Effect of Noncovalent Interactions on Vibronic Transitions: An Experimental and Theoretical Study of the C₂ H⋅⋅⋅CO₂...

We report on the experimental and theoretical infrared spectrum of the C₂ H⋅⋅⋅CO₂ complex. This complex was prepared by UV photolysis of propiolic acid (HC₃ OOH) in argon and krypton matrices. The experimental bands of C₂ H in the C₂ H⋅⋅⋅CO₂ complex are blue-shifted from those of the C₂ H monomer. The calculations on the C₂ H⋅⋅⋅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 C₂ H⋅⋅⋅CO₂ complex, we developed the theoretical approach used earlier for the C₂ H monomer. Based on the calculations, the main experimental bands of the C₂ H⋅⋅⋅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 C₂ H radical.