Assessing Aromaticity in Ions: Evaluating Induced Ring Current and UV-Vis Spectrum-Based Criteria

With increasing attention across various research fields, the aromaticity of ions has emerged as a significant topic of interest. Magnetic-based criteria, such as NMR chemical shifts, and spectrum-based criteria, such as UV-Vis spectroscopy, are commonly employed to evaluate the (anti)aromaticity of ions. While the reliability of magnetic-based criteria has been questioned through various case studies, the validity of UV-Vis-based criteria remains underexplored. In this study, we reassessed the aromaticity of 18 ions previously characterized as magnetically (anti)aromatic or nonaromatic using NMR chemical shift and/or UV-Vis criteria. This evaluation involved calculating the harmonic oscillator model of aromaticity (HOMA) index, gauge-including magnetically induced current (GIMIC), multicenter index (MCI), and isomerization stabilization energy (ISE). Our findings reveal that the magnetically based GIMIC predictions consistently align with those from UV-Vis spectra and NMR chemical shifts. However, ISE calculations indicate that four ions classified as magnetically (anti)aromatic by GIMIC and UV-Vis criteria do not exhibit additional (de)stabilization via cyclic conjugation relative to those energetically nonaromatic species, despite showing similar π-electron delocalization and bond equalization as other ions. These results underscore the importance of complementing spectrum-based criteria with stability-based metrics, such as ISE for a comprehensive assessment of aromaticity.