Electrons, delocalization number

The aromaticity of naphthalene is explained by the orbital picture in Figure 15.12. Naphthalene has a cyclic, conjugated it electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond. Since ten 77 electrons is a Hiickel number, there is tt electron delocalization and consequent aromaticity in naphthalene. 

The cyclic conjugation is continuous if the donors are on one side of the cyclic chain and the acceptors are on the other side (Scheme 14a). Electrons delocalize from a donor Dj to Aj. The electron accepted by Aj can readily delocalize to the neighbor on the other side because it is an acceptor (A ). An electron can delocalize from D, to A. The delocalization can take place along the other path. donates an electron to A. The resulting electron hole in can be supplied with an electron by the neighbor Dj. This is equivalent to the delocalization from Dj to A. Electrons can delocalize in a cyclic manner. Thermodynamic stability of continuously conjugated molecules is under control of the orbital phase property or determined by the number of n electrons. 

Almost all known inorganic heterocychc molecules, where N, O and S atoms with lone pair orbitals are donors while B atoms with vacant p orbitals are acceptors, are classified into discontinuous conjugation. The donors and the acceptors are alternately disposed along the cyclic chain. The thermodynamic stabilities are controlled by the non-cycUc electron delocalization or by the number of neighboring donor-acceptor pairs, but not by the number of % electrons [83]. In fact, both 4n % and 4n + 2% electron heterocycles are similarly known [84,85] (Scheme 33), contradicting the Hueckel rule. 

A quantitative study on local aromaticity based on n-center electron delocalization indices, wheren is the number of atoms in the ring, was performed on benzo[c]furan and benzo[b]furan. 

A more realistic use of free-electron simulation occurs with conjugated systems, assumed to be characterized by a number of electrons delocalized over the entire molecule. The simplest example is the ethylene molecule, C2H4. From the known planar structure... 

In Table 29.1, we summarized our results of electronic structure calculations for Bf -Bjj. We reported symmetry, spectroscopic state, valence electronic configuration, number of 2c-2e peripheral B-B a-bonds, number of delocalized o- and TT-bonds, and assignment of global aromaticity/antiaromaticity. The description of chemical bonding in terms of 2c-2e peripheral B-B a-bonds and nc-2e delocalized a- and tt-bonds was obtained via AdNDP method at... 

Both the V and Li NMR spectra show multiple vanadium and lithium local environments for the as-synthesized material x = 0.15), and the spectra cannot be explained by using a simple model based on the number of crystallographically distinct vanadium sites. On Li-ion intercalation, the V resonances sharpen and shift to higher frequencies (Figure 15) three sharp resonances along with two broader resonances are clearly resolved for the samples prepared at potentials of 3.4 and 3.0 V (x = 0.3 and 0.5, respectively). This behavior is consistent with solid—solution behavior in this potential range and is ascribed to the presence of localized defects at X close to 0 and electron delocalization for 1 > x > 0.05. Three lithium sites were observed in the Li... 

All materials in the Lai- r ,Coi- Fe/)3-(5 (LSCF) family of materials have electronic transference numbers approaching unity. The electronic structure LSC and LSF has often been described in terms of partially delocalized O p—Co band states based on the tg and e levels of crystal-field theory. In... 

The theory of molecular structure based on the topology of molecular charge distribution, developed by Bader and co-workers (83MI2 85ACR9), enables certain features to be revealed that are characteristic of the systems with aromatic cyclic electron delocalization. To describe the structure of a molecule, it is necessary to determine the number and kind of critical points in its electronic charge distribution, i.e., the points where for the gradient vector of the charge density the condition Vp = 0 is fulfilled. 

Aromatic cyclic 7r-electron delocalization does indeed stabilize the planar structure with bond equalization (84ZOR897)—the problem is that, in addition to that effect, there may exist some others that may eventually overshadow it. Thus, the foregoing warrants the conclusion that the preference of a planar or nonplanar geometry of heterocycle depends on a number of factors including aromaticity (antiaromaticity), which may not even be the most important. In any case, this factor should not be disregarded if one wishes to obtain a correct overall energy balance. For example, aromaticity is reflected in the values of inversion barriers. Thus, for antiaromatic 2-azirine the nitrogen inversion barrier is, as was mentioned earlier, 37.7 kcal/mol, whereas in the case of its saturated... 

Multiple Heck reactions have also been applied in a number of ways to prepare polymers (Scheme 40). One-dimensionally 7r-conjugated polymers are attractive materials because of their optical and electrical properties resulting from 7r-electron delocalization along their main chains. Among these, poly(p-phenylenevinylene) (PPV) shows high electrical conductivity, large non-linear optical responses, and electroluminescent activity. One approach... 

The concept of the hydrogen bond has a number of aspects in common with that of resonance (or electron delocalization) in molecular structures some of their qualitative effects were- well recognized before their present names were acquired and both phenomena are of very general occurrence. For their elucidation the same types of question have to be answered (i) What is the resultant electronic distribution and how can this be simply represented (ii) What stabilisation energy is achieved in particular systems in practice (id) Can this be calculated theoretically and so predicted for cases not yet directly measured ... 

Only a small number of monocyclic 1,2,3-triazines are known, so knowledge of the structure of these compounds is poor. The available data on the parent compound (1) and its methyl derivatives do not permit an evaluation of the degree of electron delocalization and aromaticity of (1) or its alkyl derivatives. Attempts at a crystallographic analysis of (1) were unsuccessful as the crystal was unstable under X-ray irradiation. 

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