Donor-acceptor model

The authors wish to acknowledge support of this work by the 3M Science Research Laboratory and the Corporate Research Laboratory. Special thanks go to Dr. Allen Sledle for helpful discussions regarding the donor-acceptor model for CO chemisorbed on Pd, and to Dr, Mark Albert for discussions regarding the hydrogen bonding of CO. 

Fig. 1. Schematic representation of the donor-acceptor model for CO adsorption.

On the basis of the donor-acceptor model the first calculational search for bonded systems between He and doubly charged cations was performed [7,11,... 

While a qualitative understanding of the structures and stabilities of Ng molecules was achieved by the donor-acceptor model, the quantitative analysis of the binding properties was provided by the analysis of the electron density distribution. 

It is weU known that many materials, whether they have originally ionic, non-ionic, or molecular lattice stmctures, are transformed into the metallic state by the application of sufficiently high pressure, and indeed this can be expected to be tme of aU materials. Even quite modest increases in pressure can affect interatomic distances, spectral transitions, formal oxidation states, and many other phenomenological parameters, e.g. can increase the coordination number. Various attempts have been made in an effort to estabhsh relationships between pressure and these phenomenological parameters but none of them accounts satisfactorily for all of the observed features. This is almost certainly because of the absence, up to now, of a model which is capable of interpreting the facts without concerning itself with too detailed an interpretation of the binding forces. However, it will be shown here, after a brief survey of the present situation, that the functional approach seems to successfully provide such a model based as it is on an electron-pair donor-acceptor model of molecular interactions. 

As with donor-acceptor model complexes, ET reactions in proteins have been induced by photoexcitation and pulse radiolysis. In pulse radiolysis experiments, it has been found that the ratio of reduction of the surface metal center to the internal protein redox center can be manipulated, depending on the choice of the mediator. With a5Ru "(His-33)Fe" cytochrome c, reduction of the Ru(III) site was 35% efficient with isopropanol as mediator and 95% efficient with pentaerythritol as mediator (74). 

A theory has been developed that suggests that ET in proteins is regulated by pathways that are optimal combinations of through-bond, hydrogen-bond, and through-space links (10, 11, 28). In this model, the molecular orbitals of the protein matrix mediate ET by a superexchange mechanism analogous to that proposed for donor-acceptor model compounds (see above). A computerized procedure (12) was employed to search for such pathways (see Fig. 16), and the ET data for His-33 of horse heart cytochrome c and His-62 of yeast iso-l-cytochrome c were found to... 

A possible explanation for the calculated trend in the metal ion binding energies to ethylene, which will be discussed for the triply bonded substrates, could lie in a consideration of the Dewar-Chatt-Duncanson donor-acceptor model for bridging-type metal-olefin complexes. Their proposed two-way interaction involves mixing of the olefin n electrons with a metal (n + l)sp a hybrid atomic orbital (L —> M, for short) and simultaneous back donation (M L) of metal nd electrons of appropriate symmetry into the olefin k molecular orbital MO. For the monocation metal ions the latter-type interaction should be less favourable due to stabilizaion of the nd electrons by the charge on the metal. L M should be favoured for the same reason stabilization of the (n + l)s and (n+ l)p orbitals by the + 1 charge. 

The electrostatic component, the attraction of a bonded hydrogen atom to a region of relatively high electron density of the base (B), is considered the dominant contribntion to most hydrogen bonds. Frontier molecnlar orbitals and the donor-acceptor model can be applied to understand the covalent contribution. 

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