Delocalisation, defined

Mx is the quantum operator associated with the transverse magnetisation. With regard to the proton magnetic relaxation, the probe determined by the end-to-end vector r, is substituted for any chemical unit attached to the chain segment. The observation is thus delocalised over the space scale defined by the distance r = 5 nm. From the spectroscopy point of view, HD represents a dispersion of non-coherent broadening frequencies and... 

Since the molecules are planar, there is a strict separation between the a-and the 7t-system. In the calculations, the o-system was handled with the usual delocalised orbitals. For the 7i-system two different models were defined. 

The delocalisation energy (AEdeioc) of the parent acids and their anions is defined as the energy difference between the localised wavefunction, < )ioc, and the delocalised ground state, ( >dei in their optimal geometry, i.e. the adiabatic delocalisation energy [104]... 

In a semiconductor, as discussed in the previous section, localisation can also occur as the width of the allowed energy band is reduced, and this was defined in terms of a limiting mobility. The Anderson model shows that disorder can lead to localisation in metals as well as semiconductors. In metals, since conduction is due only to electrons within a partially filled band, the energy in the band tail that separates localised from delocalised electron states is termed the mobility edge. The onset of localisation in a metal occurs at a minimum conductivity. This can be seen as follows. For an electron at the Fermi energy its mean free path, l, is just the scattering time, r, multiplied by the electron velocity at the Fermi energy, vF. Then, from Equations (4.1) and (4.2) it follows that ... 

The common mechanism for all these reactions may be represented as a rearrangement of the alkoxy-anion derived by the action of base on the free hydroxyl group. Without attempting to define the precise timing of events, or the detailed charge distribution, we can represent the transition state as a bond-delocalised structure of the type (ii) cf. Chapter 4, section 13). 

Initially, application of molecular orbital theory gave description of the ground state of the blue Cu site which is shown in Figure 14.2(c) in the xy plane defined by Cu, Cys S, and the two His N. Modem DFT calculations give the same description of the ground state of the blue Cu site, highly covalent with the covalency delocalised into the pTT orbital of the thiolate sulfur. 

Delocalised cations such as that shown in Scheme 6.4 are less easily handled using business rules. The substructural pattern used to define the transformation between forms matches both forms, so the transformation can be described as degenerate . Although business rules might be defined for imidazolium cations like that in Scheme 6.4, these will need to encode properties of substituents at all positions in order to be comprehensive. This is a good example of a situation in which it is best to enumerate the dilferent forms and select the highest ranking one as preferred. 

We can define an aromatic compound - - as a compound which will sustain an induced (diamagnetic) ring current" [35]. Evidence for ir-electron delocalisation in cyclic polyenes is also forthcoming from measures of coupling constants, especially the differences between the coupling constants across neighbouring bonds. If these are markedly different... 

The term bimetallic was introduced by Sinfelt to account for the fact that a catalyst may contain a muldtude of phases containing the acdve metallic components. Of these many phases, a characteristic is the binary alloy. The term alloy can describe a broad range of situations from well-defined phases or solid solutions to surface alloys in cases where bulk alloys are not thermodynamically favoured but a clearly defined surface local arrangement is obtained. Note that the novel core-shell bimetallic structures are included in this catchall term. An historical overview of the properties of alloys in connection with catalysis has been published by Ponec. At present, a broadly agreed view accepts that alloy components can be chemically recognised and, therefore, supports a somewhat localised interpretation of the alloy nature and properties. Obviously, a delocalised view loses most of its meaning in the case of clusters due to its finite dimensions. 

The C-N bonds in the peptide chains have a high degree of multiple character resulting from delocalisation of the nitrogen lone-pair into the carboxyl group. There is restricted rotation about the C-N bond and the amide groups are planar. If the twists are restricted to the C -N axes (( )), and the C -C axes ( /), the secondary structure is defined completely by the/and y values in successive amino acid units (Figure 10.14). 

Computer programs have been developed which can transform the results of molecular orbital calculations into NBOs. An optimal Lewis structure can be defined as that one with the maximum amount of electronic charge in Lewis orbitals (Lewis charge). A low amount of electronic charge in Lewis orbitals indicates strong effects of electron delocalisation [226-228]. In resonance structures, major and minor contributing structures may exist. These analyses provide results which are similar to modem valence bond theory methods. 

The largest well-defined oligomer of this type is currently the linear hexa-indium complex, 21 (Fig. 8), supported by a single ortho- y y substituted P-diketiminate ligand at each four-coordinate In centre and terminated by two iodide substituents to maintain charge balance [200]. Compound 21 displayed an absorption maximum in hexane at ca. 349 nm, which was interpreted as evidence for the existence of electronic delocalisation across the indium a framework analogous to that observed in isoelectronic catenated group 14 systems. 

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