Aromatic, or Antiaromatic, That Is the Question:

The term aromaticity in chemistry is reminiscent of symmetric, stable structures that have delocalized electrons and sustain diamagnetic ring currents in the presence of external magnetic fields. The two latter characteristics sometimes mistakenly are thought to be the same. In a groundbreaking discovery Liddle and co-workers synthesized a D3h complex with rare Th‒Th bonds. Studying the electronic structure of a neutral singlet model system, 3”, showed that the molecule has a delocalized HOMO between three Th atoms akin to a 2-electron 3-center bond. Employing nucleus independent chemical shift (NICS) computations and according to the pattern of the NICS scan plot on 3”, the authors suggested that the molecule is a σ-aromatic species as it is reflected also in the title of their paper. Unfortunately, NICS is an inappropriate tool for evaluation of the magnetic aromaticity among early and mid-transition metals (TM), the elements that share a characteristic with thorium; they all have low-energy unoccupied orbitals. In the following we explain why NICS fails to assess aromaticity of molecules containing TMs and prove that the Th3 ring is a weakly antiaromatic system. We introduce a simple protocol to examine the aromaticity of any molecule utilizing software packages, which have been used by the authors of the main article.

Nucleus-Independent Chemical Shift (NICS) as a Criterion for the Design of New Antifungal Benzofuranones

The assertion made by Wu et al. that aromaticity may have considerable implications for molecular design motivated us to use nucleus-independent chemical shifts (NICS) as an aromaticity criterion to evaluate the antifungal activity of two series of indol-4-ones. A linear regression analysis of NICS and antifungal activity showed that both tested variables were significantly related (p < 0.05); when aromaticity increased, the antifungal activity decreased for series I and increased for series II. To verify the validity of the obtained equations, a new set of 44 benzofuran-4-ones was designed by replacing the nitrogen atom of the five-membered ring with oxygen in indol-4-ones. The NICS(0) and NICS(1) of benzofuran-4-ones were calculated and used to predict their biological activities using the previous equations. A set of 10 benzofuran-4-ones was synthesized and tested in eight human pathogenic fungi, showing the validity of the equations. The minimum inhibitory concentration (MIC) in yeasts was 31.25 µg·mL–1 for Candida glabrata, Candida krusei and Candida guilliermondii with compounds 15-32, 15-15 and 15-1. The MIC for filamentous fungi was 1.95 µg·mL–1 for Aspergillus niger for compounds 15-1, 15-33 and 15-34. The results obtained support the use of NICS in the molecular design of compounds with antifungal activity.

Relating nucleus independent chemical shifts with integrated current density strengths

Indices based on the nucleus independent chemical shift (NICS) are the most frequently used in analysis of magnetic aromaticity. The magnetically induced current density, on the other hand, is a...

On the aromaticity of uracil and its 5-halogeno derivatives as revealed by theoretically derived geometric and magnetic indexes

The problem of aromaticity in heterocyclic rings of uracil and its 5-halogenoderivatives (5XU) was analyzed theoretically by calculating modified harmonic oscillator model of aromaticity (HOMA) for Heterocycle Electron Delocalization (HOMHED), nucleus-independent chemical shift parameters (NICS) and the so-called scan experiments, using helium-3 atom as a magnetic probe. The impact of halogen electronegativity on C5 atom’s NBO charges was also investigated. Water, as a polar environment, has a negligible impact on 5XU aromaticity. The most stable diketo tautomer shows a very low aromaticity while the “rare” dihydroxy form (tautomer No 6) is aromatic and resembles benzene. This is in agreement with traditional drawing of chemical formula of uracil’s six-membered ring, directly showing three alternating single and double bonds in its tautomer No 6. No good correlation between magnetic and geometric indexes of aromaticity for the studied 5XU tautomers was found. Linear correlation between the magnitude of NICS minimum, as well as the distance of the minimum above uracil ring plane center from 3He NMR chemical shift scan plot with respect to halogen electronegativity were observed. A strong linear dependence of magnetic index of aromaticity and the electronegativity of 5X substituent was observed.

