Interstellar Complex Organic Molecules (iCOMs)

• Interstellar Complex Organic Molecules (iCOMs) are carbon-bearing species with at least six atoms found in diverse astrophysical environments.
• They form primarily via radical recombination on icy grain mantles and non-diffusive processes, as supported by laboratory and theoretical studies.
• Their consistent abundance from cold cores to protostellar regions suggests direct inheritance into planetary systems with significant prebiotic implications.

Interstellar Complex Organic Molecules (iCOMs) are carbon-bearing species with at least six atoms, widespread throughout the interstellar medium (ISM) and detected in diverse astrophysical environments ranging from cold prestellar cores to protostellar envelopes, planetary disks, comets, and even external galaxies. They play a central role in the emergence of molecular complexity in space and are considered key feedstock for the chemical evolution that may lead to prebiotic and biotic molecules on planetary bodies. Their paper encompasses laboratory simulation, quantum chemical theory, astrochemical modeling, and high-sensitivity astronomical observations across submillimeter, millimeter, and infrared wavelengths.

1. Definition, Classification, and Inventory

Interstellar Complex Organic Molecules (iCOMs) are operationally defined as carbon-bearing molecular species containing at least six atoms, typically detected by their characteristic rotational or vibrational transitions. Early definitions emphasized molecular size, functional diversity, and formation mechanisms; the current consensus prioritizes atom count (≥6), but functional or architectural groupings (alcohols, ethers, cyanides, amides, aromatics, branched isomers, chiral) are widely used (Belloche, 2019, Jimenez-Serra et al., 21 Mar 2025).

A non-exhaustive selection of key iCOMs includes methanol (CH₃OH), methyl formate (CH₃OCHO), dimethyl ether (CH₃OCH₃), acetaldehyde (CH₃CHO), ethanol (C₂H₅OH), formamide (NH₂CHO), vinyl cyanide (C₂H₃CN), ethyl cyanide (C₂H₅CN), glycolaldehyde (HOCH₂CHO), ethylene glycol ((CH₂OH)₂), acetamide (CH₃CONH₂), and urea (NH₂C(O)NH₂), with up to 160 such molecules identified in the Galaxy and over a dozen in extragalactic sources (Jimenez-Serra et al., 21 Mar 2025). Diversity in functional and structural motifs is well established; e.g., linear, branched, aromatic, chiral molecules (Belloche, 2019).

2. Laboratory and Theoretical Foundations: Pathways and Kinetics

Dominant formation routes for iCOMs involve solid-state (ice mantle) chemistry and, to a lesser extent, gas-phase processes. Laboratory experiments and quantum chemistry underpin the network of elementary steps (Caro et al., 9 Dec 2025, Ferrero et al., 2023, Ferrero et al., 2023, Oberg, 2016, Enrique-Romero et al., 2019):

• Stage 1: Radical Generation UV photons (7–10 eV), secondary electrons, and cosmic rays photolyze simple ices, producing radicals/atoms (e.g., CH₃O, CH₂OH, HCO, NH₂, OH) through dissociation:

$$\mathrm{CH_3OH} + h\nu \to \mathrm{CH_2OH} + \mathrm{H}$$

$\mathrm{H_2O} + h\nu \to \mathrm{OH} + \mathrm{H}$

(see Table 1, (Caro et al., 9 Dec 2025); cross sections $\sigma$ at Ly-α $\sim 10^{-18}$–$10^{-17}$ cm²)

• Stage 2: Radical–Radical/Molecule Coupling (Solid-State Recombination)
  • CH₂OH + CH₂OH → (CH₂OH)₂
  • (ethylene glycol, barrierless or $E_a \sim 0$–5 kJ mol⁻¹)
  • HCO + CH₃O → HCOOCH₃
  • (methyl formate)
  • HCO + CH₂OH → HOCH₂CHO
  • (glycolaldehyde)
  • NH₂ + HCO → HCONH₂
  • (formamide)
  • (Caro et al., 9 Dec 2025, Enrique-Romero et al., 2019, Oberg, 2016)
• Secondary Mechanisms:
  • Radical–molecule pathways via electronically excited states (e.g., CO* + H₂ → HCO + H).
  • Acid–base chemistry (e.g., RCOOH + NH₃ → RCOO⁻·NH₄⁺) occurs barrierlessly at ≤10 K.
• Diffusion and Activation:

Radical mobility is quantified by thermal hopping rates:

$k_{\rm diff}(T) = \nu_0\,\exp(-E_{\rm diff}/k_BT)$

With typical attempt frequencies $\nu_0 \approx 10^{12}$ s⁻¹ and $E_{\rm diff}$ ∼0.3–0.7$E_{\rm bind}$. For example, at 10 K, CO is highly mobile ($k_{\rm diff}\sim10^4$ s⁻¹), while CH₂OH and HCO are nearly immobile ($k_{\rm diff}\sim10^{-3}$ s⁻¹) (Caro et al., 9 Dec 2025).

