COAR

COAr is a shorthand used in physical chemistry and molecular spectroscopy to denote a weakly bound van der Waals complex formed between carbon monoxide (CO) and an argon atom (Ar). It is a transient species that can be generated in cryogenic matrices or in supersonic jet expansions where collisions lead to the formation of a CO–Ar adduct. The interaction is dominated by dispersion forces with small electrostatic contributions from CO's dipole, resulting in binding energies typically on the order of a few wavenumbers (cm^-1). The complex is non-covalent and highly fluxional; its geometry corresponds to the Ar atom approaching near the CO molecule, with the exact preferred orientation depending on the interaction potential and experimental conditions.

Spectroscopic studies, including infrared and microwave spectroscopy, reveal shifts in CO vibrational frequencies and rotational constants relative to the free CO molecule, enabling the mapping of the CO–Ar potential energy surface. The data, often supported by high-level ab initio calculations, help benchmark intermolecular force fields used to model CO interactions with noble gases and other weakly interacting partners.

COAr is part of a broader family of CO–rare gas complexes (CO–Ne, CO–Ar, CO–Kr, CO–Xe) used to understand noncovalent interactions and collision dynamics in cold environments. While COAr itself is not a stable molecule under standard conditions, it serves as a valuable model system in physical chemistry and molecular spectroscopy.