Ablikim Kerim

A study on the aromaticity and magnetic properties of triazolephorphyrazines

The aromaticity of porphyrazine (PA) and its three triazoleporphyrazine (TAP) tautomers as well as their metal complex compounds was examined using the TRE (topological resonance energy) and the MRE (magnetic resonance energy) methods. Their local aromaticity was studied using the BRE (bond resonance energy) and the CRE (circuit resonance energy) methods. The relationship between the global and local aromaticity of TAPs and the position of the substituted nitrogen atom is discussed. The results of these calculations are compared with those provided by other aromaticity indices. Our TRE and MRE results indicate that PA and TAPs possess lower global aromaticity than porphyrin. Our BRE and CRE results show that global aromaticity is closely associated primarily with the five-membered rings which contain the –NH– pyrrolic rings, and the [18]-annulene-like conjugation pathway is not crucial for determining the aromaticity of the entire π-system. Moreover, the circuit-current susceptibility (χi) and ring current (RC) were obtained using the graph-theoretical method. The χi and RC results show that all the compounds sustain a diatropic current along the macrocycle and this current is bifurcated when it passes through every five-membered ring. Ring current maps reveal that in the five-membered rings, the ring current prefers to pass through the C–C bond when the ring contains an –NH– group, and through the C–N bond when the ring does not contain an –NH– group.

A comparative study of the aromaticity of pyrrole, furan, thiophene, and their aza-derivatives

The relative aromaticity of pyrrole, furan, thiophene, and their aza-derivatives has been examined using TRE (topological resonance energy), MRE (magnetic resonance energy), ring current (RC), and ring current diamagnetic susceptibility (χG) methods. The results obtained were compared with results obtained by others who used the energetic method ASE (aromatic stabilization energy), the geometric method HOMA (harmonic oscillator model of aromaticity), and the magnetic method NICS(1) (nucleus-independent chemical shift). The impact of nitrogen atoms on the aromaticity of the aza-derivatives of pyrrole, furan, and thiophene is discussed. An excellent correlation was found between the energetic (TRE, MRE) and magnetic (RC and χG) criteria of aromaticity for all compounds. It was expected that inclusion of a heteroatom would decrease the aromaticity relative to the cyclopentadienyl anion. Our results show that the type of the first heteroatom, which donates two electrons to the system, as well as the number of nitrogen atoms and their positions in the molecule have a strong effect on aromaticity. In general, aromaticity is enhanced when the nitrogen atom is adjacent to the first heteroatom. The magnitude of aromaticity is related closely with the uniformity of distribution of π-electrons in the molecule.

A study of the aromaticity and ring currents of azulene and azaazulenes

The global aromaticity of azaazulenes has been investigated using topological resonance energy (TRE), percentage topological resonance energy (% TRE), and magnetic resonance energy (MRE) methods. The impact of nitrogen atoms on the aromaticity of azazulenes is discussed. The aromaticity results obtained by the TRE and % TRE methods were compared with the nucleus independent chemical shift (NICS(0)) values at the cage center. We found discrepancies between the NICS(0) predictions and our results. We have found that the aromaticity of the azaazulenes depends strongly on the substituted position of the nitrogen atoms on the ring. The local aromaticity was studied using bond resonance energy (BRE) and circuit resonance energy (CRE) indices. The BRE and CRE results show that the 10π-electron peripheral rings play an important role in both the global and local aromaticity of these compounds. Ring current (RC) results show that all the azaazulenes sustain diamagnetic currents regardless of any predicted antiaromaticity.

A Study of the Aromaticity of the Heterofullerene C24–2nBnNn(n = 3 or 6) and C12N12 Isomers

The global aromaticity of the heterofullerene C24–2nBnNn(n = 3 or 6) and C12N12 isomers formed from the initial C24 fullerene with D6 symmetry, has been investigated using both the topological resonance energy (TRE) and the percentage topological resonance energy (%TRE) methods. The local aromaticity was studied using the bond resonance energy (BRE) method. Analysis was made of the effects of the types, the numbers, and the arrangements of heteroatoms on the global and local aromaticity of the molecules under consideration. Our results obtained by the TRE and %TRE methods were compared with the nucleus independent chemical shift (NICS (0)) values at the cage center. We found that NICS(0) values were not suitable for estimating the global aromaticity of these compounds. It was expected that the inclusion of heteroatoms would decrease the antiaromaticity relative to the C24(D6) cage. Our BRE results show that the antiaromaticity is primarily related to the existence of highly reactive antiaromatic bonds in the molecule. We found that when a local structure within the cage had two nitrogen and one boron atom in the same pentagon ring, it played an important role in the global aromaticity of these compounds. Finally, we predicted the kinetic stability using the minimum BRE method.

