Interstellar Stereoisomerism

The increasing detection of new molecules in the interstellar medium (ISM) shows that stereoisomerism is a fundamental contributor to interstellar molecular complexity. This work presents the first comprehensive overview of interstellar stereoisomerism. A total of 16 stereoisomeric pairs have been identified (13 conformational and 3 geometric), spanning molecules with 5-12 atoms and energy separations from ∼10 K to 2667 K. They were observed across diverse astrophysical environments with kinetic temperatures ranging from low to high values (∼7.5 to 300 K). The observed stereoisomeric ratios (OSR)defined as the column density ratio of the higher-energy isomer divided by that of the lower-energy isomervary widely (0.009-4). While systems with small energy differences (<600 K; i.e., ∼1.2 kcal mol^-1) in hot environments (>100 K) generally follow thermodynamic expectations (often assisted by tunneling-driven interconversion), many stereoisomersparticularly those in cold clouds or with larger energy separationsexhibit abundances far exceeding equilibrium values. This demonstrates that thermodynamics alone cannot explain interstellar stereoisomerism. Instead, stereoselective formation/destruction pathways (in the gas phase and/or in the surface of dust grains), photoisomerization, and chemical rearrangement during desorption must play a dominant role. Stereoisomeric ratios thus provide powerful constraints on interstellar chemical pathways, and about the physico/chemical conditions of the ISM. This review highlights the need for stereochemistry-sensitive astrochemical models. Progress in this field requires expanded laboratory spectroscopy of higher-energy stereoisomers, dedicated quantum chemical studies of isomerization processes, and the explicit inclusion of stereoselective chemistry in chemical networks. Together, these efforts will be essential for understanding the origin of stereoisomeric selectivity and molecular complexity in the ISM.