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.