Three-membered rings parameters

An interpretation of activation parameters has led to the conclusion that the bromination transition state resembles a three-membered ring, even in the case of alkenes that eventually react via open carbocation intermediates. It was foimd that for cis trans pairs of alkenes tiie difference in enthalpy at the transition state for bromination was greater than the enthalpy difference for the isomeric alkenes, as shown in Fig. 6.2. This... 

The geometric parameters of the three-membered ring sulfones and sulfoxides have been determined via X-ray diffraction techniques and gas-phase microwave spectroscopy. The accumulated data for some selected thiirane and thiirene oxides and dioxides (16-19) as well as for the corresponding thiirane (20) and the acyclic dimethyl sulfone (for the sake of comparison) are given in Table 3, together with the calculated values. 

The structures of the three-membered ring compounds obtained in these reactions are of great interest. The pertinent structural parameters are listed in Table VII. The column headed SSi gives the sum of the bond... 

Structural Parameters of Three-Membered Ring Compounds Derived... 

C-Acyl and N-acyl derivatives of three-membered rings have received little attention in recent investigations, especially with regard to up-to-date experimental techniques for determining dynamic and structural parameters of the conformational equilibrium. 

Parameter Acyclic compound Four-mem bered ring Three-membered ring... 

Table 4 summarizes the structural parameters of prismane molecules (M6H6 M = C, Si, Ge, Sn) together with the calculated values for R=H. The M—M bonds within and between the three-membered rings are denoted as a and b, respectively. The calculations predicted that a is shorter than b for R = H7,8, which is in accordance with the experimental data... 

There are molecular systems exhibiting 7r-bond fixation patterns that are entirely opposite to that induced by the Mills-Nixon effect [82,83,67]. Typical examples of this kind are provided by benzoborirene 33 and benzocyclopropenyl cation 34 (Fig. 19) These compounds represent extended 7r-systems relative to benzene itself since they encompass now empty 7r-orbitals at B and C+ atoms, respectively. The structural parameters offered by HF/6-31G [82] and MP2(fc)/6-31G [43] models are given in Table 10. Both molecules are planar. A salient feature of the aromatic CC bonds is their stretching relative to benzene at ortho and para positions. In contrast, meta bonds are more localized and shortened. Another striking property is a pronounced delocalization within the three-membered ring (aromatic pattern involving 27t electrons) as easily visualized by the resonance structures shown in Scheme 4. The same resonance mechanism is operative in benzocyclopropenyl cation. 

The energetic differences between bicyclic structures of type 9 and the oxapolyhedra of type 13 are small about 7 kcal/mol between 9u and 13u, and only about 3 kcal/mol between 9uMe and 13uMe. (This estimation is based on classical structures with interplanar angles fixed to 105.3° other parameters were optimized.) Substituent effects like those of chlorine at the three membered ring boron atom or of trimethylsilyl... 

The complete description of the catastrophes involved in the dissociation of the water molecule must clearly involve two behaviour variables and at least two control parameters. We show in the following section that the most general description of the formation or destruction of a three-membered ring structure is described by such an unfolding, the elliptic umbilic. 

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