Predicted Reversal in N-Methylazepine/N-Methyl-7-azanorcaradiene Equilibrium upon Formation of Their N-Oxides

Density functional calculations and up to five different basis sets have been applied to the exploration of the structural, enthalpy and free energy changes upon conversion of the azepine to the corresponding N-oxide. Although it is well known that azepines are typically much more stable than their 7-azanorcaradiene valence isomers, the stabilities are reversed for the corresponding N-oxides. Structural, thermochemical as well as nucleus-independent chemical shift (NICS) criteria are employed to probe the potential aromaticity, antiaromaticity and nonaromaticity of N-methylazepine, its 7-azanorcaradiene valence isomer. For the sake of comparison, analogous studies are performed on N-methylpyrrole and its N-oxide.

Synthesis of azabenziporphyrinoids by SN Ar reactions

While most of porphyrins and related macrocycles have been synthesized by classical acid-catalyzed condensation and oxidation reactions, nucleophilic aromatic substitution reactions have recently emerged as a new protocol to construct novel porphyrinoids. [Formula: see text]-Dibromotripyrrin 3 was developed as a useful building block composed of three pyrrolic segments and two meso-carbons, which was allowed to react with various nucleophiles. Herein, we applied this strategy to synthesize carbaporphyrinoids containing two nitrogen atoms at the meso-positions. Reaction of [Formula: see text]-dibromotripyrrin 3 with meta- and para-phenylenediamine gave diazabenziporphyrins 8 and 9in good yields. Their oxidized species 12 and 13 were generated by facile oxidation with lead dioxide. While diaza-meta-benziporphyrin 12 exhibited nonaromatic characteristics, diaza-para-benziporphyrin 13 was revealed to be aromatic in view of [Formula: see text]H NMR, bond-alternation, and nucleus-independent-chemical-shift calculations. Curiously, aza-meta-benzicorrole 11 was obtained in a low yield by intramolecular cyclization reaction of [Formula: see text]-anilino-[Formula: see text]-bromotripyrrin with potassium carbonate.

Bis-palladium(II) complex of doubly N-confused octaphyrin(1.1.1.1.1.1.1.1): Möbius aromaticity and chiroptical properties

A novel bis-palladium(II) complex of doubly N-confused octaphyrin (Pd[Formula: see text]-2) adopting a Möbius-twisted topological structure was synthesized and characterized. Due to the effective 36[Formula: see text]-electronic delocalization over the Möbius-twisted octaphyrin scaffold, characteristic Soret and Q-like absorption features were observed in the near-infrared (NIR) region. However, the complex Pd[Formula: see text]-2 exhibited weak aromatic character attributed to the distinct cross-conjugated resonance contribution as inferred from the 1H NMR chemical shifts as well as theoretical assessments ([Formula: see text] nucleus-independent chemical shift (NICS)). Since the bis-palladium(II) complexation of doubly N-confused octaphyrin 2 imparted significant conformational stability, topologically chiral enantiomers of Pd[Formula: see text]-2 were successfully separated as ([Formula: see text]- and ([Formula: see text]-twisted forms. The resulting isomers revealed relatively large circular dichroism (CD) responses with an absorption anisotropy factor of [Formula: see text] [Formula: see text] 0.009 in the NIR region ([Formula: see text] 823 nm). In addition, the cyclic voltammogram of Pd[Formula: see text]-2 revealed redox-rich properties due to its large [Formula: see text]-conjugated system.

Relativistic effects on the aromaticity of E3M3H3 (E = C–Pb; M = N–Bi) benzene analogues

The relativistic effects on the aromaticity of a set of benzene analogues, E3M3H3 (E = C–Pb; M = N–Bi) heterocycles, using magnetically induced current density (MICD) and the NICSzz component of the conventional nucleus independent chemical shift (NICS), is hereby examined.

Synthesis and electronic properties of meso-tetra(3,4,5-trimethoxy)phenyl-porphyrin and -tetrabenzoporphyrin and their phosphorus complexes

meso-Tetra(3,4,5-trimethoxy)phenyl-porphyrin and -tetrabenzoporphyrin and their phosphorus complexes were synthesized and their electronic absorption and magnetic circular dichroism (MCD) spectra discussed based on TD-DFT molecular orbital (MO) calculations. Nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations were also carried out for metal-free (and phosphorus) tetrabenzoporphyrins in order to discuss the aromaticity and flow of electrons in the molecule.