Electron depletion

To calculate electron production must be balanced against electron depletion. Free electrons in the gas can become attached to any of a number of species in a combustion gas which have reasonably large electron affinities and which can readily capture electrons to form negative ions. In a combustion gas, such species include OH (1.83 eV), O (1.46 eV), NO2 (3.68 eV), NO (0.09 eV), and others. Because of its relatively high concentration, its abUity to capture electrons, and thus its abUity to reduce the electrical conductivity of the gas, the most important negative ion is usuaUyOH . 

Cycloaddition of the tetrahydropyrrolopyridine-2-carbaldehyde 153 with electron-depleted alkenes in the presence of a base leads to products, the 111 NMR spectra of which are consistent with their formulation as 154 rather than 155. In the case of the acrylonitrile adduct, the initially formed pyrrolizine reacts with another molecule of acrylonitrile to give a cyanoethyl-substituted derivative <1998CHE1418> (Scheme 43). 

To understand the role of the noble metal in modifying the photocatalysts we have to consider that the interaction between two different materials with different work functions can occur because of their different chemical potentials (see [200] and references therein). The electrons can transfer from a material with a high Fermi level to another with a lower Fermi level when they contact each other. The Fermi level of an n-type semiconductor is higher than that of the metal. Hence, the electrons can transfer from the semiconductor to the metal until thermodynamic equilibrium is established between the two when they contact each other, that is, the Fermi level of the semiconductor and metal at the interface is the same, which results in the formation of an electron-depletion region and surface upward-bent band in the semiconductor. On the contrary, the Fermi level of a p-type semiconductor is lower than that of the metal. Thus, the electrons can transfer from the metal to the semiconductor until thermodynamic equilibrium is established between the two when they contact each other, which results in the formation of a hole depletion region and surface downward-bent band in the semiconductor. Figure 12.6 shows the formation of semiconductor surface band bending when a semiconductor contacts a metal. 

We consider, now, an electron-depleted space charge layer that is gradually polarized in the anodic direction. As long as the Fermi level is located away from the surface state, the interfacial capacity is determined by the capacity of the depletion layer that obeys a Mott-Schottlsy relation as shown in Fig. 5-61. 

We examine an electron transfer of hydrated redox particles (outer-sphere electron transfer) on metal electrodes covered with a thick film, as shown in Fig. 8-41, with an electron-depleted space charge layer on the film side of the film/solution interface and an ohmic contact at the metal/film interface. It appears that no electron transfer may take place at electron levels in the band gap of the film, since the film is sufficiently thick. Instead, electron transfer takes place at electron levels in the conduction and valence bands of the film. 

Halogen atoms at the tetrazole-5-position are readily displaced by better nucleophiles because of the electron-depleted character of the ring. The nucleophilic displacement was much more rapid with l-substituted-5-bromotetrazoles than with the 2-substituted isomers <84CHEC-I(5)791>. These reactions are often best carried out by warming the halotetrazole in the neat amine or hydrazine when these are liquid. Synthetic uses of the reaction are shown in Schemes 6 and 10. New routes to 5-halotetrazole derivatives have been found <95J06468>... 

Thiophene will form adducts with very strongly electron-depleted dienophiles. such as 1,2-dicyanoethyne and tetrafluoroben yne,... 

The Huang-Minlon reduction of 3-formylfuran surprisingly gave 3-methylene-2,3-dihydrofuran. The product undergoes ene reactions with a number of electron depleted alkenes and provides a route to functionalize the 3-position in furan as shown in Scheme 66 (93TL5221). 

Comparison of electron densities in the six-membered ring with those of pyridine shows the 4- and 6-positions to be the likely sites of nucleophilic attack, but they show less electron depletion than the 2- and 4-positions of pyridine. Thus only a few examples of nucleophilic attack are known and they involve displacement of halogen atoms from C-4 and C-6. 

The explanation advanced by the above authors far the relative inertness of (XL) with respect to (XXXIX) involves electron depletion by non-classical resonance. The conformation allowing such resonance is one of high energy in the case of the hydroxy add (XXXTX). but jg forcibly present in laotone (XL) by virtue of its bridged structure, Th non-classical structure envisaged by Woodward and oo-worker 18,s appears to be of the type (XLI). 

The neutral species of this parent is completely hydrated in aqueous solutions45 whereas pteridine is only 20% hydrated under the same circumstances in accord with its less electron-depleted 7r-layer. Hydrated derivatives of 7-azapteridine have since been noted.46 It has been claimed that 2-chloro-4-ethoxycarbonyl-7-azapteridine forms a 1,2-monohydrate in which the hydroxyl group and the chlorine are attached to C-2. This very unusual type of hydrate spontaneously isomerizes to the 3,4-monohydrate.47... 

In physical terms, this implies donation from the p lone pair into the antibonding o cx orbital. The oxygen atom becomes electron depleted and less basic any protonation will be at the sp2 lone pair because attack at the p orbital will diminish the anomeric stabilization.16 Conversely, the electron transfer increases the basicity of the X atom. It becomes more easily protonated, which further weakens the CX bond and encourages the X group to leave. Deslongchamps has shown that this phenomenon has important consequences in acetal chemistry (X = OR), a detailed discussion of which appears in ref. 13. 

Fig. 4.45 Schematic illustrating the electron-depleted layer at a surface of SnOz carrying chemisorbed oxygen.

The nucleophilic H20 attacks the electrophilic carbonyl carbon, which is rendered electron-depleted by the electron withdrawing attached carbonyl oxygen. Note the tetrahedral intermediates in the second and third structures in these, the carbon has the usual tetrahedral arrangement of four bonds. 

For water, with rp = (140 2) ps and t0 = (1886 2) ps, we get Tm = 580 ps. The positron-molecule lifetimes exceed this limit for all halides examined. As data analyses with positron-molecule lifetimes fixed below this limit were definitely not successful, we are confessed that the long lifetimes are a hard experimental fact. However, we have not yet come to a satisfactory explanation what makes the positrons survive this long in the neighborhood of the halide ions. Possible explanations could involve the spin similar to the effect in o-Ps, or electron depletion due to charge effects in the hydration sphere around the halide ions. 

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