Rotation and conformation

The hypothetical phase diagram in Scheme 7 assumes that the reactant and the product are at equilibrium, which is probably not true in most solid-state reactions. Since diffusion, rotation, and conformational motions are highly re-... 

B. Hindered Rotation and Conformational States of Substituents Attached... 

Barriers to rotation and conformational states around sp2-sp3 play an important role in the determination of the steric requirement of a substituent. These barriers are predominantly composed by the difference in through-space nonbonding interactions in ground and transition states. They are typically free from electronic contributions, in contrast to the quantitative approach of steric effects, which proceeds through reaction rates or (a + n)-barriers to rotation, in which the steric part is only a perturbation of the energetic content. 

Abraham, R.J. Bretschneider, E. Medium Effects on Rotational and Conformational Equilibria, in Internal Rotation in Molecules Orville-Thomas, W.J., Ed. Wi ley-Interscience New York, 1974, pp. 481-584. 

TABLE 5. Free-energy barriers (kcal mol 1 at T K) to C=C and C—N rotations and conformer populations (pz) for compounds 18... 

Table 3.1 displays the entropy changes of melting and vaporization for some pure substances. The entropy of vaporization is proportional to the ratio of the degree of randomness in the vapor and liquid phases. For a pure component, A.S. consists of translational, rotational, and conformational motion of molecules. The translational effect is the largest contribution to the entropy of vaporization. 

Not only tautomeric equilibria are subject to considerable solvent effects. Other equilibria, such as rotational and conformational equilibria [81-83], cisitrans (or E/Z) isomerization, valence isomerization [84], ionization, dissociation, and association [85] (some of which are considered in Section 2.6), complex equilibria [86, 163, 262, 263], acid/base equilibria [264, 265] etc., are also strongly affected by the medium. Only a small number of representative examples will be considered in this Section in order to give an idea of how solvents can affect these different kinds of equilibria. 

Preorganization Preorganization is a qualitative concept introduced by Cram [4] to describe the conformational similarity of a molecule in its uncomplexed relative to its complexed state. We have covalently linked the three melamines of the CA M rosette by spokes to a central benzene-based hub this linkage results in preorganization by reducing unfavorable changes in translational, rotational, and conformational entropy upon formation of the aggregate. 

Structural results on 5n and 5p reveal both compounds to be closely related to their Zr analogues Sg and 5o (56). In particular, the metal-diene portions of the molecules, which are of prime interest for the scope of this article, are truly isostructural (Tables V and VI). As Fig. 6 for 5g (Zr) and 5n (Hf) vividly shows, the similarities between both compounds even extend to the thermal behaviour of most atoms. For compounds 5o and 5p, some minor differences arise from the rotational and conformational lability of the Cp-rings and the six-membered ring system of the ligand. They leave the overall geometrical parameters of the metal-diene framework essentially unchanged, however, and are further discussed in Kriger et al. (56). 

Experiments performed in a very similar way also showed dynamic disorder for the enzyme a-chymotrypsin [16], a-Chymotrypsin is an endopeptidase acting on water-soluble polypeptides. The substrate (suc-AAPF)2-rhodamine 110 was designed to interact optimally with the binding site of the enzyme. It consists of a rhodamine 110 core that is derivatized with a succiny-lated AlaAlaProPhe peptide sequence, known to bind very specifically at the enzyme s active site (Fig. 25.2a). To further avoid potential artifacts, the enzyme was immobilized by entrapment in an agarose matrix, which restricts enzyme diffusion while still allowing free rotation and conformational dynamics of the enzyme as well as the diffusion of the substrate. 

Fig. 5A). The emission spectra of benzo[a]phenothiazine and the three methyl derivatives exhibited generally a broad and structureless band located at 500-527 nm in water/ethanol mixture (99 1, v/v), 500-507 nm in ethanol and 512-519 nm in 11P-f>-CD (Fig. 5B). Enhancement of the Stokes shift in HP-fi-CD solutions was interpreted by a greater stabilization of the tt-tc solvated excited singlet state [21], due to the reduced rotational and conformational motions of the BPHT molecules within the HP-fi-CD cavity [82], Comparable weak excitation or emission spectral shifts upon adding CD to an aqueous medium have also been reported in the case of unsubstituted phe-nothiazine [21], azure A [83] and several types of aromatic molecules [84,85]. 

The rotational and conformational isomerism in dimeric proanthocyanidines 101 was studied by NMR spectroscopy. It was found that the geometry of these important polyfla-vanoids depends on the nature of the solvent (in organic solvents and water). The effect of the Y atom and the substituents X on the planarity and the barrier to internal rotation about the aryl-Y bond were estimated by semiempirical quantum-chemical calculations... 

Effects of translational, rotational, and conformational entropy loss upon binding not included in function... 

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