Peripheral (anti)aromaticity in the singlet and triplet states of cyclopenta[fg]acenaphthylene, pyrrolo[2,1,5-cd]indolizine and 2a1 boracyclopenta[cd]indene: NICS scan approach

The π-electron count of cyclopenta[fg]acenaphthylene (1), pyrrolo[2,1,5-cd]indolizine (2) and 2a¹-boracyclopenta[cd]indene (3) reveals antiaromaticity of 1, and aromaticity of 2 and 3 in the ground electronic singlet state in accordance with the Hückel’s rule of aromaticity. This conjecture is well-corroborated from the Nucleus-Independent Chemical Shift (NICS) scan results. However, a systematic NICS scan study shows that in 1, the overall antiaromaticity in the S₀-state is predominantly due to contributions of the 5-membered ring, while the 6-membered ring reflects weak aromaticity even in the singlet-state. For 2 and 3, the aromatic character of the molecules is comprised of contributions from both the rings. On the contrary, the aromaticity of 1 and antiaromaticity of 2 and 3 in the triplet-state (according to Baird’s rule) are found to be due to contributions from both the (5- and 6-membered) rings of the molecules. In all the cases, the isotropic chemical shifts of the probe (Bq) atom have been separated into the in-plane and out-of-plane contributions to extract quantitative evaluation of the individual components. Further, the net paratropic (antiaromatic) or diatropic (aromatic) ring currents in a given ring have been analyzed from the separation of the individual components (in-plane and out-of-plane) into the paramagnetic and diamagnetic contributions. A quantitative comparison of the NICS values of 2 with those of 3 reveals greater aromaticity in the S₀-state and antiaromaticity in the T₁-state of 2.

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