• Thermal Processing:

Gradual heating during protostellar evolution (warm-up 10→100 K) substantially increases radical mobility, driving recombination with kinetics:

$R(T) = A\,\exp(-E_a/RT)$

For radical–radical reactions, $A \sim 10^{12}$ s⁻¹, $E_a \sim 0$–5 kJ mol⁻¹; radical–molecule barriers may reach $E_a \sim 10$–30 kJ mol⁻¹.

• Alternative/Excited-State and Non-Diffusive Mechanisms:

Recent astrochemical models now include proximity-driven immediate follow-on (e.g., chemical formation with immediate reaction), excited-state radical chemistry, and photodissociation-induced neighbor reactions, enabling iCOM formation even in cold (≤10 K) environments where heavy radicals cannot diffuse (Jin et al., 2020).

3. Formation Scenarios and Astrophysical Environments

The solid-state origin of iCOMs is strongly supported by both laboratory yields and astronomical correlations (Caro et al., 9 Dec 2025, Rocha et al., 2023, 2206.13270):

• Cold Prestellar Cores ($T\sim10$ K):

Radical reservoirs are generated over $10^5$–$10^6$ yr by cosmic ray-induced secondary UV photolysis. In these environments, surface chemistry is non-diffusive or exploits non-thermal mechanisms, e.g., proximity or excited-state reactions. Observed iCOM abundances (e.g., CH₃OH: $x\sim10^{-9}$, CH₃CHO: $x\sim10^{-11}$) agree with advanced non-diffusive grain models (Scibelli et al., 2020, Jin et al., 2020).

• Protostellar Envelopes and Disks (Hot Corinos, $T\sim30$–100 K):

Enhanced UV and/or internal heating mobilizes radicals, rapidly increasing iCOM formation just prior to sublimation. Observed gas-phase abundances (e.g., CH₃OH up to $10^{-7}$ fractionally) and column densities of iCOMs in hot corinos (e.g., SVS13-A, IRAS 16293-2422) directly reflect mantle radical chemistry (Bianchi et al., 2018, 2206.13270).

• Shocked Regions and Outflows:

C-type and J-type shocks (velocity 5–20 km s⁻¹) sputter or thermally desorb dust mantles, injecting iCOMs into the gas phase and triggering rapid gas-phase chemical cycling. In both low- and high-mass star-forming regions, enhanced iCOM emission traces shock morphology and correlates with shock tracers (e.g., SiO) (Rojas-García et al., 2022, Simone et al., 2020, Rojas-García et al., 2023, Vastel et al., 12 Mar 2024).

• Comets and Solar System Ices:

JWST-MIRI and Rosetta measurements reveal that cometary and protostellar iCOM abundances (relative to CH₃OH or H₂O) match to within a factor of a few, supporting direct inheritance from protostellar ices (Rocha et al., 2023). Laboratory residues formed by photoprocessing closely mirror the organic content of cometary and asteroidal material (Caro et al., 9 Dec 2025).

4. Observational Approaches and Detections

Advances in observational capabilities (ALMA, NOEMA, JWST) have enabled:

• Gas-phase Detections:

High-resolution (sub-arcsecond) mapping of primary and secondary iCOMs down to $x\sim10^{-10}$ in cold cores, hot corinos, protostellar disks, and outflows. Spectral confusion is a key observational challenge at high line densities, driving efforts to lower frequencies and line-stacking analysis (Jimenez-Serra et al., 21 Mar 2025, Bianchi et al., 2018, Simone et al., 2020).

• Ice-phase (Solid-state) Detections:

JWST has provided the first robust detections of four iCOMs (CH₃CHO, CH₃CH₂OH, CH₃OCHO, CH₃COOH) in protostellar ices, constrained by multi-band profile fitting and laboratory ice spectra (e.g., ENIIGMA tool). Derived ice-phase column densities and abundance ratios closely match cometary values (Rocha et al., 2023).

