Solid steric strains

It is finally assumed that with all force constants and potential functions correctly specified in terms of the electronic configuration of the molecule, the nuclear arrangement that minimizes the steric strain corresponds to the observed structure of the isolated (gas phase) molecule. In practice however, the adjustable parameters, in virtually all cases, are chosen to reproduce molecular structures observed by solid-state diffraction methods. The parameters are therefore conditioned by the crystal environment and the minimized structure corresponds to neither gas phase nor isolated molecule [109],... 

One example of a bonafide bis(alkyne) complex has recently been prepared. Reaction of the in situ generated olefin complex prepared by alkylation of ( -CsHs ZrC 50 with the diaryl alkyne in Equation (7) yields 253.130 In this structure, C-C coupling has not occurred, presumably a result of the steric strain associated with the zirconacyclo-pentadienyl fragment (Equation (7)). The solid-state structure further establishes the compound as a bis(alkyne) complex. Computational studies suggest that a Zr(iv) resonance structure is the most suitable representation of the compound. However, reaction of 253 with iodine in THF yields ( -CsHs Zrle 254 and the dialkyne starting material, suggesting that the zirconium center can act as a source of Zr(n) (Equation (8)). 

A nonpolar solvent favors conformation A, whereas conformation B is favored by more polar solvents (e.g. dimethylformamide, hexamethylphosphoric triamide) because the cation is more solvated (cf. Table 9, entries 1 and 2). However, this solvent effect is absent when BujP Cu" is used as counterion. Conformation A is more favored by relatively small counterions, such as the lithium and sodium ion, as compared to the larger potassium ion, due to the higher degree of association of the former. Steric strain between ASG and ASG is minimized in conformation B. Conformations A and B lead to trans- and c -substituted cyclopropanes, respectively. A study of cyclopropane esters, -in which the stereoselectivity of the reaction of polymer-supported reagents was compared with molecules of low-molecular weight, made clear that the steric and polar microenvironment of the polymer-supported reaction is not different enough in bulk to influence the selectivity substantially. Nevertheless, a specific influence of the solid phase can be observed at low temperatures. 

When conformational homogeneity is not ensured in a cis-diarylethene, the photoisomerization pathways could be rather complex. For example, for l,2-P,P -dinaphthylethene (10), the cis isomer should exist in three possible conformations (ds-lOa, ds-lOb, cis-lOc), as shown below. HT of these conformers should lead to three conformers of the trans products (traws-lOa, -10b, and -10c) (solid lines) with comparable internal steric strain. The same conformer products are also expected from OBF processes (dashed line), albeit not from the same starting conformers as in HT processes. While it is clear that the product ratio is dependent on a number of factors, this ambiguity renders such systems uninformative for elucidation of the exact nature of their photoisomerization pathways. On the other hand, it should be clear that photoisomerization, e.g., by HT, is not hkely to produce the same equilibrated mixture of conformers of the trans isomer as in the equilibrated mixture, i.e., obtained fi"om irradiation at room temperature. Thus, the claim of possible preparation of a nonequilibrated trans isomer in organic glass is also consistent with, but not a proof for, involvement of HT. 

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