Electron promotion

The lowest-energy electron promotion is from the 2 2 non-bonding n orbital to the 2b i antibonding n orbital, giving the configuration... 

In a molecule with electrons in n orbitals, such as formaldehyde, ethylene, buta-1,3-diene and benzene, if we are concerned only with the ground state, or excited states obtained by electron promotion within 7i-type MOs, an approximate MO method due to Hiickel may be useM. 

From the ground to an excited electronic state the electron promotion involved is likely to be to a less strongly bonding orbital, leading to an increase in molecular size and a decrease in rotational constants. The effect on the rotational fine structure is to degrade it to low wavenumber to give a strongly asymmetrical structure, unlike the symmetrical structure typical of vibrational transitions. 

These transitions correspond to the electronic promotion —> with the promoted electron maintaining it.s- spin unaltered. The orbital multiplicity of the configuration i.s 6 and so corresponds to two orbital triplet terms Ti, and Tjg- If, on the other hand, the promoted electron changes its spin, the orbital multiplicity is again 6 but the two T terms arc now spin triplets, T g and A weak band attributable to the spin-forbidden Mi, transition is indeed... 

At this stage, it looks as though electron promotion should result in two different types of bonds in methane, one bond from the overlap of a hydrogen ls-orbital and a carbon 2s-orbital, and three more bonds from the overlap of hydrogen Is-orbitals with each of the three carbon 2/ -orbitals. The overlap with the 2p-orbitals should result in three cr-bonds at 90° to one another. However, this arrangement is inconsistent with the known tetrahedral structure of methane with four equivalent bonds. 

On absorption of an energy A.cb one of the t2g electrons will be promoted into the Cg set, as on the right side of Fig. 3-8. As the Cg set is now full, no further electronic promotions are possible so that this corresponds to the one and only excited state of the octahedral d configuration. We thus observe a single absorption band in the d-d spectrum. The excitation is equivalent to the transfer of the hole... 

Configuration interaction, which is necessary in treatments of excited states and desirable in calculations of spin densities, is more complex with open-shell systems. This is because more types of configurations are formed by one-electron promotions. These configurations (Figure 5) are designated as A, B, Cq, C(3 G is the symbol for a ground state. Configurations C and Cp have the same orbital part but differ in the spin functions. 

The catalyst was reformulated by Alwin Mittasch, who synthesized some 2500 different catalysts and performed more than 6500 tests. They arrived at a triply promoted catalyst consisting of a fused iron catalyst, with AI2O3 and CaO as structural promoters and potassium as an electronic promoter. The process was first commercialized by BASF, with the first plant located in Oppau in Germany producing 30 tons per day in 1913. The plant initially produced ammonium sulfate fertilizer, but when the First World War broke out it was redesigned to produce nitrates for ammunition. The plant was expanded and in 1915 it produced the equivalent of 230 tons ammonium per day. 

Explain how electronic promoters assist the dissociation of molecules such as N2. 

Conductivity means that an electron moves under the influence of an applied field, which implies that field energy transferred to the electron promotes it to a higher level. Should the valence level be completely filled there are no extra higher-energy levels available in that band. Promotion to a higher level would then require sufficient energy to jump across the gap into a conduction level in the next band. The width of the band gap determines whether the solid is a conductor, a semi-conductor or an insulator. It is emphasized that in three-dimensional solids the band structure can be much more complicated than for the illustrative one-dimensional model considered above and could be further complicated by impurity levels. 

Thus, electronic promotion to an excited state may be considered in many respects analogous to partial ionization. In each case, distinctive opportunities (namely (3.169) and (3.172)) become available for strong NBO donor-acceptor delocalizations. 

Rather surprisingly, an equal contribution to the conductivity will come from positive charge carriers equal in number to the electrons promoted into the conduction band. These are vacancies in the valence band and are called positive holes or more generally holes. Each time an electron is removed from the full valence band to the conduction band, two mobile charge carriers are therefore created, an electron and a hole. 

The pioneering works of Hill and Eddows have opened the way to realize fast and efficient electron transfer of enzymes at the electrode surface. They modified a gold electrode with 4,4 -bipyrydyl, an electron promoter, not a mediator since it does not take part in electron transfer in the potential region of interest, to accomplish rapid electron transfer of cytochrome [1], Their work has triggered intensive investigation of electron transfer of enzymes using modified electrodes [2]. 

Apart from electron promoters a large number of electron mediators have long been investigated to make redox enzymes electrochemically active on the electrode surface. In the line of this research electron mediators such as ferrocene and its derivatives have successfully been incorporated into an enzyme sensor for glucose [3]. The mediator was easily accessible to both glucose oxidase and an electron tunnelling pathway could be formed within the enzyme molecule [4]. The present authors [5,6] and Lowe and Foulds [7] used a conducting polymer as a molecular wire to connect a redox enzyme molecule to the electrode surface. 

Excited singlet oxygen has one electron promoted to a higher valence orbital making an unpaired electron and a highly reactive molecule. Excited singlet oxygen will... 

Catalysts consisting of more than one component are often superior to monometallic samples. Model studies with potassium on Fe surfaces revealed, for example, the role of the electronic promoter in ammonia synthesis. A particularly remarkable case was recently reported for a surface alloy formed by Au on a Ni(lll) surface where the combination of STM... 

The occurrence of an electron-transfer reaction ensures that an electroactive species has a different number of electrons before and after reaction, so different redox states must of necessity display different spectroscopic transitions, and hence will require different energies E for electron promotion. Accordingly, the colours of electroactive species before and after electron transfer will differ. It follows that all materials will change their spectra following a redox change. 

There is a widespread opinion that stopping of projectiles at velocities below the Bohr velocity is essentially quantal. The claim appears plausible for insulators to the extent that electron promotion may be an essential sink of energy. 

The difference in the electron affinity between light and heavy isotopic isomers is, in other words, the difference in the stability of their anion-radicals. Such a difference gives a valuable tool for use in probing the chemistry of anion-radicals. The difference in the stability of the ring-deuterated and ring-nondeuterated arene anion-radicals has been employed to examine the transition states for the one-electron-promoted cleavage of naphthyl methyl phenyl ether and naphthyl benzyl ether (Guthrie and Shi 1990). In this reaction, the potassium salt of fluoranthene anion-radical was an electron donor ... 

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