situation in which, in the 0°-TR model situation, one of the ligands of the trio is displaced toward the ligands of the pair (and vice versa) to form a new trio, while the two remaining ligands (formerly of the trio) are displaced so as to form the new pair. This generates a new 0°-TR model situation and can be called a 0°-0°-TR Switch of Type I  

9 = Original Pair-Ligands O = Original Trio-Ligands 

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In the discussion of the single TR mechanism it was pointed out that one of the unique features of this concept is the TR-switch . In the idealized model for PF6 shown in Fig. 6, this situation corresponds to point (G). The mutual displacement of certain ligands at the energy level of the 0°-TR situation provides a reasonable mechanism to effect a crossover from one pair-trio combination to a different pair-trio combination. This process can be illustrated with the aid of Fig. 10. Figure 10 describes first a single TR to isomerize (1 4) (2 5) into (1 2). An attempt is made in Fig. 10 to approximate the motions corresponding to the 60° internal relative rotation of a pair 1, 5 vs. a trio 2, 3, 4. If the five ligands were identical, and if an idealized situation were involved, these motions would correspond to a clockwise 36° rotation of the pair 3... 

The spiropentaoxyphosphorane (109) in Scheme 19, with a five-membered unsaturated and a six-membered saturated ring,121 shows two signals for the CH3-C protons and a complex spectrum for the methylene protons. The single-TR mechanism explains the data, if one allows both apical-equatorial and diequatorial placement of the six-membered ring, as can be seen in Fig. 18-V and Scheme 19. The (TR)3... 

Figure 5 justifies the name turnstile rotation given to this mechanism. In the formal treatment presented in Section III, the single TR corresponds to a subclass of the class C +s. in which the permutation is of the type (a e )(ee"a), where a =apical and e =equatorial. As a consequence of the skeletal symmetry of the TBP, the following four... 

It has been suggested that in the solid state electronic energy migrates rapidly from the initially excited chromophore to excimer tr sites by either exciton diffusion or single-step mechanisms. 

The resnlts from the TR experiments presented in Fignres 3.19-3.21 above were used to determine single exponential rate constants of 5.1 X 10 s for the decay of the ketyl radical, 1.6 X 10 s for the formation of the flnoranil anion and 8.4 X 10" s for the decay of the flnoranil radical anion. These rate constants snggest that the flnoranil radical anion is forming from the ketyl radical. A reaction mechanism for the intermo-lecular abstraction reaction can be described as follows ... 

Figure 3-14. Hypothetical structures indicating possible mechanisms for transporters and channels in cell membrane (shaded region) (a) mobile carrier or porter acting as a symporter for protons (H+) and some tr ansported solute (5) (b) series of binding sites in a channel across a membrane, acting as a symporter for H+ and S (c) sequential conformations of a channel, leading to unidirectional movement of solute and (d) a protein-lined pore with multiple solute or water molecules hr single file, the most accepted version of ion or water (aquaporirr) channels.

The use of the Brown equation as a probe of reaction mechanism is essentially based on the alternative use of substituent parameters tr and a- the better correlation with one of the reference scales, i.e. in this analysis, indicates closer similarity in the mechanism or in the structure of the transition state to that of the reference reaction, solvolysis of a-cumyl chlorides [2]. While the broad applicability of the Brown treatment is widely appreciated, this (T treatment has the inevitable limitations of a single reference parameter relationship. 

Table 2.4. Binary metal/metal systems observed to exhibit solid-state amorphiz-ation by interdiffusion or other type of driven mixing (e.g. mechanical codeformation). The table lists the type of experiment, the typical reaction temperature, TR (for the case of interdiffusion reaction) and references. (B thin-film bilayer diffusion couples, M thin-film multilayer diffusion couple, S interdiffusion of polycrystalline layer of one component with single crystal of another component, MA mechanical alloying of the metals, MAT thermal reaction of a mechanically deformed composite...

Hashimoto H, Koyama Y and Mori Y (1997) Mechanism activating the 2 Ag state in all-tr a j- carotene crystal to resonance Raman scattering. Jpn J Appl Phys 36 L916-L918 Hashimoto H, Sawahara Y, Okada Y, Hattori K, Inoue T and Matsushima R (1998) Observation of solitonlike excitations in ak-/rau4-/3-carotene single crystals. Jpn J Appl Phys 37 1911-1918... 

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