Potential energy surfaces rearrangements

While this scheme is useful in helping to predict products from di-rr-methane rearrangements, all evidence indicates that the structures drawn are not intermediates in the reaction. That is, they do not represent energy minima on the potential energy surface leading from the excited state of the reactant to the ground state of the product. 

Even if full potential energy surfaces are not calculated, simple EHT calculations, skilfully coupled with orbital symmetry considerations, can provide insight into complex reactivity problems. This is well exemplified by Hoffmann and Stohrer s analysis of substituent effects on the Cope rearrangement (28). 

Figure 5.30 Alternative water pentamer isomers having partial anticooperativity, (a)-(c), or higher coordination and ring strain, (d). Labels in (a)-(c) correspond to clockwise monomer numbering from the top (see the text). (Species (a)-(c) have been optimized under the constraint of planar equilateral skeletal geometry to prevent rearrangement to Wsc [Fig. 5.29(a)] and are therefore only near-stationary points on the potential-energy surface.)...

Barbaralene [85] undergoes a rapid Cope rearrangement with a doublewell potential. The radical cation was studied using CIDNP by Roth (1987) after one-electron oxidation of [85] by y or X-irradiation. On the time-scale of the CIDNP experiment ( 10 8s), a single-minimum potential energy surface was found, i.e. bishomoaromatic structure [156] was suggested. 

The thermodynamic stabilities of phenonium ions have been determined based on bromide-transfer equilibria in the gas phase and, depending on the substituents, the bridged species (1) has been proposed as an intermediate or transition state on the potential-energy surface for the 1,2-aryl rearrangement of triarylvinyl cations (see Scheme 1). Phenonium ion (3) has been presented as an intermediate to account for the fact that lactonization of methyl 4-aryl-5-tosyloxy hexanoate (2) produces y-lactone (4) selectively under thermodynamic conditions, but affords 5-lactone (5) preferentially under kinetic conditions. It has been shown that anodic oxidation of frany-stilbene in alcohols in the presence of KF or BU4NBF4 is accompanied by its electro-oxidative rearrangement into diphenylacetaldehyde acetals. The mechanism outlined in Scheme 2 has been proposed" for the transformation. 

Scheme 16.5). Thus, the mechanism of the thermal decomposition of 7 has not been fully clarified, yet although the aryne contraction pathway has been established for some benzannellated derivatives of 4. A number of theoretical studies has been devoted to rearrangements on the CeKU potential energy surface. At... 

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