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.