Quantum chemical exploration of the binding motifs and binding energies of neutral molecules, radicals and ions with small water clusters: characterisation and astrochemical implications

ABSTRACT

Accurate binding energies of molecules to water clusters are relevant for understanding intermolecular interactions and various chemical applications. They enter models of interstellar chemical processes, as binding to icy grains influences surface reactions and thus affects calculated gas-phase abundances. Unfortunately, many astrochemical molecules (especially radicals and ions) are incompletely characterised in these models. To address this, we report computational searches for optimal structures and benchmark binding and condensation energies for sets of neutral, radical, cationic, and anionic molecules of astrochemical relevance with clusters of $N=1-4$ water molecules. These calculations utilised reliable density functionals for geometry optimisation, and coupled cluster (CCSD(T)) single point calculations with large basis sets. Four energetic binding motifs (weak, intermediate, strong or covalently bonded) were observed depending on the chemical nature of the guest molecule. Neutral closed and open-shell molecules with strong dipoles and a greater potential for hydrogen bonding are more tightly bound to water clusters compared to non-polar ones. For closed-shell cationic and anionic species, barrier-less reactions with water clusters occur, which reveals radical-free routes to molecular processing in the gas phase and on amorphous ice surfaces.

GRAPHICAL ABSTRACT

KEYWORDS:

• Binding energy
• density functional theory
• interstellar chemistry
• clusters
• intermolecular interactions

Disclosure statement

The authors declare the following competing interest: M. H.-G. is a part-owner of Q-Chem, which is the software platform in which the density functional theory calculations were carried out.

Additional information

Funding

Work at Berkeley was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy [contract number DE-AC02-05CH11231]. Support for this research was provided by the NASAs Planetary Science Division Research Program, through ISFM work package 'The Production of Astrobiologically Important Organics during Early Planetary System Formation and Evolution' at NASA Ames Research Center. Computer time from the Pleiades cluster of the NASA Advanced Supercomputer (NAS) is also gratefully acknowledged.