SD aromaticity index: a new assessment of aromaticity based on Ramsey spin dipolar contribution to NMR spin–spin coupling constants

Abstract

Aromaticity remains one of the most fundamental yet elusive concepts in chemistry, as no single observable can directly quantify it. In this work, we introduce the spin–dipolar aromaticity index (SDAI), a new descriptor based on the spin–dipolar (SD) contribution to one-bond NMR spin–spin coupling constants. Beyond providing an additional numerical index, SDAI unveils a direct physical manifestation of aromaticity through the magnetic interaction between nuclear spins mediated by delocalized π-electrons. Within the framework of density functional theory (DFT), we analyzed the behavior of the SD term across a representative series of aromatic, nonaromatic, and antiaromatic molecules, including heterocycles, substituted benzenes, fulvenes, and polycyclic hydrocarbons. The calculations show that in aromatic systems, the SD contribution to one bond coupling (¹J^SD) exhibits nearly uniform values—close to those of benzene—reflecting a collective and homogeneous spin-polarization response of the π-system. In contrast, nonaromatic and antiaromatic compounds display irregular ¹J^SD patterns, with large bond-to-bond variations that reflect localized π-electron distributions. This magnetic uniformity is therefore a necessary but not sufficient condition for aromaticity: only when the resulting ¹J^SD values remain comparable to benzene's does genuine aromatic character arise. The aromaticity trends obtained from SDAI closely reproduce those given by electronic and energetic descriptors such as HOMA, PDI, and FLU. Moreover, in molecules where magnetic and electronic criteria diverge, SDAI follows the latter, confirming its consistency with descriptors governed by π-electron delocalization. SDAI thus provides a physically grounded and computationally accessible approach to quantify aromaticity.