Electron donor-acceptor EDA complexes

In electron donor-acceptor (EDA) complexes, there is always a donor molecule and an acceptor. The donor may donate an unshared pair (an n donor) or a pair of electrons in a ti orbital of a double bond or aromatic system (a it donor). One test for the presence of an EDA complex is the electronic spectrum. These complexes generally exhibit a spectrum (called a charge-transfer spectrum) that is not the same as the sum of the spectra of the two individual molecules. Because the first excited state of the complex is relatively close in energy to the ground state, there is usually a... 

It has been shown that in certain cases (e.g., Me4Sn + I2) the reactants in an Se2 reaction, when mixed, give rise to an immediate charge-transfer spectrum (p. 102), showing that an electron donor-acceptor (EDA) complex has been formed. In these cases it is likely that the EDA complex is an intermediate in the reaction. 

This association has its counterpart that was also variously described as an encounter complex, a nonbonded electron donor-acceptor (EDA) complex, a precursor complex, and a contact charge-transfer complex.10 For electrically charged species such as anion/cation pairs (which are relevant to ion-pair annihilation), the pre-equilibrium association results in contact ion pairs (CIP)7 (equation 3)... 

Among oxo-metals, osmium tetroxide is a particularly intriguing oxidant since it is known to oxidize various types of alkenes rapidly, but it nonetheless eschews the electron-rich aromatic hydrocarbons like benzene and naphthalene (Criegee et al., 1942 Schroder, 1980). Such selectivities do not obviously derive from differences in the donor properties of the hydrocarbons since the oxidation (ionization) potentials of arenes are actually less than those of alkenes. The similarity in the electronic interactions of arenes and alkenes towards osmium tetroxide relates to the series of electron donor-acceptor (EDA) complexes formed with both types of hydrocarbons (26). Common to both arenes and alkenes is the immediate appearance of similar colours that are diagnostic of charge-transfer absorp-... 

The nitrosonium cation bears a formal relationship to the well-studied halogens (i.e. X2 = I2, Br2, and Cl2), with both classes of structurally simple diatomic electron acceptors forming an extensive series of intermolecular electron donor-acceptor (EDA) complexes that show well-defined charge-transfer absorption bands in the UV-visible spectral region. Mulliken (1952a,b 1964 Mulliken and Person, 1969) originally identified the three possible nonbonded structures of the halogen complexes as in Chart 7, and the subsequent X-ray studies established the axial form II to be extant in the crystals of the benzene complexes with Cl2 and Br2 (Hassel and Stromme, 1958, 1959). In these 1 1 molecular complexes, the closest approach of the... 

Tetranitromethane produces strongly coloured electron donor-acceptor (EDA) complexes with derivatives of the anthracene213, in dichloromethane. Specific irradiation of the charge transfer absorption band at X > 500 nm produces a rapid fading of the colour of the solutions. From these solutions, adduct 91 is obtained (reaction 24) its structure is ascertained by X-ray crystallographic diffraction. 91 is derived from an anti-addition of fragments of tetranitromethane by a multistep pathway214. 

The nitration reagents (NO2 Y) for electrophilic aromatic nitration span a wide range and contain anions Y such as nitric acid (Y = OH-), acetyl nitrate (Y = OAc-), dinitrogen pentoxide (Y = NO3-), nitryl chloride (Y = Cl-), TV-nitropyridinium (Y = pyridine) and tetranitromethane [Y = C(N02)3-]. All reagents contain electron-deficient species which can serve as effective electron acceptors and form electron donor-acceptor (EDA) complexes with electron-rich donors including aromatic hydrocarbons107 (ArH, equation 86). Excitation of the EDA complexes by irradiation of the charge-transfer (CT) absorption band results in full electron transfer (equation 87) to form radical ion... 

This event leads to the formation of an electron donor-acceptor (EDA) complex involving the formation of a coordinate link between D and M. The availability of the additional electron pair at M causes an increase in electron density at X due to further polarization of the M-X bond. It is apparent that the amount of polarization will depend on both the polarizability of the covalent bond as well as the extent of interaction between D and M. For a given substrate the latter will depend on the donor properties of the donor8). 

