Strain angular

The results obtained with methoxypyrazole (247) are attributed to an electronic effect, whereas those obtained with (246) can be explained by taking into account the angular strain in the minor isomer. Reaction of 3(5)-nitropyrazole with l-fluoro-2,4-dinitrobenzene affords exclusively l-(2,4-dinitrophenyl)-3-nitropyrazole (70JHC1237). 

The formation of the linear polymer from the cyclic monomer requires a decrease of the free energy. Because usually entropy is lost during polymerization, the main driving force for the ring-opening process is the release of the angular strain upon conversion of the cycles to linear macromolecules. Thus, a majority of three- and four-membered rings can be readily and quantitatively converted into polymers. 

Compound 471 (n = 3) possesses an abnormally high band at 1738 cm-1, which has been attributed167b to angular strain since compound 471 (n — 4) shows the corresponding band at much lower frequency (1668 cm-1). 

Strain energies of 23.5, 24.8 and 8.3 kJ mol 1 were estimated for tetrahydrofuran, pyrrolidine and tetrahydrothiophene, respectively (74Pmh(6)199). The larger sulfur covalent radius of 1.04 A lowers angular strain. 

In the case of the six-membered ring (Tables XVII and XVIII), the angular value [zLCr-Si-Ci = 105° ( 5)] cannot explain why inversion of configuration is so unfavorable compared to retention. This value suggests that the angular strain at silicon is quite similar in intermediates like 28 and 29 (Scheme 25). Thus both inversion and retention would be expected. 

The above conclusions explain the general effect of the angular strain on the stereochemistry and agree well with all the experimental facts ... 

Hence, kinetic stability towards [4 + 2] dimerization increases from siloles to stannoles. Intracyclic bond lengthening (Si—C < Ge—C < Sn—C) must give rise to a decrease in the ring angular strain and to HOMO stabilization. This inhibits dimerization by an increase in the frontier orbital (HOMO/LUMO) energy-level difference. 

At the simplest level of Mills-Nixon-type argumentation, annelation of benzene by small saturated rings should favor the Kekule structure that places the double bond outside (exo) of the strained small ring. Thus, angular strain is expected to lead to normal bond alternation, AR > 0, where the endo bond, Ri, is longer than the exo bond, R2, as shown in 24, Scheme 26. 

Figure 10. Angular strain effect on optimized geometries of benzene species and their er-frames (from ref 6). The er-frame is the QC state (see Scheme 10). (a) Stanger-benzene. (b) Triscyclobutabenzene.

Scheme 29. Rehybridization

The frequency exaltation of the Kekule mode is mirrored by the structural manifestations in the twin states, discussed with reference to Figures 16 and 17. Thus, the repulsive jr-curve in the ground state softens the potential and thereby enables the ground-state molecule to distort along the Kekule mode when angular strain is exerted. In contrast, the attractive jr-curve in the twin excited state stiffens the potential and restores the local Deh symmetry of the benzene nucleus. The two physical effects are in perfect harmony and find a natural reflection in the VB model. 

The selective cleavage of the silicon-silicon bond in the disilacyclopentane and -hexane is probably due to the concentration of internal angular strain at this bond in such smaller polygonal molecules. In case of the disilacyclo-heptane, however, it seems likely that the strain is smaller and uniformly distributed all over the ring, and hence cleavage occurs preferentially at the silicon-carbon bond by the accepted mechanism involving both an electrophilic attack on carbon and nucleophilic attack on silicon by the sulfuric acid molecule(s) 169). 

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