Delocalization definition

NMR data for 4-methyloxazole have been compared with those of 4-methylthiazole the data clearly show that the ring protons in each are shielded. In a comprehensive study of a range of oxazoles. Brown and Ghosh also reported NMR data but based a discussion of resonance stabilization on pK and UV spectral data (69JCS(B)270). The weak basicity of oxazole (pX a 0.8) relative to 1-methylimidazole (pK 7.44) and thiazole (pK 2.44) demonstrates that delocalization of the oxygen lone pair, which would have a base-strengthening effect on the nitrogen atom, is not extensive. It must be concluded that not only the experimental measurement but also the very definition of aromaticity in the azole series is as yet poorly quantified. Nevertheless, its importance in the interpretation of reactivity is enormous. 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

Electrochemistry provides routes to directly prepare nanostructures both delocalized in a random or organized way and localized at predefined surface sites with adjustable aspect ratios. Purity, monodispersity, ligation, and other chemical properties and treatments are definitely important in most cases. By delocalized electrodeposition it is possible to decorate large areas of metal or semiconductor surfaces with structures of a narrow size distribution stable nuclei-clusters can be... 

How then, can one recover some quantity that scales with the local charge on the metal atoms if their valence electrons are inherently delocalized Beyond the asymmetric lineshape of the metal 2p3/2 peak, there is also a distinct satellite structure seen in the spectra for CoP and elemental Co. From reflection electron energy loss spectroscopy (REELS), we have determined that this satellite structure originates from plasmon loss events (instead of a two-core-hole final state effect as previously thought [67,68]) in which exiting photoelectrons lose some of their energy to valence electrons of atoms near the surface of the solid [58]. The intensity of these satellite peaks (relative to the main peak) is weaker in CoP than in elemental Co. This implies that the Co atoms have fewer valence electrons in CoP than in elemental Co, that is, they are definitely cationic, notwithstanding the lack of a BE shift. For the other compounds in the MP (M = Cr, Mn, Fe) series, the satellite structure is probably too weak to be observed, but solid solutions Coi -xMxl> and CoAs i yPv do show this feature (vide infra) [60,61]. 

Despite nearly two centuries of intense scrutiny, aromaticity remains a unique research stimulus in chemistry. The concept of aromaticity is elusive it is not directly observable. Numerous indirect measures have been devised, based on the manifestations and ramifications of aromaticity. One of the most recent and widely accepted definitions [2] described aromaticity as a manifestation of electron delocalization in... 

The eigenvalues calculated either by the Merrifield or ABO method are nearly identical. As can be seen, this analysis leads to the prediction of a number of low-lying infrared absorption bands for this complex. In a discussion of the structure of the Creutz-Taube ion, I pointed out some time ago (27) that the evidence definitely favors a well-delocalized ground state (6 < 1), and predicted that these infrared bands would, thus, not be observed. In a recent publication, Krausz, et al., report that a search for the bands indeed gave negative results, and withdraw their proposed interpretation of the spectrum (48). 

Both cationic adsorption and anionic adsorption belong to what is called ionic adsorption. Covalent adsorption is due to the localized covalent bonding, and metallic adsorption is due to the delocalized covalent bonding. The distinction among these three modes of chemisorption, however, is not so definite that the transition from the covalent through the metallic to the ionic adsorption may not be discontinuous, but rather continuous, in the same way as the transition of the three-dimensional solid compounds between the covalent, metallic, and ionic bonding. 

A model was adopted in which the delocalizing power of the substituent was defined analogously to Fischer s increments (Fischer, 1964) and Arnold s o -scale (Arnold, 1986). Experimental delocalization parameters S"P were calculated according to (10). By definition, 5 is zero for the... 

Spin-crossover, definition, 32 2 Spin delocalization, polynuclear molecules, 43 182-183... 

The definition of aromaticity conceived by Hiickel strictly applies to monocyclic ring systems, but indole, constructed from the fusion of benzene and pyrrole, behaves as an aromatic compound, like quinoline and isoquinoline. The ring fusion, however, affects the properties of both components. This is reflected in the valence bond description of indole, shown in Scheme 7.1, where one canonical representation shows electron density shared between N-1 and C-3 in the pyrrole unit (implying enamine character). Note that although other canonical forms can be drawn, where the lone-pair electrons are delocalized into the benzenoid ring, their energy content is relatively high and they are of limited importance. 

Not surprisingly, bifunctional species preceded the definition of the term distonic, both in the gas and condensed phases. For example, Hammond et al. studied the cage effect for radical pairs generated by decomposition of azo compounds. Among their targets was the doubly protonated amidine (27), whose decomposition yielded a pair of distonic radical cations (28 +).Attachment of the spin-bearing carbon in the 2-position of the diazaallyl function ensures minimal delocalization of unpaired spin into the latter. 

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