Electron nature

Properties and Mature of Bonding. The metaUic carbides are interesting materials that combine the physical properties of ceramics (qv) with the electronic nature of metals. Thus they are hard and strong, but at the same time good conductors of heat and electricity. 

Polarization and dipole moment studies for alkyl-, aryl-, carbonyl- hydroxy- (keto-) and amino-isoxazoles have been compiled and likewise support the low electron nature of the ring 63AHC(2)365, 62HC(l7)l,p. 177). More recent studies predict the order of electrophilic substitution to be 5>4> 3 on frontier electron density values of 0.7831, 0.3721 and 0.0659, respectively 7lPMH(4)237,pp.245,247). This contrasts with earlier reports of 4>5>3 on density values of —0.09, -t-0.14 and -t-0.18 in that order 63AHC(2 365). 

For cyclopentanone, cyclohexanone, and cycloheptanone, the K values for addition are 48, 1000, and 8 M , respectively. For aromatic aldehydes, the equilibria are affected by the electronic nature of the aryl substituent. Electron donors disfavor addition by stabilizing the aldehyde whereas electron-accepting substituents have the opposite effect. 

The addition of imidazole to the ethyl hemiacetal of tnfluoroacetaldehyde provides 1 -(1 -hydroxy-2, 2, 2 -tnfluoroethyl)imidazoles in yields depending upon the electronic nature of the substiments [5] (equation 5) (Table 1)... 

Now, since the R-D bond is shorter than the R-H bond, but the electronic natures of H and D are identical, there must be a greater electron density at R in R-D than in R-H. Thus, solely as a consequence of the quantum mechanical zero-point energy... 

Although cTi estimates by different methods or from different data sets may disagree, it is generally held that the inductive effect of a substituent is essentially independent of the nature of the reaction. It is otherwise with the resonance effect, and Ehrenson et al. have defined four different ctr values for a substituent, depending upon the electronic nature of the reaction site. An alternative approach is to add a third term, sometimes interpreted as a polarizability factor, and to estimate the inductive and resonance contribution statistically with the added parameter the resonance effect appears to be substantially independent of reaction site. " " ... 

Figure 8-7 illustrates schematically the electronic natures of the polar ground state (an ion pair) and the less polar excited state. This is, therefore, a case of p.g. 

There are three main criteria for design of this catalytic system. First, the additive must accelerate the cyclopropanation at a rate which is significantly greater than the background. If the additive is to be used in substoichiometric quantities, then the ratio of catalyzed to uncatalyzed rates must be greater than 50 1 for practical levels of enantio-induction. Second, the additive must create well defined complexes which provide an effective asymmetric environment to distinguish the enantiotopic faces of the alkene. The ability to easily modulate the steric and electronic nature of the additive is an obvious prerequisite. Third, the additive must not bind the adduct or the product too strongly to interfere with turnover. 

The reactivity of an acid derivative toward substitution depends both on the steric environment near the carbonyl group and on the electronic nature of the substituent, Y. The reactivity order is acid halide > acid anhydride > thioester > ester > amide. 

The electronic nature of TsHs (and other simple alkyl substituted TsRg species) has been investigated experimentally by UV absorption and photolumines-... 

Mataka and coworkers further studied the exo/endo selectivity of outside attack products in the reactions of 96 and 97 with A-(5-X-phenyl)maleimides [56]. They found that the endo/exo selectivity is markedly dependent on the electronic nature of the substituent X (Scheme 47). The electro-withdrawing substituents such as NOj and Cl enhance enrfo-selectivity. The relative order of the enr/o-selectivity is NOj > Cl > H > OCH3. 

Among the Friedel-Crafts alkylations of aromatic compounds with (chlorinated alkyl)silanes, the alkylation of benzene with (tt>-chloroalkyl)silanes in the presence of aluminum chloride catalyst was generally affected by two factors the spacer length between the Cl and silicon and the electronic nature of substituents on the silicon atom of (w-chloroalkyl)silanes. As the spacer length between the C—Cl and silicon increases from (chloromethyl)silane to (/i-chloroethyl)silane to (/-chloropropyl)silane, the reactivity of the silanes increases. As the number of chloro-groups on the silicon decreases from (chloromethyl)trichlorosilanes to (chloromethyl)methyldichlorosilanes to (chloromethyl)trimethylsilanes, the... 

Phenazine leucos are capable of providing yellow, orange, red, and magenta images whereas thiazine and oxazine leucos are normally restricted to turquoise, blue, and purple colors.19 Color depends on the electronic nature of substituents R1 to R4, as shown in Table 3. 

The derivatives 15a-i exhibit characteristic 31P NMR spectroscopic data (see Table I), which are distinctly different from those observed for the P-organo-substituted derivatives 12 and 13 (8(3IP) = 65.8-136.0).27 However, the similar Si chemical shifts and. /(Si, P) coupling constants observed for 12,13, and 15 clearly show the identical electronic nature of the low-coordinate silicon centers in these derivatives. 

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