Jr-interactions

FIGURE 1.11 Systematic optimization of structural binding and stereorecognition increments of amino acid side chain (Rsa) carbamate residue (Rso)- Dependency of separation factors on the steric bulkiness of amino acid and carbamate residues as quantified by their corresponding Taft s steric parameters Es sa Eg so- (1) ionic interaction (2) jr-jr-interaction (3) hydrogen bonding (4) steric interaction. (Reproduced from M. Lammerhofer et al., J. Sep. ScL, 29 1486 (2006). With permission.)... 

Leventis R, Silvius JR. Interactions of mammalian cells with lipid dispersions containing novel metabolizable cationic amphiphiles. Biochim Biophys Acta 1990 1023(1) 124-132. 

Haxo, H. E., Jr. Interaction of Selected Lining Materials with Various Hazardous Wastes. II. Proceedings of the Sixth Annual Research Suymposium. U.S.E.P.A. pp. I6O-I8O. March 198O. 

When a CSP is applied, the separation mechanism is based on the differences in the interaction between the chiral selector in the stationary phase and the enantiomers of the solute. Depending on the nature of the selector and the type of the solute, the stereoselective interaction can be based on interactions of one or more different types such as inclusion complexation, Tr-jr-interaction, dipole stacking, hydrogen bonding, electrostatic interaction, hydrophobic interaction, and steric interaction [35]. In order to obtain chiral discrimination between the enantiomers, a three-point interaction is required between at least one of the enantiomers and the CSP [36]. The interactions can be of attractive as well as repulsive nature (e.g., steric and electrostatic interactions). 

Due to the confined environment and cation-jr interactions within the zeolite cavities as well, it is expected that the regioselectivity in the photooxygenation of trisubstituted... 

The cyclopentadienyl /3-diketiminate 249 and its 4-terf-butylpyridine adduct 250 have been prepared and structurally characterized (equation 19). An X-ray crystal-structure determination of 249 showed that the cyclopentadienyl group is /j -bonded to magnesium. On the basis of the observed bonding parameters of magnesium with the -diketiminate skeleton [Mg-N 2.006(2) and 2.021(2) A, Mg-C 2.729(3) and 2.826(3) A and Mg-C 2.689(3) A] this bonding is described in terms of a jr-interaction. However, in 250 the /3-diketiminate is iV,iV -chelate bonded to magnesium. 

This selectivity was attributed to the perpendicular geometry of the vinylic hydrogen atom to the olefinic plane. In such a conformation, the vinylic hydrogen is activated considering the large interactions between the vinyl C—H bond and the reacting C=C double bond (Scheme 25). 

It has already been mentioned (Section III) that the study of the diastereoselection in the electrophilic addition of singlet oxygen to the n face of chiral alkenes is of primary interest for the achievement of a selective oxyfunctionalization reaction. Zeolite confinement and cation-jr interactions might be expected to affect significantly the diastereoselectivity in the photooxygenation of chiral alkenes. 

Pt(5<4)-CO(jr ) interaction. Charge transfer between the Pt and CO fragments is dominated by the CO- Pt a dative bond at larger Pt—CO distances and by Pt— CO n back-bonding at shorter bond lengths.307... 

The symmetry of transformation (107) is identical to that of the alkyl car-bonyl-to-acyl conversion, (5), and therefore the difference in the propensities of these insertions to occur must relate to differences in the orbital energetics of the two processes. Since the metal-nitrosyl jr-interaction is a dominant feature of the electronic structure in NO+ complexes, and since it will be weakened by the formation of a nitroso species, one can expect (107) to be correspondingly less favorable than acyl formation in (5). If reaction (107) does occur, however, one can readily envisage tautomerism of the nitroso ligand to an oxime. The reactive nature of the oxime may then create difficulties in the identification and isolation of organonitrogen products, and indeed, this may have obscured general observation of transformation (107) in the past. 

Benzylsilanes and allylsilanes are easily oxidized anodically compared with alkylsilanes and arylsilanes. Benzylsilanes exhibit irreversible cyclic voltammetric waves. It is notable that their oxidation potentials (Ep) are markedly less positive than those of the unsilylated parent compounds owing to the a-jr interaction (Table 3)10a. It is interesting that a-trimethylsilylation of xylenes markedly decreases their oxidation potential while additional a -trimethylsilylation makes a little change (Table 3). It has also been reported that a a, a-interacting system (the neighboring C—Si bonds) in addition to a a-ir interaction caused a significant decrease of the oxidation potentials1013. 

Since silicon stabilizes an a-carbanion by d jr-p jr interaction, silicon should promote electron transfer to a carbon-halogen bond which generates the a-carbanion. In fact,... 

Substituted benzylideneanilines form molecular complexes187 with 106 and with 2,5-dichloro-3,6-dihydroxy-p-benzoquinone (110) (chloranilic acid). These complexes were investigated by IR and 111 NMR. The complexes between benzylideneanilines and 106 are CT complexes (111), while complexes with 110 are formed through electron and proton transfer187, as illustrated in 112. In 111, a refers lo n -> jr interaction and b is a jr -> jr interaction, sp2 N-atoms of some heterocyclic imines (such as 2,2 -bipyridyl, 1,10-phenantroline) are also n-donors towards acceptors (tetracyanoethylene, p-benzoquinone, chloranil)188. 

The more distant the two extrema, the greater the final ground state s displacement and the larger its bond alternation This scenario corresponds to starphen-ylene, 5, and the model system 28 (Schemes 14 and 27) where the jr— -interaction between the annelating ring and the benzene nucleus is obviously present. The reason the -maximum exhibits a negative displacement Ar < 0 is explained later. 

To understand the directionality of the -displacement, we now consider the effect of an annelating ring that maintains ji—jr-interaction with the Kekule structures of benzene, shown in Figure 12. At its top, the figure shows the Kekule structures, K2 with the double bonds in the endo-positions and Ki with the... 

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