Vicinal effect

Vicinal effects can also play a part in the course of the reaction utilizing Oppenauer conditions. 1,3-Diols or j5-amino alcohols may not react, presumably on account of format on of an aluminum complex. If oxida-... 

Vicinal effects in compounds with one endocyclic heterosubstitutent can be estimated by comparison with corresponding carbocycles. In Scheme 51 a-SCS values in 9-heterobicyclo[3.3.1]nonanes 178 to 180 and 182 (197,324,325) and oxaadamantanes 184 (326) are thus compared with those in 177, 181 (327), and 183, respectively (232). 

A cluster ion beam, which is composed of atomic particles such as and Auj/, where n is the number of atoms in the cluster [7], or molecular ions such as OH and AlO, has attracted a great deal of interest in atomic physics [8,9] and in materials science [10]. The cluster ion beam has distinctive properties of vicinity effects caused in substances, which is... 

As a result, free-radical chlorination of alkanes is a nonselective process. Except when only one type of replaceable hydrogen is present (methane, ethane, neopentane, unsubstituted cycloalkanes), all possible monochlorinated isomers are usually formed. Although alkyl chlorides are somewhat less reactive than alkanes, di- and polychlorinations always occur. The presence of a chlorine atom on a carbon atom tends to hinder further substitution at that carbon. The one exception is ethane that yields more 1,1-dichloroethane than 1,2-dichloroethane. The reason for this is that chlorination of an alkyl chloride occurs extremely slowly on the carbon atom adjacent to the one bearing the chlorine atom (vicinal effect).115... 

Alkanes. The chlorination of ethane known to produce more 1,1-dichloroethane than 1,2-dichloroethane is explained by the so-called vicinal effect.115 One study revealed285 that this observation may be explained by the precursor 1,2-dichloroethane radical (the 11 2-chloroethyl radical) thermally dissociating into ethylene and a chlorine atom [Eq. (10.54)]. Indeed, this radical is the major source of ethylene under the conditions studied. At temperatures above 300°C, the dissociation dominates over the chlorination reaction [Eq. (10.55)], resulting in a high rate of ethylene formation with little 1,2-dichloroethane ... 

Vicinal effects can also play a part in the course of the reaction utilizing Oppenauer conditions. 1,3-Diols or / -amino alcohols may not react, presumably on account of formation of an aluminum complex.5 5 46b> If oxidation were to take place it would probably be followed by dehydration to give an unsaturated ketone. Retro-aldol cleavage has been found to occur with a 17,21-dihydroxy steroid.32 The 11 -hydroxyl group which is generally inert to Oppenauer oxidation will react if a hydroxyl group is present on the... 

The circular dichroism (CD) spectra of optically active di-, tri-, and tetranuclear complexes of chromium(III) and cobalt(III) have been reported and used to establish the complexes absolute configurations (55 59, 111, 115, 116, 152-157). The changes in circular dichroism resulting from ion pairing have been studied for the tetranuclear hexol Co j(OH)2Co(NH,)4J, h+ and have been shown to be attributable to the vicinal effect of the chiral oxygen centers produced stereospecifically by the ion-pair formation (56). For a series of trinuclear cobalt (III) amine complexes, cis-Co(CN)2[(OH)2Co(N4)2 J3 +, it was shown that the main CD contributions due to the two chiral Co(OH)4(CN)2 and Co(N4)(OH)2 centers are additive (155). In the case of the related tetranuclear complex Co((OH)2Co(en)2J,< + this postulate of additivity of CD spectra proved unsatisfactory (57). 

Solid-state CD can provide information on solute-solvent interactions when compared with the solution spectra in various solvents. The effects of solvents on the rotatory power are often the results of the formation of some kind of coordination compound between the solvent and the optically active molecules concerned in solution [10,18]. This may affect the optical activity of the molecule by way of conformation alteration in the case of flexible compounds, or through vicinal effects. In contrast, in the solid state, molecules are densely packed and are under a much stronger influence of neighboring molecules. In one sense, this situation can be regarded as an extreme case of the solvent effect [11]. Thus an unusual conformation of a chiral molecule that is unstable in solution may be... 

Figure 3.47 shows the evolution of the heating process of the composite block and how it attains a complex steady state structure with the surface zones covered by complicated isothermal curves (see also Fig. 3.46). Secondly, this figure shows how the brick with the higher thermal conductivity is at steady state and remains the hottest during the dynamic evolution. As explained above, this fact is also shown in Fig. 3.46 where all high isothermal curves are placed in the area of the brick with highest thermal conductivity. At the same time an interesting vicinity effect appears because we observe that the brick with the smallest conductivity does not present the lowest temperature in the centre (case of curve G compared with curves A and B). The comparison of curves A and B, where we have X = 0.2, with curves C and D, where X = 0.4, also sustains the observation of the existence of a vicinity effect. In Fig. 3.48, we can also observe the effect of the highest thermal conductivity of one block but not the vicinity effect previously revealed by Figs. 3.46 and 3.47. If we compare the curves of Fig. 3.47 with the curves of Fig. 3.48 we can appreciate that a rapid process evolution takes place between T = 0 and T = 1. Indeed, the heat transfer process starts very quickly but its evolution from a dynamic process to steady state is relatively slow.

A classic paper by Corey and Bailar (7) provided the basis of understanding stereospecific effects in chelate complexes through conformational analysis of the chelate rings. Dwyer (18j 58) obtained all of the mixed en-(—)pn complexes with Co(III). The energy differences proved consistent with predictions from the Corey-Bailar treatment. Contributions to the optical activity from the vicinal effect of the pn and from the spiral of the chelate rings have been studied and found to be additive (10). Conformational aspects of chelate rings have been reviewed recently (23). 

The examination of additive contributions (1, 3, 5, 6, ) such as configurational, conformational, and vicinal effects of optically active ligands has been useful in the correlation of stereochemical effects and CD spectra. The ligand-polarization model (20, 21, 22, 24) of optical activity depends upon the polarizability of the perturbing groups which constitute the dissymmetric environment around the symmetric chromophore. Phenyl subsitutents which have large anisotropic polarizability can make contributions with signs reversed from those expected (20). 

Additivity of Circular Dichroism of d—d Transitions The Vicinal Effect in a Homologous Series of Triethylenetetraaminecobalt(IH) Amino Acid Complexes... 

Figure 4. Algebra involved in calculating vicinal effects...

Figure 5. The average vicinal effects calculated for Compounds 2, 3,4, and S...

Figure 10. Vicinal effect for /3,-bound (-) alanine derived from the CD spec-...

Figure 11. Vicinal effect for ffi-S-proline derived from Complexes 9S and 9n...

This report describes a method of experimentally obtaining the vicinal effects of amino acid anions bound to a tetraamine-cobalt(III) moiety by dealing exclusively with A-B2 complexes of both R and S-amino acids. Additivity of circular dichroism of both the configurational and vicinal effects for - transitions is verified experimentally. It is demonstrated that the vicinal effect not only contains information as to the chirality of the bound amino acid but also as to the mode of binding, i.e., Bi vs. b2-... 

Most CD spectral studies of cobalt(III) complexes have been undertaken to investigate various sources of optical activity such as distribution of chelate rings, conformation of chelate rings, vicinal effect due to asymmetric carbon in an optically active ligand, and vicinal effect due to an asymmetric donor atom. Extensive reviews on these subjects have been written by Fujita and Shimura (1), Hawkins (2), and Mason (3). 

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