Atomic localization

For example, the three NH bonding and three NH antibonding orbitals in NH3, when symmetry adapted within the C3V point group, cluster into ai and e mos as shown in the Figure below. The N-atom localized non-bonding lone pair orbital and the N-atom Is core orbital also belong to ai symmetry. 

Table 1-4 gives some calculated reactivity indices free valence or Wheland atomic localization energies for radical, electrophilic, or nucleophilic substitution. For each set of data the order of decreasing reactivity is indicated. In practice this order is more reliable than the absolute values of the reactivity indices themselves. 

It is notable that pyridine is activated relative to benzene and quinoline is activated relative to naphthalene, but that the reactivities of anthracene, acridine, and phenazine decrease in that order. A small activation of pyridine and quinoline is reasonable on the basis of quantum-mechanical predictions of atom localization encrgies, " whereas the unexpected decrease in reactivity from anthracene to phenazine can be best interpreted on the basis of a model for the transition state of methylation suggested by Szwarc and Binks." The coulombic repulsion between the ir-electrons of the aromatic nucleus and the p-electron of the radical should be smaller if the radical approaches the aromatic system along the nodal plane rather than perpendicular to it. This approach to a nitrogen center would be very unfavorable, however, since the lone pair of electrons of the nitrogen lies in the nodal plane and since the methyl radical is... 

A second theoretical index, and one for which there appears to be more justification in its application to free-radical reactions, is the atom localization energy. This index is a measure of the energy required to localize one electron of the 7r-electron system in the aromatic molecule at the point of attack of the radical. The formation of the intermediate adduct in a free-radical aromatic substitution may be regarded as the sum of two processes one, the localization of an electron at the point of attack and the other, the pairing of this... 

It is of interest that both the methyl affinities and the reactivities of aromatic compounds toward the pheny radical are correlated both by Froax and by atom localization energies. Dewar has shown that the energy required to remove one atom from conjugation (in a hydrocarbon containing an even number of carbon atoms) is greater... 

Figure 2. Minimum atomic localization energy in benzenoid hydrocarbons plotted against energy of the highest occupied molecular orbital (HOMO).

Since there is no good physical framework in which the measured hardness versus temperature data can be discussed, descriptions of it are mostly empirical in the opinion of the present author. Partial exceptions are the elemental semiconductors (Sn, Ge, Si, SIC, and C). At temperatures above their Debye temperatures, they soften and the behavior can be described, in part, in terms of thermal activation. The reason is that the chemical bonding is atomically localized in these cases so that localized kinks form along dislocation lines. These kinks are quasi-particles and are affected by local atomic vibrations. 

The mechanism by which dehydrogenation reactions occur has been clarified by Vennesland et al. (FI, V2) in 1953. In experiments with isotopes using deuterium as an indicator it has been found (FI) that the H-atoms localized in the hydrogenated reaction product are the same as those derived from the hydrogen donor, thus excluding an electron transfer. In this case, they would not be the same since they derive from the hydrogen ions of the water-containing medium. The... 

Those results show that HREM combing with image processing software is an alternative way (but depending on sofisticted equipment like PEG TEM) to arrive to similar results regarding light atom localization. 

Albert, M.R. (2000) Qualitative evidence for adsorbate-induced edge metal atom localization the charaderization of lifted backbond degeneracy in adsorbed CO through the model adsorption of methyl isocyanide on Pd/Al203. J. Catal., 189, 158-69. 

Few examples are known for this type of intramolecular dihydrogen bond. One of them is the trimetallic osmium cluster shown in Structure 5.15 [28]. This compound, described well by various methods, has revealed a number of problems connected with characterizations of dihydrogen-bonded complexes that deserve separate discussion. The hydrogen atoms localized in the x-ray molecular structure of complex 1 of Figure 5.17 provide a formulation of interaction N-H -... 

Table A5-23 Bond Distances Connecting Heavy Atoms. Local Density Models... 

A well understood case is that of quinoline reaction at position 2 is kinetically favored as compared with reaction at position 4, but the adduct from the latter is thermodynamically more stable. This situation, where the site of attack leading to the more stable adduct is the y position, is analogous with those regarding the formation of Meisenheimer adducts from benzene and pyridine derivatives and RCT nucleophiles. Presumably, with quinoline kinetic control favors the position that is more strongly influenced by the inductive effect of the heteroatom. The fact that position 2 of quinoline is the most reactive toward nucleophilic reagents is probably related to the lower 71-electron density at that position.123 However, the predominance of the C-4 adduct at equilibrium can be better justified by the atom localization energies for nucleophilic attachment at the different positions of quinoline. Moreover, both 7t-electron densities and atom localization energies indicate position 1 of isoquinoline to be the most favored one for nucleophilic addition. 

Somewhat more involved is the computation of further reactivity indices Wheland s atom localization energy, A6 exact super-delocalizability, S7 various polarizabilities, ir8 Brown s factor, Z° Dewar s reactivity number, Ap10 and the approximate superdelocali-zability, S 7... 

The letters A and B in parentheses indicate the empirical parameter used in the HMO calculation (A 8jr = 0.5 B 8k+ = 2.0). Af denotes the atom localization energy. c Correlation coefficient. d Number of data points. 

It was found years ago136 that atom localization energies are correlated with free valences and autopolarizabilities. 

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