Binding Energies of Astrophysically Relevant Molecules on CO 2 Clusters: Benchmarking and the Donor-Acceptor Origin of Surface-Dependent Binding

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The gist

The gist The binding energies of twelve astrophysically relevant molecules on CO2 clusters were calculated using DFT calibrated against CCSD(T)/CBS, revealing that binding strength depends on whether

In short

The study calculated binding energies for twelve molecules on CO2 clusters using high-level quantum chemistry methods. Results show binding strength depends on whether a molecule needs to donate a hydrogen bond, which CO2 cannot do. This explains why mixed water-CO2 ice separates into CO2-rich domains and helps understand volatile partitioning in interstellar ices.

Key concepts

Binding Energy (BE)
BE is a measure of the strength of attraction between a molecule and the surface it is adsorbed on. The researchers calculated these using DFT methods calibrated against highly accurate CCSD(T)/CBS data to get reliable values for different molecules on CO2 clusters.
Hydrogen Bond (H-bond) Donation
This refers to a situation where a molecule gives up a hydrogen atom to another molecule, creating an attraction. The study found that the binding strength on CO2 is heavily influenced by whether the adsorbate needs to donate this H-bond, which CO2 itself cannot provide.
Desorption Temperature (Tpeak)
This is the temperature at which a molecule thermally leaves or desorbs from a surface. The calculated BEs were used with the Polanyi–Wigner equation to predict these temperatures, showing they range widely from 11 K to 132 K depending on the molecule and cluster size.

Terminology used across episodes

This episode discusses

The paper

Binding Energies of Astrophysically Relevant Molecules on CO 2 Clusters: Benchmarking and the Donor-Acceptor Origin of Surface-Dependent Binding · Read on arXiv

Aneesa Ahmad, Catherine Walsh, Stefan Vogt-Geisse

School of Physics and Astronomy, University of Leeds · Departamento de Físico-Química, Facultad de Ciencias Químicas, Universidad de Concepción

Carbon dioxide (CO 2) is among the most abundant interstellar ices and has been observed by JWST in water-rich mixtures and pure CO 2 environments. Astrochemical models adopt binding energies (BEs) measured on water ice because few BEs are available for CO 2 ice. We calculate BE distributions for twelve adsorbates (N 2, CH 4, HCl, C 2 H 2, H 2 S, HF, HCN, H 2 CO, HCOOH, HNC, CH 3 OH, H 2 O) on CO 2 clusters containing two to five molecules using DFT calibrated against CCSD(T)/CBS, a coupled-cluster treatment extrapolated to the complete-basis-set limit. Binding strength depends on whether the adsorbate requires the surface to donate a hydrogen bond (H-bond). CO 2 can accept an H-bond but cannot donate one, and all six species compared with water ice from Ahmad et al. (2026) bind more weakly on CO 2. The ratio of the BE on CO 2 to water ice ranges from 0.66 for H 2 CO and 0.75 for H 2 O to 0.87 for CH 3 OH, 0.89 for HCOOH and 0.92 for C 2 H 2. Formaldehyde is predicted to desorb near 57 K from CO 2-rich ice, compared with 85 K from water-rich ice, and could contribute to cold gas-phase H 2 CO. Although the mean binding energies are changing with cluster size for several species, the differences can be traced to local binding interactions likely to remain important in larger ice models. The BEs vary by more than 500 K between sites, while the desorption peaks range from 11 K for CH 4 to 132 K for HCOOH. The limited H-bonding capacity of CO 2 may favour separation of mixed H 2 O:CO 2 ice into CO 2-rich domains.

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Binding Energies of Astrophysically Relevant Molecules on CO 2 Clusters".

Jocelyn: The gist The binding energies of twelve astrophysically relevant molecules on CO2 clusters were calculated using DFT calibrated against CCSD(T)/CBS,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So we started by looking at this paper, "Binding Energies of Astrophysically Relevant Molecules on CO two Clusters: Benchmarking and the Donor-Acceptor Origin of Surface-Dependent Binding" <ref:2610.12055#pg1,Clusters: Benchmarking and the Donor-Acceptor Origin of Surface-Dependent Binding>. The main idea is that because we don't have many binding energy numbers for CO two ice, the authors calculated them using DFT calibrated against CCSD(T)/CBS <ref:2610.12055#pg1>.

Jocelyn: They focused on twelve adsorbates: N two CH four HCl, C two H two H two S, HF, HCN, H two CO, HCOOH, and so on <ref:2610.12055#pg1>. Their core claim is that binding strength depends entirely on whether the adsorbate needs the surface to donate a hydrogen bond <ref:2610.12055#pg3>.

Subrahmanyan: This implies that CO two can accept a hydrogen bond from a molecule, but it cannot provide one back to the adsorbate <ref:2610.12055#pg3>. That’s a crucial physical distinction for modeling how these things stick together.

Vera: Exactly, and they showed that all six of those species bind more weakly on CO two than they do on water ice, even though the difference varies a lot between the species <ref:2610.12055#pg3>.

Jocelyn: They quantified this spread by showing ratios, like H two O having a ratio of zero point seven five and CH three OH reaching zero point eight seven compared to water ice <ref:2610.12055#pg3>. This spread is what makes the result interesting, it’s not just a single average value for everything <ref:2610.12055#pg3>.

Subrahmanyan: And they also discussed how the binding energy distribution itself matters, saying that the whole range of energies across all sites is important for astrophysics because that tells you about the thermal desorption temperature <ref:2610.12055#pg3>.

Vera: Plus, they predicted formaldehyde will desorb at fifty-seven Kelvin from CO two-rich ice, which is lower than the eighty-five Kelvin we see for water-rich ice <ref:2610.12055#pg1>. That low temperature could be relevant for cold gas phase species <ref:2610.12055#pg3>.

Jocelyn: So, to sum up, the paper establishes a framework where binding strength is dictated by the donor-acceptor interaction between the molecule and the CO two surface <ref:2610.12055#pg3>. This sets a new standard for how we should approach these calculations <ref:2610.12055#pg1>.

Subrahmanyan: It’s about shifting the focus from just comparing total binding energy numbers to understanding the specific structural requirements for those bonds, which is where the donor-acceptor origin comes in <ref:2610.12055#pg3>.

Conclusion: Vera: So, wrapping up this talk on "Binding Energies of Astrophysically Relevant Molecules on CO two Clusters: Benchmarking and the Donor-Acceptor Origin of Surface-Dependent Binding", we've seen how binding strength isn't uniform across different molecules <ref:2610.12055#pg3>.

Jocelyn: The authors, Ahmad, Walsh, and Vogt-Geisse did a lot of heavy lifting here by using DFT calibrated against CCSD(T)/CBS to get these binding energies for twelve different species on CO two clusters <ref:2610.12055#pg1>.

Subrahmanyan: The big implication is that astrochemical models can’t just treat all volatile molecules the same way when they're sitting on a CO two surface <ref:2610.12055#pg3>. They have to account for those specific hydrogen bond needs of each molecule <ref:2610.12055#pg3>.

Vera: It confirms that the difference between CO two ice and water ice isn't a single constant we can just subtract from everything <ref:2610.12055#pg3>. It’s a species-dependent effect <ref:2610.12055#pg3>.

Jocelyn: It also reinforces the idea that CO two might naturally segregate into pure regions when it mixes with water ice because of these weak binding differences <ref:2610.12055#pg3>.

Subrahmanyan: This work gives us a more detailed tool for understanding how molecules are distributed in the complex chemical environments found in disks and envelopes <ref:2610.12055#pg3>. We have to use these site-dependent binding energies when we build those models <ref:2610.12055#pg3>.

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