Beyond Hydrogen Bonding: π···π Stacking Directed Self‐Assembly of Carboxylic Acid Clusters in the Gas Phase

ABSTRACT Carboxylic acids critically influence atmospheric chemistry by modulating acidity, new particle formation, and aerosol growth. The hydrogen‐bonding capability of the −COOH group drives the assembly of structurally diverse gas‐phase clusters with atmospheric species. Despite their importance, experimental data on larger carboxylic acid clusters remain limited, and computational predictions of their global minimum structures lack consensus. Here, we employ high‐resolution microwave spectroscopy to determine the geometries of formic acid and propiolic acid clusters, identifying three trimers and two tetramers. A total of 34 isotopologues were analyzed to robustly confirm the cluster structures. Symmetry‐adapted perturbation theory and many‐body expansion analyses demonstrate a fundamental transition in stabilization mechanisms: trimers rely on conventional hydrogen bonds, whereas tetramers exhibit cooperative π–π stacking interactions that drive a structural transformation from single‐layer to double‐layer architectures. These findings resolve long‐standing ambiguities in cluster configurations and establish essential benchmarks for modeling carboxylic acid‐driven atmospheric nucleation processes.