Isomerization Upon Desorption

• Isomerization upon desorption is a process where excess energy from desorption activates molecular interconversion, affecting observable isomer distributions.
• Non-equilibrium energy redistribution during desorption overcomes high activation barriers, enabling formation of less stable isomers in astrochemical and catalytic contexts.
• Kinetic models and experimental analyses reveal that desorption-driven isomerization critically influences surface chemistry, cluster reconfiguration, and interstellar molecule ratios.

Isomerization upon desorption refers to the process in which molecular isomers interconvert as a direct consequence of desorption, typically from a solid surface or grain, and is often driven or enabled by the excess energy released during the desorption event. This concept has become central to understanding isomer distributions in surface chemistry, cluster physics, astrochemistry, and photoprocessing, especially in environments where non-equilibrium or non-thermal energy sources predominate. The phenomenon connects molecular isomerization kinetics, energy transfer mechanisms, and the role of surface-induced processes in determining the observable population of specific isomeric forms.

1. Fundamental Mechanism of Isomerization upon Desorption

Isomerization upon desorption (often abbreviated IUD) involves interconversion between molecular isomers facilitated by the release of excess energy when a molecule detaches from a surface or cluster. In icy grain chemistry, astrochemical modeling, and cluster transformations, desorption can impart substantial internal energy to the molecule, activating pathways that would otherwise be inaccessible at the prevailing environmental temperature.

For example, formation of formic acid (HCOOH) on interstellar dust grains via the hydrogenation of the HOCO radical yields both cis and trans forms. Upon chemical desorption, the process splits into two branches,

$\ce{t-HOCO(i) + H(i) -> c-HCOOH(g)}$

and

$\ce{t-HOCO(i) + H(i) -> t-HCOOH(g)}$

where desorption can activate isomerization, resulting in the gas-phase production of cis-HCOOH even though it is thermodynamically less stable (Molpeceres et al., 22 Sep 2025).

A plausible implication is that the desorbing molecule’s internal energy can promote isomerization that would not occur in thermal equilibrium, thereby modifying gas-phase isomer ratios absent subsequent gas-phase rearrangement.

2. Energy Redistribution and Thermodynamic Role

The IUD process is fundamentally governed by both the energy landscape of the isomers and the energetic partitioning during desorption. The activation barrier for isomerization (ΔH°_iso or ΔG‡) can be substantial: for interstellar molecules, computed Gaussian-4 enthalpy barriers have been reported up to 67.4 kcal/mol (Etim, 2023). Nevertheless, energetic events (cosmic rays, shock waves, or the thermal spike from desorption itself) can be sufficient to overcome barriers that are otherwise insurmountable at ambient conditions.

Thermodynamics dictates that the most stable isomer (lowest H°_f) is typically most abundant, but under non-thermal conditions enabled by desorption, less stable isomers can be populated if the rate of energy dissipation is slow. The IUD mechanism thus serves as a kinetic window into molecular distributions inaccessible by equilibrium population analysis.

Process Aspect	Typical Energy Range (kcal/mol)	Mechanistic Driver
Isomerization barrier	0.0–67.4	Shock waves, chemical desorption
Excited isomerization	variable (up to tens of kcal/mol)	Excess energy from desorption

3. Kinetic Effects and Temperature Dependence

Experimental and modeled kinetics reveal that IUD is highly effective in cold environments where unimolecular isomerization rates are negligible. For instance, the trans-to-cis ratio of formic acid in TMC-1 (ratio ≈ 17.5) is reproduced by models only when the IUD mechanism is included with high efficiency (70–90%), implying nearly all c-HCOOH is produced on grain surfaces and survives desorption due to chemical activation (Molpeceres et al., 22 Sep 2025).

At higher temperatures, quantum tunneling and direct unimolecular rearrangement may become non-negligible, allowing the gas-phase isomerization to eventually drive ratios toward the thermodynamic equilibrium value. RRKM theory with tunneling corrections is typically employed for microcanonical rate constant analysis in these regimes.

4. Surface Chemistry and Cluster Transformations

Isomerization upon desorption also features prominently in the reversible transformations of inorganic clusters and in surface-bound molecular systems. In carboxylate-capped CdS clusters, desorption of hydroxyl-bearing species and the consequent change in ligand binding motif triggers a diffusionless, displacive reconfiguration of the cluster core (Williamson et al., 2019). FTIR and kinetic analysis reveal first-order transformation kinetics analogous to molecular isomerization, with activation energies around 1 eV and excitonic gap shifts of 140 meV.

Similarly, in catalytic and surface assembly contexts, the presence of coordinating adatoms (e.g., Fe on bipyridine assemblies) can drive conformational isomerization upon desorption or during surface migration, fundamentally altering supramolecular architectures and chirality (Freund et al., 2018).

5. Astrochemical Implications and Observational Signatures

Astrochemical surveys have emphasized the critical role of IUD in explaining the detection of high-energy isomers in cold dark clouds. Observed ratios of cis- and trans-formic acid in sources like TMC-1 and L483 match only when grain-surface formation and subsequent desorption with chemical activation are modeled—gas-phase isomerization is negligible at ~10 K (Molpeceres et al., 22 Sep 2025). Similar processes are proposed for cyanide/isocyanide pairs, ketenes, and larger prebiotic molecules, where desorption can supply the energy necessary for rearrangement (Etim, 2023).

Photoprocessing experiments and spectroscopy show that polycyclic aromatic hydrocarbon (PAH) ions (e.g., C₁₁H₉⁺) in the gas phase undergo extensive interconversion among long-lived isomeric forms below the dissociation threshold after desorption-like excitation, substantiated by MD simulations and photophysical measurements (Lozano et al., 9 Jul 2025). The resultant isomer distribution shapes both rotational and electronic spectral signatures in astrophysical surveys.

6. Theoretical Frameworks and Modeling Approaches

Modeling of IUD phenomena typically employs multi-channel reaction networks split by isomeric form, with desorption-induced isomerization treated either as a kinetic branching process or with explicit RRKM/tunneling analysis. In surface and solid-state systems, Monte Carlo and Ising lattice simulations incorporating dual spin dynamics (nonconserved isomerization vs. conserved diffusive segregation) reveal that phase segregation can disrupt simple Arrhenius behavior of isomerization reactions—suggesting that similar disruption may occur during desorption coupled with surface diffusion (Thwal et al., 2023).

Modeling Approach	System Regime	Key Insight
RRKM + tunneling	Gas phase, >10 K	Quantum-enabled equilibrium approach
Branching networks	Grain chemistry, <20 K	Non-thermal ratios set by IUD
MC/Ising dual-dynamics	Surfaces/clusters	Kinetics may deviate from Arrhenius

7. Broader Impact and Future Directions

Isomerization upon desorption has broad ramifications for molecular detection, catalysis, material design, and interstellar chemical models. Recognition of non-equilibrium population dynamics leads to more accurate astrochemical modeling and enhances prediction of spectral features for remote sensing. In practical terms, understanding and harnessing IUD may allow selective synthesis of high-energy or functionally desired isomers by controlling desorption pathways and energy partitioning—important for supramolecular assembly, cluster engineering, and prebiotic chemistry. Further systematic studies integrating multidimensional spectroscopy, isotope labeling, and detailed surface or cluster characterization are likely to elucidate the role of IUD in more complex chemical environments.