Effect of Noncovalent Interactions on Vibronic Transitions: An Experimental and Theoretical Study of the C2H⋅⋅⋅CO2 Complex

We report on the experimental and theoretical infrared spectrum of the C₂H⋅⋅⋅CO₂ complex. This complex was prepared by UV photolysis of propiolic acid (HC₃OOH) in argon and krypton matrices. The experimental bands of C₂H in the C₂H⋅⋅⋅CO₂ complex are blue‐shifted from those of the C₂H monomer. The calculations on the C₂H⋅⋅⋅CO₂ structures were performed at the RMP2/aug‐cc‐pVTZ level. The relative stability of the complex structures was evaluated by using the RCCSD(T)/aug‐cc‐pVQZ level. To simulate the spectrum of the C₂H⋅⋅⋅CO₂ complex, we developed the theoretical approach used earlier for the C₂H monomer. Based on the calculations, the main experimental bands of the C₂H⋅⋅⋅CO₂ complex are assigned to the most stable parallel structure. Almost all the strong bands predicted by theory (with intensities >30 km mol⁻¹) are observed in the experiment. To our knowledge, it is the first study of the effect of noncovalent interactions on vibronic transitions and the first report on an intermolecular complex of the C₂H radical.