Rotational excitation of interstellar complex organic molecules: the example of vinyl cyanide
- Rafal Tekreeti
- Jun 23
- 3 min read
By: Karina Sogomonyan, Malek Ben Khalifa, Jérôme Loreau
According to our current knowledge, the interstellar medium is non-uniform. It consists of numerous astrophysical environments varying in chemical complexity, such as diffuse molecular clouds, dense molecular clouds, or protostars.
The recent surge in detections of complex organic molecules (COMs) in the interstellar medium asserts the importance of investigating their excitation processes. Collisional excitation with the background gas (H2 molecules and He atoms) plays a significant role for molecules like vinyl cyanide, which have been detected in environments such as the giant molecular cloud Sagittarius B2 with prevailing non-Local Thermodynamic Equilibrium conditions. Collisional properties, such as cross-sections and rate coefficients for interactions with the most abundant interstellar species, are crucial for determining molecular abundances in these cold environments, thereby providing insight into the ISM's physical conditions and the emergence of chemical complexity in space.
Computational approach:
Collisional excitation studies generally consist of two parts: the generation of a highly accurate ab initio potential energy surface that represents the interaction between the molecule and the collider, and scattering calculations to investigate the dynamics of the collision. With the increasing size of molecular systems, these calculations become more and more challenging. The interaction between the COM and collider (in this case, a helium atom) is usually weak by nature, allowing only highly accurate methods such as coupled-cluster theory to provide results of desirable quality. The scattering calculations employ current state-of-the-art methods to compute molecular collision properties, such as the fully quantum Coupled States and Close-Coupling techniques. They provide accurate collisional cross-sections for various collisional energies. However, as the size of the system increases, these methods become computationally demanding.
Key findings:
In our study, we computed the first ab initio potential energy surface for the interaction between vinyl cyanide and He using the explicitly correlated coupled-cluster theory (CCSD(T)-F12). The resulting PES was found to be highly anisotropic with a global minimum of -53.5 cm-1 and several local minima, thus requiring an increased number of ab initio calculations.
Employing the obtained PES in scattering calculations, we aimed to provide the collisional cross-sections for kinetic energies up to 100 cm-1. Such calculations scale non-linearly with the increasing kinetic energy value; hence, the computational demand can only be satisfied by HPC. The resulting cross-sections were thermally averaged to obtain collisional rate coefficients for temperatures up to 20 K. A propensity favoring the transitions with Δka=0 was found, which is a common occurrence in asymmetric top molecules. Once published, these rate coefficients can be used by astronomers to model observed spectra more accurately, thereby improving our understanding of the physical conditions in space.
Read the full article in ACS Publications here
🔍 Your Research Matters — Let’s Share It!
Have you used VSC’s computing power in your research? Did our infrastructure support your simulations, data analysis, or workflow?
We’d love to hear about it!
Take part in our #ShareYourSuccess campaign and show how VSC helped move your research forward. Whether it’s a publication, a project highlight, or a visual from your work, your story can inspire others.
🖥️ Be featured on our website and social media. Show the impact of your work. Help grow our research community
📬 Submit your story: https://www.vscentrum.be/sys
