Mulliken charge-transfer theory

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). 

A nearly universal feature of EDA complexation is the presence of new absorption bands in the electronic spectrum of the complex that are not found in the spectrum of uncomplexed donor or acceptor [137-140]. These spectral bands are observed even in cases where no other evidence of complexation exists, i.e., where Keda is too small to measure. The charge-transfer resonance theory of Mulliken [141] was originally formulated to account for these striking spectral features. According to Mulliken, the ground-state wave function for the complex can be formulated as... 

The authors [33] have elucidated the linear dependence of Ao0 (z-dep) on E for the polyanions by a quantum chemical consideration. A model Hamiltonian approach to the charge transfer (CT) interaction between a polyanion and solvents has been made on the basis of the Mulliken s CT complex theory [34]. 

It is important to recognize that the intermolecular long-distance bonding with the participation of halogen derivatives represents a specific example of the broad general area of donor/acceptor interactions. Moreover, the complexes of molecular iodine, bromine and chlorine with aromatic donors represent classic examples of charge-transfer compounds [26-28] that are vital for the development of Mulliken theory of intermolecular association [29-31]. The latter thus provides the convenient framework for the... 

In the same year as that of the proposal of the frontier-electron theory, the theory of charge-transfer force was developed by Mulliken with regard to the molecular complex formation between an electron donor and an acceptor 47>. In this connection he proposed the "overlap and orientation principle 48> in which only the overlap interaction between the HO MO of the donor and the LU MO of the acceptor is considered. 

Electron-transfer mechanism for nucleophilic addition. In accord with Mulliken theory, irradiation of the charge-transfer band of [Py+, BMeT] directly affords the radical pair via one-electron transfer (equation 46). 

The UV-vis spectral analysis confirms the appearance of a new charge-transfer absorption band of the complexes of colorless a-donors (R3MH) and the n-acceptor (TCNE). In accord with Mulliken theory, the absorption maxima (Act) of the [R3MH, TCNE] complexes shift toward blue with increasing ionization potential of the metal hydrides (i.e., tin > germanium > silicon) as listed in Table 8. 

Electron transfer mechanism. In accord with the Mulliken theory, photoexcitation of the charge-transfer band effects the formation of a contact ion radical pair, which undergoes a rapid mesolytic scission of the M—H bond (equation 52). 

Similar vivid colorations are observed when other aromatic donors (such as methylbenzenes, naphthalenes and anthracenes) are exposed to 0s04.218 The quantitative effect of such dramatic colorations is illustrated in Fig. 13 by the systematic spectral shift in the new electronic absorption bands that parallels the decrease in the arene ionization potentials in the order benzene 9.23 eV, naphthalene 8.12 eV, anthracene 7.55 eV. The progressive bathochromic shift in the charge-transfer transitions (hvct) in Fig. 13 is in accord with the Mulliken theory for a related series of [D, A] complexes. 

The electron-transfer paradigm in Scheme 1 (equation 8) is subject to direct experimental verification. Thus, the deliberate photoactivation of the preequilibrium EDA complex via irradiation of the charge-transfer absorption band (/ vCT) generates the ion-radical pair, in accord with Mulliken theory (equation 98). 

The extent to which steric effects adversely affect the attainment of such intimate ion-pair structures would be reflected in an increase in the work term and concomitant diminution of the inner-sphere rate. This qualitative conclusion accords with the reactivity trend in Figure 16. However, Marcus theory does not provide a quantitative basis for evaluating the variation in the work term of such ion pairs. To obtain the latter we now turn to the Mulliken theory of charge transfer in which the energetics of ion-pair formation evolve directly, and provide quantitative informa-... 

According to Mulliken (1952a,b Mulliken and Person, 1969), the formation of the ion radical pair occurs upon the irradiation of the charge-transfer band of the EDA complex (2). The experimental proof of Mulliken theory is... 

IT Complexes may be described in terms of Mulliken s charge-transfer theory (23-25). This theory attributes the complex formation of stable aromatic molecules to an electron donor (D)-electron acceptor (A) interaction which is approximately represented by the combination of three wave functions ... 

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). 

The positive S.P. observed when gases are adsorbed on a metal surface has been atrributed to (a) polarization of the adsorbate by the electron field of the metal double layer 73) and (6) charge-transfer effects 103). The importance of charge-transfer forces has been stressed by Mulliken 87) in his general theory of donor-acceptor interaction. If, as suggested, these charge-transfer forces contribute to the van der Waals attraction, then they probably take part in the physical adsorption process. The complex M X resulting from the adsorption of an inert gas on a metal surface M has been described as essentially no-bond with a small contribution from the structure As seen in Table VI, the S.P., and hence... 

According to Mulliken charge-transfer theory (7), the separation of AhvCT = 0.61 eV for the two series represents the constant difference in the donor properties (ionization potential) of Co(CO)4- and I- in salt pairs with the same acceptor cation. The latter is a corollary of the Mulliken... 

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... 

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