Isomers resonance structures compared

Nuclear y-ray resonance spectra of solid solutions of Fe and Co in /3-rh boron give inconclusive results, although the large isomer shifts as compared to Fe metal indicate that the accommodation of Fe atoms in the boron structure is associated with changes in the electronic state. The magnitudes of the shifts are... 

A perusal of the / values in Table 1 also shows that/aa is higher in the [3,2-6]-annelated systems than in the corresponding [2,3-6] isomers. For example / , in thieno[3,2-6]thiophene (3) and N-benzylthieno[3,2-6]pyrrole (18) is 1.55 and 1.3 Hz compared to 1.17 and 1.0 Hz for the corresponding [2,3-6 ]- annelated systems (7) and (19). This relatively high value for / - in the [3,2-6]-fused systems is a consequence of the transmission of the spin interactions between the protons through the v- framework rather than the cr-framework. In qualitative terms this means that several resonance structures can be written for such systems wherein electronic perturbations at C-2 are effectively transmitted to C-5 rather than C-6 (Scheme 4). 

As was the case for dinitrobenzene, the meta and para nitroaniline isomers have essentially the same gaseous enthalpy of formation. In the gaseous phase, it is surprising to find that despite the more attractive quinonoid resonance structures for the para isomer (58) than for the meta (59) the meta and para nitroaniline have essentially the same gas-phase enthalpy of formation. In the solid and liquid states the intemolecular stabilization lowers the enthalpy of formation of the para isomer relative to the meta. Interestingly, the gas-phase intramolecularly hydrogen-bonded ortho isoma- is of comparable stability to its isomers. In contrast, it is considerably less stable than its isomers in the solid state because it can form fewer intermolecular hydrogen bonds. All isomers of nitroaniline are more stable than calculated by additivity. 

Finally, the (lO.C) and (lO.E) minima, located, respectively, at 0.73 and 2.27 kcal mol of the most stable structure, correspond to the most stable planar configurations. In both cases, their predominant resonance structures are based on the Au6 template. The largest stability of the (lO.C) isomer compared to the (lO.E) isomer may be explained by a larger delocalization of valence electrons. 

Consider 1,3-pentadiene and 1,4-pentadiene. Which, if either, would benefit from the type of resonance described above Draw appropriate resonance contributors for this isomer. Indicate the likely importance of different zwitterionic structures which you might draw. Compare the energies of 1,3-pentadiene and 1,4-pentadiene. Which one is more stable ... 

Pyridyne (98) is the most stable of the three isomers, and, in contrast to 100 and 102, has a higher preference for the singlet ground state than 13. This change suggests the participation of nitrilium ion structure 98B and possibly 98C in the resonance hybrid, which is also evident from the shortening of the N—C2 bond compared to the N—C6 bond. 

Chia and Simmons388 calculated the resonance energies (ER)20 of four mono- and dibenzotetraazapentalenes (Scheme 24). Values are comparable with those of o-condensed aromatic systems (naphthacene, ER = 110 kcal mol-1 chrysene, ER = 116.5 kcal mol-1), and, like these carbocyclic systems, angularly-shaped molecules are more stable than linear ones. HMO calculations of delocalization energies (DE) show that the tetraazapentalene structure 15 is more stable than the tetra-azacyclooctatetraene valence isomer 324 (Scheme 14, Section IV,B,2) whether 324 is planar or tub-shaped. Calculations of electrophilic reactivity (Section IV,C,4,d), electronic spectra (by the PPP method employing all singly excited configurations), and bond orders have been carried out, and they confirm the aromatic nature of these systems. 

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