Aromaticity and kinetic stability of fullerene C36 isomers and their molecular ions

The aromatic character of fullerene C36 isomers was examined by the Hess-Schaad resonance energy (HSRE), topological resonance energy (TRE) and the percentage topological resonance energy (%TRE) models. According to the nucleus-independent chemical shift (NICS) at the cage center, C36 fullerene isomers must be highly aromatic with negative values. However, they are predicted to be antiaromatic with negative HSREs and negative TREs. The TRE method revealed that they all are aromatized by acquiring two or more electrons. NICSs at the cage center and the 2(N + 1)2 rule cannot be used as an indicator of the aromatic stabilization for C36 isomers and their molecular ions. We utilized the bond resonance energy (BRE) model to estimate the kinetic stability of C36 isomers and their molecular ions. C36 isomers are only stabilized kinetically in penta- and hexavalent molecular anions. All the results indicate that aromaticity and kinetic stability are closely related to the cyclic motion of π electrons.

Aromaticity of Heterofullerenes C18BxNy (x + y = 2) and Their Molecular Ions

The aromaticity of all possible substituted fullerene isomers of C18N2, C18B2, C18BN, and their molecular ions which originate from the C20 (Ih) cage were studied by the topological resonance energy (TRE) and the percentage topological resonance energy methods. The relationship between the aromaticity of C18BxNy isomers and the sites where the heteroatoms dope at the C20 (Ih) cage is discussed. Calculation results show that at the neutral and cationic states all the isomers are predicted to be antiaromatic with negative TREs, but their polyvalent anions are predicted to be aromatic with positive TREs. The most stable isomer is formed by heteroatom doping at the 1, 11-sites in C18N2, C18B2, and C18BN. Heterofullerenes are more aromatic than C20. The stability order in the neutral states is C18N2 > C18 BN > C18 B2 > C20. The stability order in closed-shell is C18B28− > C206− > C18BN6− > C18N24−. This predicts theoretically that their polyvalent anions have high aromaticity.

A study on the aromaticity and kinetic stability of fullerene C38 isomers and their molecular ions

ABSTRACT The aromaticity of the fullerene C38 isomers and their molecular ions is examined by the topological resonance energy (TRE) method. The aromaticity order obtained by the TRE method is compared with relative energies found in DFT calculation results reported in literature. It is found that all C38 isomers exhibit antiaromaticity when in the neutral state. However, they are highly aromatic in the hexaanionic states. The minimum bond resonance energy (min BRE) method is utilized to estimate the kinetically stability of the C38 isomers and their molecular ions. According to the min BRE method, C38 isomers are only have kinetic stability in penta- and hexavalent molecular anionic states.

A study of the aromaticity of heteroannelated cyclooctatetraene derivatives

The aromaticity of the rings of thiophene, pyrrole, furan, and benzene annelated cyclooctatetraene (COT) derivatives and of their double charged ions was studied using the graph-theoretical theory of aromaticity. On the basis of topological resonance energy, it was found that the global aromaticity is dependent upon on the arrangement of heteroatoms in the given molecule. Relative stability of these molecules when in different charged states can been explained in terms of the topological charge stabilization rule. We expect that fusing the COT ring with an increasing number of aromatic rings will lead to an increase in the aromaticity of the molecule. According to the bond resonance energy (BRE) and circuit resonance energy (CRE) indices, local antiaromaticity of the COT ring is weakened as the number of fused rings increases, and these changes play a significant role in the global aromaticity of the molecule. For some compounds, our BRE and CRE indices do not predict the same order of magnitude of the local aromatic character of certain rings that the nucleus independent chemical shift (NICS(0) and (NICS(1)) methods predict. Finally, for the available compounds, correlations between the diatropic and paratropic chemical shifts of the protons and our ring current results were analyzed and good agreement was found.

A study on the aromaticity and ring currents of dithienopyridines and dithienobenzene

The global aromaticity of dithienopyridine and dithienobenzene isomers was investigated using the topological resonance energy (TRE) and percentage topological resonance energy (%TRE) methods. The effect of variations in the positions of sulfur and nitrogen atoms on π-electron delocalization is analyzed. The local aromaticity of these isomers is described based on the bond resonance energy (BRE) and circuit resonance energy (CRE) methods. Our BRE and CRE results show that structure of the central six-membered rings has a strong effect on global aromaticity. The aromaticity of these dithienopyridine isomers is enhanced when a complete pyridine unit exists in their middle ring structure, while the aromaticity of the dithienobenzene isomers is enhanced when a complete benzene unit exists in their middle ring structure. For dithienopyridines, our results obtained using the TRE method correlate well with the Bird aromaticity index as reported in the literature. Our ring-current results show that all these comp...