Substituted systems transition

The two transition states have approximately the same energy according to 3-2IG calculations. It is from this reference that M8-9j results on substituted systems need to be measured . ... 

No limits on the content of transition metal ion were found for Fe-substituted systems. As mentioned above, and in line with crystallographic literature,45 a monophasic sample with magnetoplumbite structure was obtained for the completely substituted BaFe O. This sample showed excellent CH4 combustion activity upon calcination at 700 °C, but it markedly deactivated either upon increasing the calcination temperature or upon treatment at high tempera-... 

Similar nonbonded interactions occur between C(4) and C(7) substituents in the c/s-fused 1,6,8-nona-triene transition state (Figure 13). ° The very hi trans selectivity exhibited by trienes (42), (43), (44) and (46) is striking in view of the data summarized in Figure 2 for less highly substituted systems. The results with (44) and (45) imply that the extent of destabilization of the c/r-fused transition state depends on the steric requirements of the C(7) substituent, a conclusion also supported by other examples cited in the references. Nevertheless, these interactions are large enough to reverse the stereoselectivity of the IMDA reactions of internally activated nonatrienes (44) and (47) relative to the usually observed cis manifold (compare (5) and (7), Figure 2). 

The calculated hyperfine coupling constants (B3LYP/6-31G //MP2/6-31G ) for the type B transition state and the distorted minimum clearly show that this species must be considered a type A structure. The hyperfine coupling pattern of the lowest-energy minimum [ai = —1.43 mT a = 1.98 mT) shows a trend similar to the experimental splittings of the fran.s-l,2-dimethylcyclopropane radical cation a, 2 = —1.19 mT a = 2.18 mT), whereas the pattern calculated for the transition state (fl2,3 = 0.55 mT) is incompatible with that model (Figure 18). The distorted structure type calculated for the methyl-substituted systems seems to prevail also under other conditions (see below). 

By a kinetic study Schneider and Rau made it evident that Cope rearrangements of this type had to pass via a boat-like transition state as depicted in equation 162. Of course, this stereoelectronic requisite has certain consequences for synthetic applications, particularly for highly substituted systems. 

Since the preparation of the Cp(OC)2Fe- [2], Cp(OC)3W- [3a], and Cp(OC)2(Me3P)MoAV-substituted [3b, 4] pentachlorodisilanes and their pentahydrido-derivatives, our interest is focussed on extending this series to the CsMe5(OC)2Fe- and CsR5(OC)2Ru-substituted systems (R = H, Me) These investigations are motivated by the study concerning the electronic effect of transition metal centers on the chemical properties and reactivity of the a- and p-silicon in disilanyl-complexes. 

The materials with C4 and C spacers exhibited only a smectic A phase, whereas the tetramer with a Cn spacer shows, upon heating, a transition to an additional unidentified phase at 38.7 °C. When the number of methylene units is increased, the isotropization temperatures increase accordingly. The material with the Ce spacer has the lowest solidification temperature (-14.7 °C). Thus these symmetrically substituted systems show low melting... 

To provide a baseline from which to compare the substituted system, we first compute the n energy of the unsubstituted starting material on the left in Fig. 6.53 a and the fully aromatic version of the corresponding transition structure on the right. The starting material will have the n energy of two independent % bonds,... 

The effect of axial -substitution has been studied for methyl58 and hydroxy substituents37. Compared to the unsubstituted system, axial attack is now enhanced, which is attributed to destabilization of the equatorial transition states by additional steric effects58. As in the case of equatorial substitution, shielding seems to be larger for methyl- than for hydroxy-substituted systems. 

As a starting point for the mechanistic discussion, it is useful to review the structural features of the excited states. The first singlet excited state of butadiene. Si, can be approximated as the i >2->--LUMO tt-tt transition. The lack of fluorescence from this excited state indicates that a very facile path exists for nonradiative energy transfer. The S2 state has doubly excited character and relaxes to a structure with ionic character that can rotate at the pyramidal carbon but not at the allyl fragment. The minimum energy of the Tj state corresponds to the allyl-methylene diradical with a nearly 90° twist and slight pyramidalization at the methylene carbon. In substituted systems, one or the other of the zwitterion structures (e.g., allyl cation versus allyl anion) may be favored. ... 

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