Electron donors/acceptors, chemical reactions

A reasonable model has been proposed to accommodate these results (2/y 23). The presence of quinoid functions in lignin would give rise to electron donor-acceptor complexes with existing phenolic groups. These complexes, like quinhydrone, would form stable radical anions (semiquinone anions) on basification, according to the scheme shown below. Both biological and chemical oxidation would create more quinone moieties, which in turn would increase the contribution of Reactions 1 and 2. Alternately, enzymatic (< ) and/or alkaline demethylation 16) would produce... 

Electron transfer profoundly affects chemical reactivity by inverting normal electron densities in electron donor/acceptor pairs and therefore activating previously inaccessible reaction modes. The basic principles have been widely discussed in several reviews [69-72]. 

Photoinitiated electron transfer reactions are among the earliest photochemical reactions documented in the chemical literature and (ground state) electron donor-acceptor interactions have been known for over one hundred years. Some aspects of plant photosynthesis were already known to Priestly in the eighteenth century. The photooxidation of oxalic acid by metal ions in aqueous solution was discovered by Seekamp (UVI) in 1803 and by Dobereiner (Fe,n) in 1830. The electron donor-acceptor interactions between aromatic hydrocarbons and picric acid were noticed by Fritzsche in the 1850s the quinhydrones are even older,... 

Photochemical dehydrofragmentation has also been observed by Whitten et al. [16, 17] in PET reactions of aminoalcohols. The reaction is restricted to the geminate pair and the complimentary roles of reduced acceptor and oxidized donor facilitate chemical reaction in competition with back ET. The rapid fragmentation is dependent on the acceptor anion-radical induced deprotonation of the donor cation radical in the contact ion-pair and is strongly dependent on the structure of A [16]. The chemical transformation converts the aminoalcohol into the free amine, aldehyde and reduced electron acceptor. The efficiency of the PET induced fragmentation is affected by the stereochemistry of the aminoalcohol as well as the solvent [18]. Both the thioindigo (TI) and dicyanoanthracene (DCA) sensitized reactions are more efficient in nonpolar solvents such as benzene and... 

The [3 + 2] photocycloaddition (Scheme 6.79) usually involves the ground-state alkene and the Si excited state of an electron-donor substituted benzene derivative, often via an exciplex intermediate.807,809 811,816 The discrimination between the ortho- and metacycloaddition pathways is dependent on the electron donor acceptor properties of the reaction partners and the position and character of the reactants substituents.807 The reaction typically produces many regio- and stereoisomers however, a suitable structure modification can reduce their number. Intermolecular and intramolecular versions of the reaction are presented in Scheme 6.88 (a) photolysis of the mixture of anisole and 1,3-dioxole (226) leads to the formation of two stereoisomers, exo- and endo-221, in mediocre ( 50%) chemical yields 830 (b) four different isomers are obtained in the intramolecular photocycloaddition of an anisole derivative 228. 831... 

The most difficult chemical step in precursor Z synthesis is the cleavage of the C2 -C3 bond with subsequent insertion of the C8 atom. While the mechanistic details of this process are unknown, it appears that strict radical transfer from dAdo to 5 -GTP must occur with significant rearrangement of active site molecules in a manner that prevents unwanted side reactions. The C-terminal [4Fe S] cluster may play an important, heretofore unidentified role during catalysis, either functioning as an electron donor/acceptor system or coordinating reaction intermediates. 

Examination of the chemical, electrochemical, and spectroscopic behaviour of R3EM(C0)3L (R = Ph or Me, E = Ge or Sn, M === Mn or Re, L = phen or bpy) reveals that the lowest state is one in which an electron has been transferred from the HOMO, which has sigma E—M bonding character to the LUMO which is mainly localized on The rhenium complexes emit in fluid solution at room temperature, and the luminescence may be quenched by both electron acceptors and electron donors, as in reactions (5) and (6). No net photochemistry is observed... 

Chemical reactions can be studied at the single-molecule level by measuring the fluorescence lifetime of an excited state that can undergo reaction in competition with fluorescence. Reactions involving electron transfer (section C3.2) are among the most accessible via such teclmiques, and are particularly attractive candidates for study as a means of testing relationships between charge-transfer optical spectra and electron-transfer rates. If the physical parameters that detennine the reaction probability, such as overlap between the donor and acceptor orbitals. 

In bulk chemical reactions, an oxidizer (electron acceptor) and fuel (electron donor) react to form products resulting in direct electron transfer and the release or absorption of energy as heat. By special arrangements of reactants in devices called batteries, it is possible to control the rate of reaction and to accomplish the direct release of chemical energy in the form of electricity on demand without intermediate processes. 

The properties of electron transfer proteins that are discussed here specifically affect the electron transfer reaction and not the association or binding of the reactants. A brief overview of these properties is given here more detailed discussions may be found elsewhere (e.g.. Ref. 1). The process of electron transfer is a very simple chemical reaction, i.e., the transfer of an electron from the donor redox site to the acceptor redox site. 

Frontier Orbitals and Chemical Reactivity. Chemical reactions typically involve movement of electrons from an electron donor (base, nucleophile, reducing agent) to an electron acceptor (acid, electrophile, oxidizing agent). This electron movement between molecules can also be thought of as electron movement between molecular orbitals, and the properties of these electron donor and electron acceptor orbitals provide considerable insight into chemical reactivity. 

The first step in constructing a molecular orbital picture of a chemical reaction is to decide which orbitals are most likely to serve as the electron donor and electron acceptor orbitals. It should be obvious that the electron donor orbital must be drawn from the set of occupied orbitals, and the electron acceptor orbital must be an unoccupied orbital, but there are many orbitals in each set to choose from. 

Orbital energy is usually the deciding factor. The chemical reactions that we observe are the ones that proceed quickly, and such reactions typically have small energy barriers. Therefore, chemical reactivity should be associated with the donor-acceptor orbital combination that requires the smallest energy input for electron movement. The best combination is typically the one involving the HOMO as the donor orbital and the LUMO as the acceptor orbital. The HOMO and LUMO are collectively referred to as the frontier orbitals , and most chemical reactions involve electron movement between them. 

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