Substituent interaction, intramolecular

This approach is admissible because SCSs are well known to be additive unless (i) the conformational equilibrium of the molecule is altered by progressive substitution, and/or (ii) there is no intramolecular substituent interaction whatsoever. Thus, if application of eq. [18] to a rigid molecular system affords ICS values other than zero, such an interaction must exist. In earlier publications (216,221) the abbreviation NAE (nonadditivity effect)—a merely descriptive term—was employed. 

It was suggested that the bulky 4- -butylphenyl substituents render the extended conformation more stable due to the intramolecular steric interactions, which predominate at higher temperatures, while at lower temperatures,... 

A further question which may be answered by X-ray is that of planarity. Recent papers206,207 disclose that of the thousands of possible PAHs with 10 rings, the vast majority should be nonplanar, even chiral, structures resulting from steric factors. Highly notable examples of nonplanar PAHs include helicenes, fullerenes, bowl-shaped components of fullerenes, and nanotubes.206,207 Several intriguing species have been prepared in which the rings are forced out of planarity due to intramolecular steric interaction between substituents. These interactions become important when the angles by which substituents protrude from the sp2 periphery are such that they would approach adjacent substituents closer than allowed by van der Waals radii. 

The intramolecular dipolar interaction in solution should decrease with increasing size of the substituents in the order pyridiniimi > N (CH3)2 > N (C2H5)2 [34]. The potentiometric titration curves of polybetaines resemble those of the titration of weak acids and weak bases. Measurements of the... 

Third-row anomeric effects of arylselenenyl substituents were studied in 2-arylseleno-l,3-dithianes (5) (Scheme 3). This effect favours the axial conformer by 6.7-9.2 kJ/mol, and an intramolecular se interaction... 

In Table III, the positive interaction between IMZ stearate (6) and the hydroxybenzoate stabilizer UV-Chek AM-340 is shown. This degree of lifetime enhancement was not achieved in any of the HALS compounds synthesized in which the hydroxybenzoate group was present as an intramolecular substituent. The proximity of the hindered phenolic group may in some way affect generation of the nitroxide radicals from the amine nitrogen. 

In contrast to (Ph2Zn)2, both bis(pentafluorophenyl)zinc and bis[2,4,6-tris(trifluoromethyl)phenyl]zinc have a two-coordinate monomeric solid-state structure approaching linearity, which is certainly a result of electronic and steric (see Steric Effect) properties of the orffto substituents (Figured). In the crystal structure of both compounds, the aromatic rings are almost orthogonal to one another. This special situation alleviates intramolecular (see Intramolecular) steric interactions and allows intramolecular stacking interactions between rings on adjacent molecules. 

The high proportion of muco-inositol formed in the isomerization is surprising. The isomerization of mj/o-inositol with acetic acid-sulturic acid (see Section VI, p. 190) gives an equilibrium mixture of myo-, DL-chtro-, and mwco-inositol in the ratios of 54 41 5. These ratios correspond to the relative thermodynamic stabilities of the three inositols as determined on the basis of intramolecular, steric interactions of the substituents, whereas the isomerization in hydrogen fluoride gives the thermodynamically least-stable isomer (muco) in the highest yield. This discrepancy can be rationalized if it is postulated that the acetoxonium ion in the mwco-inositol system is further stabilized as a di-cation of structure 138, formed because... 

These data can be explained in terms of the high stabilization energy resulting from solvation of the excited t state. High p values in these cases indicate that the polar solvent-solute intermolecular stabilization ofthe zwitterionic excited t state is very sensitive to intramolecular substituent effects. In contrast, there is no dependence of Stokes shifts on a-constants in cyclohexane, which is nonpolar aprotic solvent, where the vibrational relaxation ofthe Franck-Condon state plays a primary role in stabilizing the excited state. This implies that the vibrational relaxation is not sensitive to intramolecular donor-acceptor interactions. The observations showed that the ultrafast intra- and intermolecular electronic polarization plays a major role in determining the position of the Franck-Condon zwitterionic state and its sensitivity to the relaxation of polar-substituted stilbenes. 

An alternative scenario in which complexation of the base and proton abstraction occur simultaneously in a kinetically controlled one-step reaction was postulated by Schleyer (Fig. 26.4a) [13, 14]. In this kinetically enhanced metalation (KEM) model, this is not precomplexation that is important but the existence of a stabilizing metal-substituent interaction at the rate-limiting transition structures [13]. The fact that KIEs of inter- and intramolecular lithiation of anisole by -BuLi in Etp are identical (kjkj =2.5 0.2) [63] supports this one-step mechanism. 

This addition to the aromatic ring is believed to be eoncerted, since the relative geometry of the substituents on the alkene is retained in the product. Lesser amounts of products involving addition to 1,2- or 1,4-positions of the aromatic ring are also formed in such photolyses. ° This type of addition reaction has also been realized intramolecularly when the distance between the alkene and the phenyl substituent is sufficient to permit interaction. 

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