Antiaromaticity of Cycloheptatrienyl Anions: Structure, Acidity, and Magnetic Properties

Investigations of the nature and degree of antiaromaticity of cycloheptatrienyl anion derivatives using both experimental and computational tools are presented. The ground state of cycloheptatrienyl anion in the gas phase is triplet, planar and Baird-aromatic. In DMSO, it assumes a singlet distorted allylic form with a paratropic ring current. The other derivatives in both phases assume either allylic or diallylic conformations depending on the substituent pattern. A combination of experimental and computational methods was used to determine the pKa values of 16 derivatives in DMSO, which ranged from 36 to -10.7. We revealed that the stronger stabilization of the anionic system, which correlates with acidity, does not necessarily imply a lower degree of antiaromaticity in terms of magnetic properties. Conversely, the substitution pattern first affects the geometry of the ring through the bulkiness of the substituents and their better conjugation with a more distorted system. Consequently, the distortion reduces the cyclic conjugation in the π-system and thereby decreases the paratropic current in a magnetic field, which manifests itself as a decrease in the NICS. The triplet-state geometries and magnetic properties are nearly independent on the substitution pattern, which is typical for simple aromatic systems.

Adaptive σ aromaticity in the rhenacyclopropene rings

Species generally exhibit one-state aromaticity either in the lowest singlet state (S0 ) or the lowest triplet state (T1 ) according to the Hückel's and Baird's rules. Hence, it is rare for species exhibit two-state aromaticity in both the S0 and T1 states (termed as adaptive aromaticity), let alone adaptive σ aromaticity. Here, we report adaptive σ aromaticity in unsaturated rhenacyclopropene rings via density functional theory (DFT) calculations. Various aromaticity indices including NICS, ACID, EDDB together with isodesmic reactions support the adaptive σ aromaticity in these rhenacyclopropene rings. As the T1 state of these species is formed by the ππ* excitation, the σ-aromaticity of these three-membered rings in the S0 state could hold in the T1 state. In addition, the aromaticity effect of the fused rings is also examined. Our findings expand the family of adaptive σ aromaticity, enriching the metallaaromatic chemistry.

Current Density and Spectroscopy—A Themed Issue in Honor of Professor Riccardo Zanasi on the Occasion of His 70th Birthday

It is our great pleasure to introduce the Festschrift of Chemistry to honor Professor Riccardo Zanasi (Figure 1) on the occasion of his 70th birthday and to recognize his important contributions to quantum chemistry, particularly in the field of magnetic response and chiroptical spectroscopies [...]

Valence non-Lewis density as an approach to describe and measure aromaticity of organic and inorganic molecules

Based on the linear combination of atomic orbital-molecular orbital by the natural bond orbitals (NBO) theory, the attractive donor-acceptor superposition interaction between filled (Lewis-type) and vacant (non-Lewis-type) orbitals provide a general mechanism for quantal energy lowering. This interaction has a direct impact on the quantity of the second-order stabilization energy. Therefore, the valence non-Lewis density (VNLD) index, the electron density of unoccupied valence nonbonding and antibonding orbitals, is introduced as an approach to describe and measure aromaticity. This index is based on the frontier orbital concept. To investigate the validity of the proposed aromaticity index, we selected several test sets of organic and inorganic molecules such as different ring sizes in cyclic and heterocyclic hydrocarbons, and all-metal and semimetal clusters, and compared our findings with previous aromaticity analysis. According to the results, VNLD values are well correlated and anticipated the order of aromaticity with the formerly introduced criteria. Furthermore, VNLD reveals that the rings with π-sextet electrons localized in a ring are more aromatic than the other rings, thus, it is in line with Clar's rule. Our proposed aromaticity index has advantages such as, easy to obtain from NBO analysis, and does not require reference molecules which made it more applicable for realizing the aromaticity order in many organic and inorganic compounds.