How to evaluate aromaticity under pressure? Benzene as a benchmark system.

Aromaticity is a fundamental concept in chemistry, central to understanding chemical stability, reactivity, and electronic structure. While traditionally examined under ambient conditions, recent studies have begun to probe its behavior in unconventional regimes, reflecting the diversification of reaction environments. Here, we investigate aromaticity under hydrostatic pressure, extending the range of external perturbations considered so far in aromaticity research. Using state-of-the-art quantum chemical methodologies designed to simulate pressure at the single-molecule level, we systematically analyze the evolution of aromaticity in benzene, the prototypical aromatic compound. Structural and electronic indices consistently indicate a modest loss of aromaticity under pressure, while magnetic descriptors point towards an enhanced aromaticity under pressure. This underscores the enduring need for a multidimensional framework consisting of complementary descriptors. During the analysis, particular emphasis is placed on the performance, adaptability, and limitations of widely used aromaticity indices under compression, leading to proposed refinements and guidelines for their reliable application. The results are interpreted within different theoretical perspectives on aromaticity, providing new insights into the robustness of aromaticity descriptors under extreme conditions and outlining avenues for future research.