A common way to determine Kid values is to measure sorption isotherms in batch experiments. To this end, the equilibrium concentrations of a given compound in the solid phase (Cis) and in the aqueous phase (CIW) are determined at various compound concentrations and/or solid-water ratios. Consider now the sorption of 1,4-dinitrobenzene (1,4-DNB) to the homoionic clay mineral, K+-illite, at pH 7.0 and 20°C. 1,4-DNB forms electron donor-acceptor (EDA) complexes with clay minerals (see Chapter 11). In a series of batch experiments, Haderlein et al. (1996) measured the data at 20°C given in the margin. 

If either D or A is optically active, the CT transition is expected to exhibit CD, because the optically active species perturbe the CT transition. Firstly, Briegleb 54) observed the CD of the electron donor-acceptor (EDA) complex of various kinds of optically active ketones with tetracyanoethylene (TCNE). He concluded that the CT and CD are attributable to the n-> n transition, where n is the lone-pair orbital of the optically... 

Scheme 4. PET in electron donor — acceptor (EDA) complexes (D+ A--) . .vertical" ion pair (D+ A- ), adiabatic ion pair AGS free enthalpy of solvation (cf. [42]).

NACs adsorb preferentially to the siloxane surface of the mineral. The adsorbed NACs on the siloxane site are oriented coplanar to the surface. There are two points of view on the adsorption mechanism the formation of electron donor-acceptor (EDA) complexes between basal oxygens of the siloxane surface and nitroaromatic compound and on the other side H-bonding of NACs to water ligands of exchangeable cations or direct coordination of N02 groups to such cations. The strength of adsorption depends on the structure of the mineral and the characteristics of compound (i.e., number, type and position of substituent) as well as on the type of exchangeable cation of the mineral. 

Spontaneous copolymerizations are encountered much more frequently, particularly when monomers of opposite polarity are mixed [9-10]. Early workers noticed that, upon mixing of certain electron-rich and electron-poor olefins, spontaneous polymerizations occurred without added initiator [99, 124 128]. Mixing electron-rich olefins with electron-poor olefins almost always results in brightly colored solutions. The colors are due to the CT excitation (hvCT) of the electron-donor-acceptor (EDA) complex [129], Theories for these spontaneous polymerizations mostly center around the charge-transfer complexes (CT or EDA complexes) [128]. 

The thionine function, possessing a relatively high redox potential, acts as an electron acceptor upon irradiation with visible light, which is considered to first form the electron-donor-acceptor (EDA) complex (exciplex) with the electron donor (D reductant). The electron in the exciplex further added to the thionine function to produce the colorless leucothionine and the electron acceptor (A). The A may be an oxidized form of D. The dark reaction may take place either by back electron transfer to the thionine function via the EDA complex or by air oxidation of the leucothionine when atmospheric oxygen is present. 

Kochi and co-workers have recently identified and characterized the weak charge transfer complexes between tropylium ion and a series of substituted arenes in acetonitrile solution [74], Photoexcitation of these electron donor acceptor (EDA) complexes leads to an electron transfer from the arene donors to the tropylium ion in accord with Mulliken s theory [75]. Time resolved spectroscopic observation of the arene radical cations (formation within the 30 ps laser pulse) has confirmed their intermediacy. The subsequent decay of the photogenerated radical cation and the concomitant regeneration of the ground state EDA complex occurs with a rate constant, kBET > 4 x 1010 s 1 (Scheme 11). This fast back electron transfer... 

Synonyms for EPDjEPA complex are electron donor acceptor (EDA) complex [50], molecular complex [57, 58], and charge-transfer (CT) complex [51]. Since normally the term molecular complex is only used for weak complexes between neutral molecules, and the appearance of a charge-transfer absorption band does not necessarily prove the existence of a stable complex, the more general expression EPDjEPA complex, proposed by Gutmann [53], will be used here. This will comprise all complexes whose formation is due to an interaction between electron-pair donors (Lewis bases) and electron-pair acceptors (Lewis acids), irrespective of the stabilities of the complexes or the charges of the components. 

In the catalytic combination of nitrogen and hydrogen, the molecules lose their translational degrees of freedom by fixation on the catalyst surface. This drastically reduces the required energy of activation, for example, to 103 kj/mol on iron [100], The reaction may then proceed in the temperature range 250-400 °C. In 1972, it was discovered that electron donor - acceptor (EDA) complexes permit making ammonia with measurable reaction rate at room temperature. 

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