• Spatial Distribution:

Mapping in both gas and solid phase reveals that iCOM abundance enhancements are associated with shock fronts, disk–envelope interfaces, accretion streamers, and centrifugal barriers, often on scales of 10–1000 au (Codella et al., 2019, Vastel et al., 12 Mar 2024, Rojas-García et al., 2023).

5. Reaction Kinetics and Theoretical Constraints

Quantum chemical simulations and laboratory studies determine the kinetic parameters controlling iCOM formation:

• Barrier Heights and Tunneling:

For radical–radical coupling, barriers are typically ≤10 kJ mol⁻¹; strong site- and functional-dependence exists. On water-rich clusters, many channels are barrierless (e.g., HCO + CH₃ → CH₃CHO), but site variability can introduce small barriers, exponentially suppressing rates below 30 K (Enrique-Romero et al., 2019, Álvarez-Barcia et al., 2018). Hydrogen transfer steps (addition/abstraction) show that abstraction from aldehyde CH is always faster than addition to the same carbon (Álvarez-Barcia et al., 2018).

• Gas-phase vs. Solid-state Pathways:

For some iCOMs (e.g., acetaldehyde), gas-phase production via neutral-neutral reactions (e.g., CH₃CH₂ + O → CH₃CHO + H; $k(T)\sim 1.4\times10^{-10}(T/300)^{-0.4}$ cm³ s⁻¹) is important in shocked or warm post-desorption gas and may dominate over cold-surface routes where barriers frustrate recombination (Simone et al., 2020, 2206.13270).

• Alternative Surface Channels:

Atomic C can induce non-energetic, barrierless iCOM synthesis on water-dominated ice via the formation of $^3$C–OH₂ centers, enabling immediate conversion to methanol, ethanol, methanediol, etc., depending on available co-reactants (Ferrero et al., 2023). On CO-rich surfaces, atomic C plus sequential H ➞ ketene ➞ acetyl ➞ CH₃CHO/CH₃CH₂OH, with branching ratios depending on tunneling efficiency and local H atom densities (Ferrero et al., 2023).

6. Evolutionary Trends and Inheritance into Planetary Systems

Comprehensive surveys show that iCOM abundance ratios (e.g., CH₃OCHO/CH₃OH, CH₃CH₂OH/CH₃OH) remain remarkably constant from cold prestellar environments, through protostellar evolution, accretion disks, and into cometary soils (2206.13270, Rocha et al., 2023). This compositional continuity suggests inheritance of the molecular inventory from the protostellar phase into planetesimals, maintaining chemical signatures that can be used to trace planetary system histories and potentially the seeding of prebiotic chemistry.

Discrepancies between model predictions and observations in extremely cold cores (≤10 K) have been resolved by invoking fast, non-diffusive mechanisms, enhancing iCOM production at low temperatures and extending chemical complexity much earlier in star formation than previously recognized (Jin et al., 2020, Scibelli et al., 2020).

7. Implications for Prebiotic Chemistry and Future Directions

Laboratory and observational data consistently indicate that a broad spectrum of iCOMs, including prebiotically relevant molecules (amino acid, sugar, and nucleobase precursors), are synthesized and preserved in interstellar ices. Delivery via cometesimals, asteroids, and dust to the early Earth could have contributed significantly to the molecular inventory required for the emergence of life (Caro et al., 9 Dec 2025, Oberg, 2016).

Future research will target:

• Expansion of ice-phase iCOM inventory with JWST, including systematic abundance measurements in different star-forming environments.
• Incorporation of non-diffusive, excited-state, and multi-body reaction mechanisms in astrochemical models to further refine the chemical networks and evolutionary predictions.
• Spatially resolved ALMA imaging (≲10 au scales) to disentangle the physical sites and timescales of iCOM formation and release.
• Laboratory measurements and quantum chemical studies of reaction barriers, branching ratios, and rates under astrophysically relevant conditions.

The field is rapidly clarifying the kinetic framework and inheritance pathways bridging solid-state and gas-phase iCOMs, establishing their foundational role in astrochemistry, the origins of planetary systems, and the molecular basis for prebiotic evolution (Caro et al., 9 Dec 2025, Rocha et al., 2023, 2206.13270).