Electronic structure three postulates

The nature of the metal-to-dioxygen bonding is not well understood.21 Undoubtedly both a and % orbitals of the oxygen atoms play some role. In the most irreversibly formed complexes, i.e. those with the longest O—O bonds, the electronic structure can perhaps be described fairly accurately by postulating a set of three single bonds, two M—O and one O—O. However, this may be too simple a picture, since it has been claimed that electron-spectroscopic results imply the transfer of ca. 1.4 electrons to 02 in... 

It is difficult to give a localized orbital description of the bonding in a period 3 hypervalent molecule that is based only on the central atom 3s and 3p orbitals and the ligand orbitals, that is, a description that is consistent with the octet rule. One attempt to do this postulated a new type of bond called a three-center, four-electron (3c,4e) bond. We discuss this type of bond in Box 9.2, where we show that it is not a particularly useful concept. Pauling introduced another way to describe the bonding in these molecules, namely, in terms of resonance structures such as 3 and 4 in which there are only four covalent bonds. The implication of this description is that since there are only four cova-... 

Turning to non-metallic catalysts, photoluminescence studies of alkaline-earth oxides in the near-ultra-violet region show excitation of electrons corresponding to three types of surface sites for the oxide ions which dominate the surface structure. These sites can be described as having different cation co-ordination, which is normally six in the bulk, depending on the surface location. Ions on a flat surface have a co-ordination number of 5 (denoted 5c), those on the edges 4 (4c), and the kink sites have co-ordination number 3 (3 c). The latter can be expected to have higher chemical reactivity than 4c and 5c sites, as was postulated for the evaporation mechanism. 

In the present studies we have postulated three additional framework structures closely related to the mordenite (Cmcm) structure (11) and have examined how these structures might be distinguished from mordenite by x-ray and electron diffraction studies. We then critically re-examined the published literature and various available specimens to verify the existence of these other structures. Since verification was established, we examined some specimens further and considered the consequences of the various structures. 

The reactive species generated by the photoexcitation of organic molecules in the electron-donor-acceptor systems are well established in last three decades as shown in Scheme 1. The reactivity of an exciplex and radical ion species is discussed in the following sections. The structure-reactivity relationship for the exciplexes, which possess infinite lifetimes and often emit their own fluorescence, has been shown in some selected regioselective and stereoselective photocycloadditions. However, the exciplex emission is often absent or too weak to be identified although the exciplexes are postulated in many photocycloadditions [11,12], The different reactivities among the contact radical ion pairs (polar exciplexes), solvent-separated radical ion pairs, and free-radical ions as ionic species... 

Benzene has two major resonance structures that contribute equally to the resonance hybrid. These are sometimes called Kekule structures because they were originally postulated by Kekule in 1866. You may also encounter benzene written with a circle inside the six-membered ring rather than the three double bonds. This representation is meant to show that the bonds in benzene are neither double nor single. However, the circle structure makes it difficult to count electrons. This text uses a single Kekule structure to represent benzene or its derivatives. You must recognize that this does not represent the true structure and picture the other resonance structure or call upon the MO model presented in Section 16.3 when needed. 

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