Interstellar stereoisomerism

Abstract: 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 (7.5-300 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 (1.2 kcal/mol) 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.

• The paper provides a comprehensive analysis of 16 interstellar stereoisomeric pairs, challenging classical thermodynamic models with observed ratios that defy minimum energy predictions.
• It identifies multiple formation and destruction mechanisms, including ice chemistry, photoprocessing, and gas-phase pathways, that govern non-equilibrium isomer distributions.
• The review emphasizes the need for isomer-specific astrochemical models and targeted spectroscopic searches prioritizing high-dipole, higher-energy species.

Interstellar Stereoisomerism: Patterns, Mechanisms, and Implications for Astrochemical Complexity

Introduction

The manuscript "Interstellar stereoisomerism" (2603.24848) provides a comprehensive and critical review of the current landscape of stereoisomerism in the interstellar medium (ISM). Employing both observational and theoretical perspectives, the work categorizes and examines all confirmed conformational and geometric stereoisomeric pairs detected in various interstellar environments. It rigorously evaluates the origin of their observed abundance ratios and addresses the limitations of classical thermodynamic paradigms. The work further articulates the necessity of stereochemistry-resolving models for accurate predictions of interstellar chemical complexity.

Systematics of Interstellar Stereoisomerism

A detailed compilation identifies 16 confirmed interstellar stereoisomeric pairs, encompassing 13 conformational and 3 geometric cases, spanning molecular sizes from 5 to 12 atoms. These pairs are distributed among oxygen-, nitrogen-, and sulfur-bearing species, detected across diverse environments with gas kinetic temperatures from 7.5 K to 300 K.

The observed stereoisomeric ratios (OSRs)—defined as the column density ratio of the higher- to lower-energy isomer—span a wide dynamic range from 0.009 to 4. Notably, OSRs ≈ 1 or even >1 are reported for systems such as ethanol, allyl cyanide, and crotononitrile, where the higher-energy stereoisomer rivals or exceeds the lower-energy form in abundance. This observation directly contradicts a strict application of the Minimum Energy Principle (MEP), which would predict negligible population of the higher-energy form in thermal equilibrium for large energy separations.

Quantitative data further highlight that stereoisomers with large energy separations (ΔE > 600 K, up to ~2667 K) are detected with non-negligible abundance—a point sharply at odds with equilibrium expectations. This is vividly illustrated in the case of trans-methyl formate (ΔE ~2667 K), cis-formic acid (ΔE ~2033 K), and cis-crotononitrile (ΔE ~1143 K), where higher-energy forms are detected despite prohibitively low expected populations at ISM kinetic temperatures.

Mechanistic Insights and Deviations from Thermodynamics

For stereoisomeric pairs with ΔE < 600 K in environments with T_kin > 100 K, the OSRs align with Boltzmann statistics and are compatible with rapid interconversion by tunneling, as detailed for imines and select alcohols [GarciadelaConcepcion2021, GarciadelaConcepcion2022]. Multidimensional quantum tunneling is identified as a key mechanism allowing equilibrium to be approached for systems with sufficiently low effective barriers. These rare cases provide stringent empirical constraints on isomer energy differences—potentially exceeding the precision of current electronic structure methods.

In stark contrast, for ΔE > 600 K or in cold environments, OSRs universally deviate from thermodynamic control. The necessary explanation shifts to consideration of non-equilibrium, stereoselective formation and destruction processes. The work identifies:

• Ice Chemistry and Isomerization-Upon-Desorption (IUD): Laboratory and simulation results show that isomeric distributions can be set by surface reaction networks and then 'frozen in' upon non-thermal desorption, or reshuffled by excess energy transfer during desorption events [Molpeceres2025]. The OSR may therefore reflect the physical-chemical history of grain chemistry rather than instantaneous gas-phase equilibrium.
• Photoprocessing: In high-UV fields (e.g., Orion Bar), interconversion by photoexcitation and subsequent isomerization has been demonstrated to efficiently populate otherwise inaccessible higher-energy forms, as for cis-formic acid [Cuadrado2016].
• Gas-Phase Stereoselective Pathways: Ion–molecule and radical–molecule reactions can feature energy barriers or transition-state surfaces favoring formation of specific geometric or conformational isomers; for example, the branching in the C$_3$H$_6$ + CN reaction governing crotononitrile isomerism [Mallo2025].
• Sequential Acid-Base (SAB) Mechanisms: Secondary chemical cycles (e.g., involving proton transfer and recombination) can lead to efficient re-equilibration or even population inversion between stereoisomers [GarciadelaConcepcion2023].
• Stereoselective Destruction: The relative dipole principle (RDP) has been invoked, whereby more polar forms experience faster H-atom destruction rates, modulating steady-state OSRs [Shingledecker2020], though this effect generally yields subdominant modifications compared to formation biases.

No universal mechanism can account for all observed ratios; the OSR emerges as a nuanced probe of local and historical kinetic, photochemical, and desorption-driven chemical dynamics.

Implications for Spectroscopy and Detection Strategies

A salient conclusion of the review is the frequently higher dipole moments of metastable, higher-energy stereoisomers. This property disproportionately enhances the detectability of these species in rotational surveys, sometimes to the extent that only the higher-energy form is observed (e.g., cis–trans HOCOOH [Sanz-Novo2023]), while the lowest-energy isomer remains spectroscopically inaccessible due to negligible or zero dipole moment.

As a direct implication, the review proposes a targeted search strategy for new interstellar species: prioritize higher-energy stereoisomers when their dipole moments are substantially larger, even at the cost of thermodynamic population penalty. This approach is especially valuable for nonpolar molecules and for amino acids (e.g., glycine and alanine), which are not directly observable in their lowest-energy form but whose higher-energy conformers may be accessible due to superior $|\mu|$ values.

Theoretical and Modeling Imperatives

Current astrochemical models generally neglect stereoisomerism, treating isomers as a lumped population or exclusively tracking the global minimum. The review forcefully argues for the explicit inclusion of stereochemistry, requiring both spectroscopic measurements and quantum chemical computations of all kinetically accessible stereoisomers and their reaction networks. Incorporating isomer-specific kinetics, destruction rates, and surface/gas-phase conversion efficiencies is essential for accurate population predictions.

The review further demonstrates that for several species (e.g., $n$-propanol, propyl cyanide), benchmarking with isomer-specific OSRs can provide direct empirical calibration points for quantum chemical energetics, offering more discriminating constraints than traditional laboratory measurements.

Conclusion

The review positions stereoisomerism as a critical diagnostic of interstellar molecular complexity and chemical evolution. It highlights the empirical breakdown of the MEP in the ISM and quantifies the need for stereoselective mechanisms—operating in both gas and condensed phases, and mediated by quantum and non-equilibrium effects. Immediate priorities for the field include:

• Expanded laboratory spectroscopy of high-energy stereoisomers
• Quantum dynamics studies of interconversion and tunneling processes for key systems
• Refinement of astrochemical models to treat stereochemistry at network level
• Focused molecular searches prioritizing accessible, high-dipole and high-energy isomers when symmetry precludes direct observation of global minima

Stereoisomeric ratios thus provide a uniquely sensitive probe of the ISM’s chemical histories, and their systematic characterization will furnish critical constraints on the mechanisms driving prebiotic chemical complexity in astronomical environments.