F-electron delocalization

Fig. 4, Schematic partial density of states scheme for light actinide metals (s and p states are counted together) with f electrons delocalized...

Fig. 6. Schematic partial density of states scheme for an NaCl-type (binary) compound (with UN as an example) with f electrons delocalized and unhybridized. Uranium is on the left and nitrogen on the right. In ascending order nitrogen valence band f-band tied to the Fermi level the d conduction band. The Fermi level is at zero on the energy scale. The unhybridized band centres, Qi, are shown on the right. This unhybridized model corresponds to the fully ionic model...

The first phase transition in Pm from dhcp to fee has been reported at 10 GPa. The next transition occurred around 18 GPa and Pm is shown to exhibit a dfee structure which in this study has been stable at 60 GPa. The authors point out changes in diffraction patterns between 50 and 60 GPa and attribute them to a possible indication of further phase change. Upon releasing the pressure, the dfee structure reverted to fee and with some hysteresis, back to dhep. Figures 15 and 16 show the diffraction patterns at various pressures and EOS of Pm to 60 GPa, respectively. The authors point out the smooth nature of the fit and the absence of large discontinuities in the volume to come to the conclusion that the f-electron delocalization that is seen in the neighboring elements of Pm is not seen in its case up to 60 GPa. 

While many of the above systems form orientational intermolecular bonds, other are characterized by weakly directional interactions [53,54], and in some cases, such as in triphenyl phosphite, the dominating interaction is expected to be nondirectional [55]. Hence it is possible that liquid polymorphism may also occurs in materials characterized by nondirectional interactions. This possibility is supported by a recent observation of a transition between two amorphous polymorphs in CessAUs, a metallic glass with nondirectional bonds, in which the transition is caused by pressure-induced f-electron delocalization [56]. 

In order for a substitution to occur, a n-complex must be formed. The term a-complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the hydrogen that is displaced. As the term implies, a a bond is formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a-complex is a four-7t-electron delocalized system that is electronically equivalent to a pentadienyl cation (Fig. 10.1). There is no longer cyclic conjugation. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive chargex)f the cation is located at the positions ortho and para to the site of substitution. 

The same mixture of H and I was obtained starting with either of the geometrically isomeric radical precursors E or F. A possible explanation is based on the assumption of a common radical conformer G, stabilized in the geometry shown by electron delocalization involving the radicaloid p-orbital, the p-peroxy oxygen and Jt of the diene unit. The structure of the compounds H and I were determined by H NMR spectra and the conversion of H to diol J, a known intermediate for the synthesis of prostaglandins. 

X-ray analysis of 2-methoxy-4-hydroxy-5//-l-benzazepin-5-one (a benzazatropolone), prepared by methylation of the corresponding 4-hydroxy-l-benzazepin-2,5-dione with Meerwein s reagent, demonstrates the presence of a planar seven-membered ring but, in contrast to tropolone, little 71-electron delocalization.17 Likewise, ll//-dibenz[f>,e]azepin-ll-ones display no significant aromatic character.18 In contrast, 7-chloro-8//-thieno[3,2-c]azepin-8-one (12) has azepine ring hydrogen resonances at 8.7 and 9.02 ppm that indicate a substantial contribution from the polar zwitterionic mesomer 13.19... 

Bretta F. King and F. Weinhold, Structure and spectroscopy of (HCN)n clusters Cooperative and electronic delocalization effects in C H N hydrogen bonding. J. Chem. Phys. 103, 333 347 (1995). 

The third example in Scheme 3.64 represents the cation-radical of 1,3,6,8-tetraazatricyclo [4.4.1.F ]dodecane. Zwier et al. (2002) prodnced evidence of instantaneous electron delocalization over the four equivalent nitrogen atoms. This extensive delocalization in a completely saturated system is a principal featnre of the third example and reveals the consequences of orbital interactions throngh space and bonds. The space—bond delocalization can serve as a driving force for the cation-radical rearrangements as it has recently been exemplified by transformation of the phenyl-honsane cation-radical into a mixtnre of phenylbicyclononenes (Gerken et al. 2005). 

Sherman, D.M. (1985) Electronic structures of Ee " coordination sites in iron oxides application to spectra, bonding and magnetism. Phys. Chem. Min. 12 161-175 Sherman, D.M. (1987). Molecular orbital (SCF-Xa-SW) theory of metal-metal charge transfer processes in minerals I. Application to the Fe vpe charge transfer and electron delocalization in mixed-valenced iron oxides and si-licates.Phys Chem Min 70 1262-1269 Sherman, D.M. (1990) Crystal chemistry, electronic structure and spectra of Fe sites in clay minerals. Applications to photochemistry and electron transport. In Coyne, L.M. McKeever, S.W.S. Blake, D.F. (eds.) Spectroscopic characterization of minerals and their surfaces. A.C.S. Symposium Series 415, 284-309... 

ASED (Atom Superposition and Electron Delocalization) molecular orbital calculations on the formation of monomeric 1-triazolylborane by the process BH3-I-triazole H2-f H2B(Tz) indicate... 

The rest of the chapter is structured in such a way as to give the reader the possibility to get convinced of this description. The description originates from the peculiar properties of the 5 f electron states. Therefore, in Part II, these states will be examined in free actinide atoms. In Part III, the meaning of the atomic information for solids will be discussed the puzzling problem of atomic-like localized states, which, however, spread enough to form narrow bands and allow a delocalized description. In this part, the conceptual tools to understand the puzzle are provided. 

In a) and P) the non bonding-hypothesis for 5 f electrons is retained, differences in cohesive energy being only due to promotion of outer electrons from one to another orbital state and ionization energies (or electron affinities) due to the different valence states attained. Therefore, any further discrepancy found with experimental values, is indicative of the metallic bonding introduced by delocalization of the 5f electrons (point y). 

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