Bond-switch process

These observations must be taken into account when considering the mechanism of halogen addition They force the conclusion that a simple one step bond switching process of the following type cannot be correct A process of this type requires syn addi tion It IS not consistent with the anti addition that we actually see... 

It has been argued that this rearrangement may occur via a bond-switch process in 167. In this process the sulfur acts as a nucleophilic center, which is opposite to the electrophilic behavior of the pivotal sulfur in, for example, the conversion of 149 into 150. The alternative intermediacy of a bipolar sulfur tetra-azapentalene structure or a ring-opened intermediate in... 

Although boron is more accurately described as a metalloid rather than a metal, this section is concluded by two papers that describe the structures and bonding in several organoboron/organophosphorus compounds that display ylidic character. The X-ray structure of 9-borylanthracene (88) shows that only one of the diisopropylphosphine moieties is bonded to the boron in the solid state. However, H NMR evidence shows that an intramolecular bond-switching process takes place very rapidly in solution. The structures of a series of borabenzene adducts of phosphorus ylides, iminophosphoranes and tertiary phosphines have also been determined. Treatment of l-chloro-3,5-dimethyl-2-(trimethylsilyl)-l,2-dihydroborinine (89) with methylenetriphenylphosphorane (90) produces (triphenylphosphonio)methanide-3,5-dimethylborabenzene (91). However, if the reaction sequence is reversed and (90) is treated with (89), then (trimethylsilyl)(triphenylphosphonio)methanide-3,5-dimethylborabenzene (92) is obtained (Scheme 26). Treatment of an isomeric mixture of l-chloro(trimethyl-silyl)dihydroborinines (93) with N-(triphenylphosphoranylidene)aniline (94) produces iV-(triphenylphosphonio)anilide-borabenzene (95) (Scheme 27). Crystal structures of (91), (92) and (95) show that the P-C or P-N bonds are... 

Cyclooctatetraene offers another example of fluxional behavior. In an operation distinct from boat-boat ring reversal depicted in 5-14, the locations of the single and double bonds are switched. Anet and co-workers used the same molecule to examine the bond-switching process, whose antiaromatic transition state is 5-25b. (The transition state to ring reversal... 

A number of other carbocations similar to C5H5 have been studied, the most notable being Cr.Mel. It has been demonstrated that the structure of this cation is Csi,. 11.66. rather than a rapidly equilibrating series of classical structures, 11.67, where a bond-switching process permutes three two-centcr-two-electron... 

There is something more unusual about the electronic structure of the open" isomer, redrawn in 11.62. A bond-switching process converts 11.62 into 11.63 by way of 11.64. This is a munetry-allowed reaction. Notice that the symmetry of... 

Also the prediction has proved correct of a molecular configuration, in which intramolecular alkyl shifts are realized, i.e., an intramolecular nucleophilic substitution at the tetrahedral carbon atom. It has been found [6] that a fast (10 s at 25°C) bond-switching process associated with the rupture-formation of the C—S bonds of the anchored center does occur in the degenerate rearrangement of the l,8-fcis-(arylthio)-anthracene-9-carbinyl cation XIV in solution ... 

As in Saveant s model they describe the R—bond by a Morse potential and introduce an effective switching operator describing the bond breaking process ... 

Like in the dissociation process, no caging of the four-center reaction products was found in our simulation. On the other hand, the cluster can provide a cage for the reactants, i.e. early on when the cluster is very compressed. The role of the caging depends on whether the collision of the two reactants did or did not lead to bond switching. If reaction occurred, the repulsion between the products causes them to rapidly recoil from one another. The role of the cluster atoms is to cool the internal excitation, as shown above. If at the first try bond rearrangement did not occur, the cage insures a second chance (cf. Fig. 18). 

The structure is relaxed and the energy is calculated after each bond switch at all stages in the process, both during the original randomization and the subsequent annealing. 

The randomizaton takes place by a a diffusion of atoms that is implicit in our earlier description of the initial randomization process as being akin to melting [1]. Later it was shown that the root-mean-square displacement of each atom must be of the order of the nearest-neighbor distance in order that the network lose all memory of the original crystal structure as measured by the structure factor S q) [21]. In this context, the melting point can be defined as that temperature for which the mean square displacement increases linearly with time. It appears, though, that a sequence of bond switches as illustrated in Fig. 1 is not the primary mechanism for self-diffusion in silicon [31,32]... 

For generating models of about 512 atoms or more, the geometrical region over which local relaxation is made will be small compared with the model size. It would then be possible to simultaneously generate bond switching in several well-separated regions and thus speed up the entire process. This lends itself to parallel computation. We have not done this, but O Mard has implemented such a parallel modeling system [5]. 

In Table 1, we list the irreducible ring statistics for two crystal structures, FC-2 and BC-8, and for four models of amorphous silicon. When a bond-pair switch is introduced into the otherwise perfect FC-2 structure, the number of irreducible rings is conserved. Four 5-folds and eight 7-folds are created, but twelve 6-folds are eliminated. This conservation rule holds until the regions of bond-switching overlap, and is not grossly violated even then in the randomization process. Note that the total number of irreducible rings per atom is exactly two for the FC-2 structure and it is almost two for all the amorphous structures. 

Clearly, a bond-switching mechanism of the type in Equation 1.2 is unrelated to the electronic structure of the excited state and does not provide a basis for understanding the reaction or predicting further excited-state processes. 

Solvation plays a key role because the magnitude of the electrostatic ion-polar molecule interaction is large, say 10-20 kcal mol , comparable to the barriers for concerted bond-switching reactions. Consider the Sn2 reaction OH + CH3CI Cl -h CH3OH in the gas phase. The reaction is concerted so the new (CH3-OH) bond forms as the old (CH3-CI) bond is stretched. This process has an activation barrier but it must be significantly lower than the energy needed to break the old bond. On the other hand, as the reactants approach, the first interaction they experience... 

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