Long range electronic effect

Long range electronic effects sometimes can be discerned. Thus, the striking difference in basicity [4] of 4-cyclopropylpyridine [pKa 6.44] and 4-(2-carbethoxy cyclopropyl)pyridine [pKa 5.58] is attributable to the field effect and through-bond interaction. 

Substituted norsnoutanes (61) have been introduced as substrates with sterically unbiased re-faces, which allow electronic effects in re-facial selectivity of nucleophilic additions to be evaluated.96 Examples indicate how this system allows separation of long-range electronic effects into orbital and electrostatic contributions. 

In recent years further novel classes of compounds were added to cyclophane chemistry. The multilayeredphanesiS derived from [2.2]paracyclophane contain coaxially stacked benzene rings connected by ethano bridges para to one another. The first members of this series were described in 196416. Quadruple layered phane hydrocarbons 3 and 4 reveal in their UV-spectra long range electronic effects penetrating several arene units. 

Several approaches were used to quantify inhibition effects of metal adlayers. These involved calculations of currents of H2 evolution based on the order-disorder theory of alloys (141, 144, 145], and simulations based on geometric [150, 151, 153] and long-range electronic effects [150, 151], Verification of the models used in some simulations seems... 

By increasing the complexity of the cluster the long range electronic effect seems to occur in a completely delocalised way for instance a kind of collective property has been shown to occur in many anionic cluster compounds (see Table 6). 

Direct evidence of similar long range electronic effects has been reported in chemisorption over various films of transition metals [110-113]. In this way, CO can be used as a molecular probe to indicate the electronic changes at the surface of very small particles. When the surface of a crystallite is covered with a small amount of CO, any further chemisorption of a Lewis base will result in an electronic effect very likely through metal atoms of surface of the crystallite (Figure 24). 

Kurnikov I V and Beratan D N 1996 Ab initio based effective Hamiltonians for long-range electron transfer Hartree-Fock analysis J. Chem. Phys. 105 9561-73... 

Donath11 has extended these calculations to cyclohexane and cyclopentane and obtained comparably good agreement. It seems safe to conclude that these long-range electron correlation effects account for the isomerization energies of the paraffins which had heretofore remained unexplained. 

It is worth noting that each Na atom appears to perturb the electron density of the Pt(lll) surface over large ( 12) atomic distances. This can explain nicely the observed long-range promotional effect of Na on Pt surfaces. It is strongly reminiscent of the IR spectroscopic work of Yates and coworkers who showed that a single adsorbed alkali atom can affect the IR spectra of up to 27 coadsorbed CO molecules.80... 

The enthalpies of reaction for nucleophilic carbencs depend on the stereoelec-tronic properties of the ligands affecting the availability of the carbene lone pair. An example of electronic influence is the 3.5 kcal/mol enthalpy difference between the isosteric pair IMes and IMesCI that shows the electron-withdrawing nature of Cl compared to H. This trend again is in line with electron donor/withdrawing ability of arene substituents. The effect in this la.st case is a long range electronic... 

Some substituents induce remarkably different electronic behaviors on the same aromatic system (8). Let us consider, for example, the actions of substituents on an aromatic electron system. Some substituents have a tendency to enrich their electronic population (acceptors), while others will give away some of it (donors). Traditionaly, quantum chemists used to distinguish between long range (mesomeric) effects, mainly u in nature, and short range (inductive) effects, mainly a. The nonlinear behavior of a monosubstituted molecule can be accounted for in terms of the x electron dipole moment. Examples of donor and acceptor substituents can be seen on figure 1. 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

We note the unique, highly ordered titania nanotube array structure enables the conductive electrolyte, in this work IM KOH, to permeate the entire internal and external surfaces, hence there is a constant electrostatic potential along the length of the tubes (no RC ladder effect). Therefore long-range electron transport is dominated... 

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