Weak Interactions in Interstellar Chemistry: How Do Open Shell Molecules Interact with Closed Shell Molecules?

The interstellar medium (ISM) is home to several open shell and closed shell molecules considered to be unusual or highly unstable on Earth. The chemistry of these molecules in the ISM has been widely studied over many decades. However, the concept of weak chemical interactions, which is terrestrially well-studied, has not been highlighted much in interstellar chemistry. In this study, we illustrate the wide variety of possible weak interactions in the ISM occurring between a carefully chosen set of open shell (OH, SH, CN, NO, NH₂, and HO₂) and closed shell (H₂O, H₂S, HF, HCl, NH₃, PH₃,HCN, HNC, HCP, CH₃OH, and CH₃SH) molecules, which are important in interstellar chemistry. We expound upon the structural and energetic features of the weak interactions by employing electronic structure calculations [CCSD­(T) and density functional theory]. The nature of the weak interactions is further probed by three different techniques, viz., the atoms-in-molecules (AIM) method, the transfer of spin densities, and the natural bond orbital (NBO) method. The astrochemical implications of the weak interactions are subsequently discussed, and it is suggested that the weak interactions could impact the molecular abundances in the ISM.