Transition states, frozen

For 10, the differences of axial and equatorial Si-P bond lengths are attributable to a "normal" cis-influence of the methyl groups, whereas for 9, the difference Si-Pax/Si-Peq is drastic. In fact, in the solid state, 9 rather appears as "a frozen transition state" between four- and hexacoordination (omitting pentacoordination). In solution, 9 is fluctional. 

DFT calculation was performed using Gaussian 03 package [11]. The geometry optimizations were carried out using the B3LYP hybrid functional and the triple-f basis set. H atoms were kept frozen through all calculations. Transition states were localised... 

Until now, the isotopic effect was discnssed only in relation to the reactants. In electron-transfer reactions, the solvent plays an eqnally important role. As mentioned, different solvate forms are possible for reactants, transition states, and products. Therefore, it seems important to find a reaction where the kinetic effect resulting from the introduction of an isotope would be present for solvents, but absent for reactants. For a published work concerning this problem, refer Yusupov and Hairutdinov (1987). In this work, the authors studied photoinduced electron transfer from magnesium ethioporphyrin to chloroform followed by a dark recombination of ion-radicals in frozen alcohol solutions. It was determined that the deuteration of chloroform does not affect the rate of transfer, whereas deuteration of the solvent reduces it. The authors correlate these results with the participation of solvent vibrational modes in the manner of energy diffraction during electron transfer. 

The structures of the intriguing dications 156 and 158 were also computed by DFT calculations. The C NMR chemical shifts were also calculated using both GIAO and IGLO methods. Both dications 156 and 158 can also be characterized as 4c/2e a-bishomoaromatic rectangular cyclobutane dications as well as frozen Woodward Hoffmann transition state analogs. 

The presence of lattice coupling in direct dissociation complicates somewhat the definition of V and how V extracted from measurement should be compared to DFT calculations. There are two well defined limits for DFT calculations of V, for a frozen lattice (Ts = 0) and for the case where the lattice is fully relaxed at the transition state (Ts = oo). Since experiments are performed at finite Ts, the best V description is probably intermediate between these two extremes. 

Figure 13.8 A 25-atom quantum subsystem embedded in an 8863-atom classical system to model the catalytic step in the conversion of D-2-phosphoglycerate to phosphoenolpyruvate by enolase. What factors influence the choice of where to set the boundary between the QM and MM regions Alhambra and co-workers found, using variational transition-state theory with a frozen MM region that was selected from a classical trajectory so as to make the reaction barrier and thermochemistry reasonable, that the breaking and making bond lengths were 1.75 and 1.12 A, respectively, for H, but 1.57 and 1.26 A, respectively, for D...

It is interesting to consider further the relationship between situations 3 and 4. Situation 3 will be reached if the transition state in 4 is sufficiently stabilized so that its relative energy drops below those of the valence tautomeric forms 42a and 42c. In other words, situation 3 corresponds to a frozen transition state 7. A no-bond homoaromatic compound is simply the realization of a frozen TS. 

Concepts such as the frozen transition state 47 or the frozen reaction path (Biirgi-Dunitz reaction path)48 were developed to obtain direct experimental information on transition states. So far, all attempts have failed to realize a frozen transition state experimentally, Cremer and coworkers49 58 have shown that a no-bond homoaromatic compound (PES situation 3), such as the homotropenylium cation, corresponds to a frozen transition state, and therefore its investigation provides ample information about the properties of transition states. 

The description of no-bond homoaromatic systems as frozen transition states is in line with the observation that their PES is rather flat in the direction of the interaction distance. This means that (a) homoaromatic stabilization energies are small (see Table 2 and the discussion presented above) and (b) relatively small energy increases lead to relatively large... 

Actually, this question focuses on the generation of a frozen transition state situation. Can each valence tautomeric equilibrium between an unconstrained cyclopropyl homo-conjugated compound and its open monocyclic counterpart be manipulated in such a way that a frozen transition state is obtained There are results pointing in this direction (see the following chapter3), however at the moment one is far from being able to generalize any of these observations. 

Baxter, Cowley and coworkers have reported a solid state investigation of the structure of a 9-phosphabarbaralane and suggested this has a symmetrical structure corresponding to a frozen transition state 265. In solution the molecule exists as a classical structure. 

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