The aromatic character of the transition state structures (TSs) involved in pseudocyclic reactions of fluorinated compounds

The application of aromaticity and antiaromaticity to reaction mechanisms

Aromaticity, in general, can promote a given reaction by stabilizing a transition state or a product via a mobility of π electrons in a cyclic structure. Similarly, such a promotion could be also achieved by destabilizing an antiaromatic reactant. However, both aromaticity and transition states cannot be directly measured in experiment. Thus, computational chemistry has been becoming a key tool to understand the aromaticity-driven reaction mechanisms. In this review, we will analyze the relationship between aromaticity and reaction mechanism to highlight the importance of density functional theory calculations and present it according to an approach via either aromatizing a transition state/product or destabilizing a reactant by antiaromaticity. Specifically, we will start with a particularly challenging example of dinitrogen activation followed by other small-molecule activation, C-F bond activation, rearrangement, as well as metathesis reactions. In addition, antiaromaticity-promoted dihydrogen activation, CO2 capture, and oxygen reduction reactions will be also briefly discussed. Finally, caution must be cast as the magnitude of the aromaticity in the transition states is not particularly high in most cases. Thus, a proof of an adequate electron delocalization rather than a complete ring current is recommended to support the relatively weak aromaticity in these transition states.

Planarity does not always mean higher aromaticity - Intriguing metalloaromaticity of three Al13+ isomers

A pair of Al₁₃⁺ clusters, one perfectly planar (CI) and another quasi-planar structure (CII) have been reported recently by our group [Guin et al. Journal of Molecular Modeling, 2018, 24, 344]. Both these clusters are rare examples of metal-aromatic systems having unique aromatic character. In these clusters, localized strong anti-aromatic deltas are embedded within a strong aromatic sea. The quasi-planar CII structure is a true minimum structure with zero imaginary frequency, but the planar CI structure has been found to have three imaginary frequencies. Further search for a possible transition structure (CT) with single imaginary frequency was successful, which shows that CT structure is also quasi-planar but the tail region of this cluster is puckered downward as compared to that of the CII cluster for which tail region is puckered upward. A comparative electronic structural analysis of these three clusters have been carried out which has provided insight into the way the transformation among the three states take place. Harmonic frequency analysis of these clusters reveals that transition from CT to CII occurs through the planar cluster CI that appears to have an intermediate geometry between the downwardly puckered CT and the upwardly puckered CII. A comparative NICS, ELF, AIM and LOL analysis of all three clusters reveal that the global aromatic nature increases in the sequence CT, CI, CII. A TDDFT study reveals that the oscillator strength of both CT and CII cluster is quite close to each other, which are one order of magnitude higher than the planar CI cluster.