“…This diverse range of behavior is relatively simply attributed to the hyperconjugative effects of C-F σ* orbitals and of C-Si σ orbitals, as discussed by von Schleyer and coworkers 14 , 15 . Nevertheless, given that substituent effects on ring currents in aromatic systems are typically quite small 16 – 20 , the present dramatic variations as a function of substituents at the saturated meso carbon are unusual indeed. Figure 6 Integrated current densities ( a , c , and e ) and current density plots ( b , d , and f ) for Ni[10-F-isoCor], Ni[10-isoCor], and Ni[10-(Me 3 Si) 2 -isoCor].…”
Density functional theory calculations of magnetically induced current densities have revealed high diatropic ring currents in unsubstituted isocorrole consistent with homoaromatic character. An examination of the Kohn-Sham molecular orbitals showed clear evidence of homoconjugative interactions in four occupied π-type molecular orbitals as well as in the LUMO. Remarkably, substituents at the saturated meso position were found to exert a dramatic influence on the overall current density pattern. Thus, whereas bis(trimethylsilyl)-substitution strongly enhanced the peripheral diatropic current (consistent with enhanced homoaromaticity), difluoro-substitution engendered a strong, net paratropic current (consistent with antihomoaromaticity). In this respect, isocorroles stand in sharp contrast to benzenoid aromatics, for which substituents typically exert a small influence on the current density distribution.
“…This diverse range of behavior is relatively simply attributed to the hyperconjugative effects of C-F σ* orbitals and of C-Si σ orbitals, as discussed by von Schleyer and coworkers 14 , 15 . Nevertheless, given that substituent effects on ring currents in aromatic systems are typically quite small 16 – 20 , the present dramatic variations as a function of substituents at the saturated meso carbon are unusual indeed. Figure 6 Integrated current densities ( a , c , and e ) and current density plots ( b , d , and f ) for Ni[10-F-isoCor], Ni[10-isoCor], and Ni[10-(Me 3 Si) 2 -isoCor].…”
Density functional theory calculations of magnetically induced current densities have revealed high diatropic ring currents in unsubstituted isocorrole consistent with homoaromatic character. An examination of the Kohn-Sham molecular orbitals showed clear evidence of homoconjugative interactions in four occupied π-type molecular orbitals as well as in the LUMO. Remarkably, substituents at the saturated meso position were found to exert a dramatic influence on the overall current density pattern. Thus, whereas bis(trimethylsilyl)-substitution strongly enhanced the peripheral diatropic current (consistent with enhanced homoaromaticity), difluoro-substitution engendered a strong, net paratropic current (consistent with antihomoaromaticity). In this respect, isocorroles stand in sharp contrast to benzenoid aromatics, for which substituents typically exert a small influence on the current density distribution.
“…For nitro carbazole derivatives, substitution of the nitro group causes pronounced changes in aromaticity (as shown by the HOMA index) in the pyrrole (middle) ring, but only weak changes in the benzene rings, as shown in Fig. 18 [119]. Moreover, aromaticity of the pyrrole ring is more significantly affected by the N-substitution than the Csubstitution.…”
Section: •••Hf H•••fh N•••hf and Nh•••f −mentioning
Numerous studies on nitro group properties are associated with its high electron-withdrawing ability, by means of both resonance and inductive effect. The substituent effect of the nitro group may be well described using either traditional substituent constants or characteristics based on quantum chemistry, i.e., cSAR, SESE, and pEDA/sEDA models. Interestingly, the cSAR descriptor allows to describe the electron-attracting properties of the nitro group regardless of the position and the type of system. Analysis of classical and reverse substituent effects of the nitro group in various systems indicates strong pi-electron interactions with electron-donating substituents due to the resonance effect. This significantly affects the pi-electron delocalization of the aromatic ring decreasing the aromatic character, evidenced clearly by HOMA values. Use of the pEDA/sEDA model allows to measure the population of electrons transferred from the ring to the nitro group. Keywords Nitro group. Substituent effects. Molecular modeling. Sigma and pi electron structure. Substituent effect stabilization energy. Charge of the substituent active region This review is dedicated to our friend and mentor Professor Tadeusz Marek Krygowski of the Department of Chemistry of the Warsaw University in gratitude for his guidance, support, and engagement in our research.
“…Also, it can be used to quantify the aromaticity of extended macrocyclic pathways, which is more difficult with nucleus-independent chemical shift calculations 7,26 . We used the DFT-computed bond lengths to calculate the HOMA indices 55,56 , because it is not feasible to accurately extract absolute or even relative bond lengths from the AFM measurements due to substantial distortions in the images [57][58][59] . The calculations confirm the two-fold symmetry of the aromatic system and reveal two types of pyrrole rings with different aromaticity, in accordance with the model introduced in Fig.…”
Section: Effect Of Aromaticitymentioning
confidence: 99%
“…Here, n is the number of bonds in aromatic pathway, V is a normalization constant (257.7 Å −2 for C-C bonds and 93.52 Å −2 for C-N bonds), d i is the respective bond length and d opt is the 'optimal' aromatic bond length 51,55 . We determined the optimal aromatic bond lengths from the average DFT-computed C-C and C-N bond lengths in benzene and pyrrole, respectively: 1.396 Å for C-C bonds and 1.375 Å for C-N bonds.…”
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