Direct Estimation of Aromatization Energy from ¹H NMR and UV-Vis Absorption Data of Homodesmotic Molecules

This study delves into the ring-opening reaction of two distinct diaryl-ring-pyran systems, referred to as drp and drp, where the term 'ring' encompasses aromatic, nonaromatic, or antiaromatic motifs. These systems transform into the corresponding cis-ortho quinonoid systems, denoted as c-drq and c-drq. Homodesmotic pairs (drp, drp) and (c-drq, c-drq) are categorized as (aromatic, nonaromatic), (aromatic, partially aromatic), (antiaromatic, nonaromatic), and (nonaromatic, nonaromatic), with their energy difference representing aromatization energy (E_aroma). Using reliable density functional theory, E_aroma is assessed for various aromatic and antiaromatic ring motifs, including borderline cases and nonaromatic structures. For example, benzene exhibits an E_aroma of 23.4 kcal/mol, indicating 3.9 kcal/mol aromatic stabilization per CC bond, while cyclobutadiene shows -29.9 kcal/mol, indicating a 7.5 kcal/mol destabilization of the CC bond. This approach extends to evaluating global and local aromatic stabilization effects in polycyclic hydrocarbons, nonbenzenoid systems, and heterocyclic compounds. Additionally, variation in ¹H NMR chemical shift (δ_avg) correlates with E_aroma, suggesting that a -1.0 ppm shift corresponds to 24.2 kcal/mol aromatization energy. UV-vis absorption maxima difference (Δλ_avg) correlates linearly with E_aroma, enabling direct assessment of aromatization energy from UV-vis spectra using suitable homodesmotic pairs. This comprehensive approach enhances our understanding of structural, energetic, and spectroscopic aspects of aromatic and antiaromatic systems.