Mulliken Charge Transfer Complexes

Typical for perylene and many other n-systems is the relatively high energy of the highest occupied molecular orbital (HOMO). Typical for the bromine molecule, on the other hand, is the low energy of the lowest unoccupied molecular orbital (LUMO). Therefore, in the experiment of Akamatu et al., a Mulliken charge transfer complex is formed between perylene and bromine, where perylene is the electron donor (D) and bromine is the acceptor (A). 

The theory for Mulliken charge transfer complexes is essentially the same as the Marcus theory applied to excited state electron transfer (ET). Typical for the absorption spectrum of a Mulliken charge transfer complex is additional absorption at quite low energy, absorption that does not exist in the pure substances (Figure 18.5). 

The small value of AG for ET and conductivity in a Mulliken charge transfer complex is because the complex gains energy after the electron has been transferred by simple Coulomb attraction. The donating power of D may be measured by its... 

Reaction paths of the base-catalysed hydrolysis of iV-ethylbenzamide PhCONHEt were traced by DFT calculations and the results showed that the number of elementary processes was four and the rate-determining step was the breakdown of the TI, the TS of which led to a novel Mulliken charge-transfer complex Ph(HO) C=0—NH2Et7 A Brpnsted plot of the buffer-catalysed hydrolysis of A-hydroxyphthalimide obtained in buffers covering a range of -1.74 to 8.33 at 323 K was linear and had = 0.29 0.5. The value was 35.8 x 10 s . ... 

The first example of a donor-acceptor molecular complex was noted in 1949 by Bensei and Hildebrand [137] in their studies involving charge transfer complexes between benzene and molecular iodine. Subsequently such complexes were studied by Mulliken [138] and now more recently have been used by Stoddart et al. [16,139] in designing novel self-assembling systems. 

Evaluation of the Work Term from Charge Transfer Spectral Data. The intermolecular interaction leading to the precursor complex in Scheme IV is reminiscent of the electron donor-acceptor or EDA complexes formed between electron donors and acceptors (21). The latter is characterized by the presence of a new absorption band in the electronic spectrum. According to the Mulliken charge transfer (CT) theory for weak EDA complexes, the absorption maximum hv rp corresponds to the vertical (Franck-Condon) transition from the neutral ground state to the polar excited state (22). 

Mulliken s general concept of charge-transfer complexes can be given explicit and quantitative reformulation in the NBO framework. This allows one to recognize the essential electronic continuity that relates CT complexes of different types, including H-bonded species (n-a CT complexes). Particular attention was paid to the interesting 7t-7t CT complexes of NO+ and related pi-acids, which exemplify the distinctive quantal dependence on the shapes of donor and acceptor orbitals. 

Some people argue that Lewis acids should be called electron-pair acceptors and the bases electron-pair donors. This may be concentrating the attention on electron pairs in chemistry more than they deserve M.O. theory suggests a much more nuanced view, and there is no sense in which exactly six electron pairs are involved in the bonding of CrFs" and IrFe according to spectroscopic observations (20). On the other hand, to call Lewis acids and bases simply acceptors and donors invite to confusion with Mulliken s t57pical charge-transfer complexes 37). 

Nitro compounds form charge-transfer complexes with aromatic donors due to polarization of the nitro group as in I and II. According to Mulliken [6, 7] these FI complexes involve hybrid structures with only a dative and no bond. It has been more recently suggested [8] that in the for-... 

For monographs, see Foster Organic Charge-Transfer Complexes Academic Press New York, 1969 Mulliken Person, Molecular Complexes Wiley New York, 1969 Rose, Molecular Complexes Pergamon Elmsford, NY, 1967. For reviews, see Poleshchuk Maksyutin, Russ. Chem. Rev. 1976, 45, 1077-1090 Banthorpe Chem. Rev. 1970, 70, 295-322 Kosower Prog. Phys. Org. Chem. 1965,3, 81-163 Foster Chem. Br. 1976,12, 18-23. 

The term, charge transfer adsorption, has also been applied to adsorption which resembles the charge transfer complexes of Mulliken. 

Arenes spontaneously form intermolecular 1 1 complexes with a wide variety of electrophiles, cations, acids, and oxidants that are all sufficiently electron-poor to be classified as electron acceptors. Spectral, structural, and thermodynamic properties of these donor/ acceptor associates are described within the context of the Mulliken charge-transfer (CT) formulation. The quantitative analyses of such CT complexes provide the mechanistic basis for understanding arene reactivity in different thermal and photochemical processes. 

Mulliken s Quantitative Description of Intermolecular (Charge-Transfer) Complexes... 

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... 

While TT-complexes play, as we shall see, a rather subsidiary role in the sequence of events leading to initiation in typical cationic systems, the formation of charge-transfer complexes can be directly responsible for the synthesis of active species. A charge-transfer complex, in Mulliken s meaning of the term, arises from the interaction of a donor mdecule having a low ionisation potential with an acceptor molecule... 

Most importantly, the organometallic donor-acceptor complexes and their electron-transfer activated reactions discussed in this review are ideal subjects to link together two independent theoretical approaches, viz. the charge-transfer concept derived from Mulliken theory [14-16] and the free-energy correlation of electron-transfer rates based on Marcus theory [7-9]. A unifying point of view of the inner-sphere-outer-sphere distinction applies to charge-transfer complexes as well as electron-transfer processes in organometallic chemistry. 

Direct excitation of electron-transfer states may yield surprising results. This is the case with the bimolecular benzene iodine charge-transfer complex. In solutions this system is the prototypical case of charge transfer as reported by Mulliken [262]. The characteristic 280 nm absorption band of the benzene-iodine system is distinct from any absorption features of neat iodine or benzene. It has been identified as being due to a promotion of the HOMO benzene n electron to a a LUMO orbital on iodine resulting in benzene iodine electron transfer. 

We start our discussion with simple concepts from the band theory for solids, discuss what can break the symmetry of one-dimensional systems, introduce electrical conductivity and superconductivity, present the Mulliken charge transfer theory for solution complexes and its extension to solids, then discuss briefly the simple tt electron theory for long polyenes. Other articles in this volume review the detailed interplay between structure and electronic properties of conductors and superconductors [206], and electrical transport in conducting polymers [207],... 

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