Bond delocalization

Delocalization (Section 1 9) Association of an electron with more than one atom The simplest example is the shared electron pair (covalent) bond Delocalization is important in conjugated tt electron systems where an electron may be associated with several carbon atoms... 

The precise geometrical data obtained by microwave spectroscopy allow conclusions regarding bond delocalization and hence aromaticity. For example, the microwave spectrum of thiazole has shown that the structure is very close to the average of the structures of thiophene and 1,3,4-thiadiazole, which indicates a similar trend in aromaticity. However, different methods have frequently given inconsistent results. 

The description of the nonclassical norbomyl cation developed by Wnstein implies that the nonclassical ion is stabilized, relative to a secondary ion, by C—C a bond delocalization. H. C. Brown of Purdue University put forward an alternative interpreta-tioiL He argued that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical ion. The 1,2-shift that interconverts the two ions was presumed to be rapid relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown proposed that die rapid migration would lead to preferential approach of the nucleophile fiom the exo direction. 

In the ring molecules containing a n bond, delocalization of tt electrons occurs through the interaction with o orbitals [19]. 

Notably, the bicyclic radical 41 is localized, in sharp contrast to the allylic-type delocalization of cyclotrigermenyl radical 39 and cyclotetrasilenyl radical 40. The obvions reason for snch a distinction is the absence of the neighboring to the Ge-radical center tt-bond necessary for the effective throngh-bond delocalization of the unpaired electron in the radical 41, whereas the through-space radical-C=C bond interaction is not sufficiently strong to indnce the effective delocalization of the unpaired electron. 

A particularly attractive group of substrates are the annulenes, since electron transfer allows an interconversion of (4n + 2)n- and (4n)rr-systems, and thus a switch between 7>bond delocalization and rc-bond localization (Mullen, 1984). 

Cluster Configuration Peripheral Bonds Delocalized (r-honds Delocalized ir-bonds Antiaromaticity... 

Fragments in compounds 155—157 exhibit aromatic bond delocalization. The lowest aromaticity is calculated for Af-pyridinium cyclopentadienide 157, with the interfragmental C—N bond shorter than the corresponding one in 155 and 158. The phenolate moiety in 159 has a high NICS value (—4.6 ppm), in agreement with the one for deprotonated phenol (—6.2 ppm compared to —9.7 ppm for benzene, as cited),196 while the acceptor pyridinium counterpart has a NICS value of —5.5 ppm, showing aromatic delocalization. 

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). 

CONFORMATIONS Conjugated double bonds, DELOCALIZATION Conjugate force,... 

Thus, the RE determined from the energy of an isodesmic reaction of bond separation is in fact QMRE-like and represents an estimate of various effects of electron delocalization. By contrast, the use of the homodesmotic reaction leads to a Dewar-type RE (75TCA121) allowing the evaluation of the contribution by precisely the cyclic electron (bond) delocalization. 

By contrast, the Dewar resonance energy represents solely the contribution coming from the cyclic electron (bond) delocalization since the model reference structure is represented not by a system of isolated 7r-bonds, but by a hypothetical cyclic polyene with the number of tr- and tr-bonds equal to that in a given molecule. Making use of the additivity of bond energies in acyclic polyenes (65JA692), one may calculate the total energy... 

The energy of the homodesmotic reaction does not exclusively reflect the effect of cyclic (bond) delocalization. The reference structure is hypothetical and one cannot write the equation of a reaction, where a cyclic and an acyclic structure participate, for which the difference between the energies of products and reactants was determined by a single factor, namely, aromatic stabilization (antiaromatic destabilization) (75TCA121). 

The HSE values estimate the contribution by cyclic (bond) delocalization, whereas the AE values for the isodesmic reaction (ISE) [76JCS(P2)1222] refer to the stabilization energy associated with conjugation as a whole clearly, the latter values turn out appreciably larger, cf. HSE (4), (6) and ISE (3), (5). 

Structural indices constructed in this fashion are, in essence, phenomenological, and one is entitled to ask whether the specific features in the geometry of the aromatic and antiaromatic molecules are indeed determined, and if so, to what degree, by the cyclic electron (bond) delocalization. 

Since the aromaticity is defined as the stabilization due to cyclic electron (bond) delocalization, the data on thermodynamics and kinetics of various reactions leading to removal of cyclic delocalization system (or, conversely, to its formation) may in principle be used for assessing aromatic stabilization or antiaromatic destabilization. 

The aromatic stabilization of a molecule is the energy contribution due to the cyclic bond delocalization. This contribution is defined as the resonance energy (RE)... 

The problem in determining resonance energies is to single out of the total energy of the molecule the contribution from the cyclic bond delocalization. 

In attempting to approximate (CNDO/2) the nature of the 1,2,5-thiadiazole ring (2a vs. 2b), a nontraditional view has been proposed <83IJC(B)802> with two C=N bonds and one three-center two-electron 7t-bond delocalized over the N—S—N system. This would account for the electron-rich N—S—N nature of the molecule. Classical structures (2a, 2b) are viewed as unlikely versus a hybrid structure (2c). 

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