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

Abstract

We report on the experimental and theoretical infrared spectrum of the C H⋅⋅⋅CO complex. This complex was prepared by UV photolysis of propiolic acid (HC OOH) in argon and krypton matrices. The experimental bands of C H in the C H⋅⋅⋅CO complex are blue-shifted from those of the C H monomer. The calculations on the C H⋅⋅⋅CO 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 C H⋅⋅⋅CO complex, we developed the theoretical approach used earlier for the C H monomer. Based on the calculations, the main experimental bands of the C H⋅⋅⋅CO complex are assigned to the most stable parallel structure. Almost all the strong bands predicted by theory (with intensities >30 km mol ) 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 C H radical.

Spectroscopic characterization of the complex of vinyl radical and carbon dioxide: Matrix isolation and ab initio study

“…36 The C 2 H 2 • • • CO 2 complexes can be obtained by UV photolysis of PA/Ng matrices. 27 In accordance with the previous results, 27 26,27 Prolonged irradiation produces the HC The UV-irradiated PA/Ng matrices were annealed at 25 and 35 K for Ar and Kr matrices, respectively, which led to some decrease of the amount of C 2 H 2 • • • CO 2 . Simultaneously, new absorptions appeared in the FTIR spectrum at the positions, which slightly differ from the bands of monomeric C 2 H 3 and CO 2 [trace (b) in Fig.…”

“…This difference may explain the higher stability of the C 1 structure as compared to the C s structure. 26 The structural assignment of the experimental species is not straightforward because of rather small shifts of the observed vibrational bands (Table II). Matrix-site splitting of the experimental bands further complicates the comparison.…”

“…The other example is the C 2 H• • • CO 2 complex, which exhibits remarkable vibronic interactions due to peculiarities of the electronic properties of the ethynyl radical (C 2 H). 26 It is not an easy task to produce intermolecular complexes of free radicals in sufficient concentrations in cryogenic matrices. For example, the HCNH• • • CO 2 complexes were obtained by the reactions of H atoms with the HCN• • • CO 2 complexes prepared by deposition of the HCN/CO 2 /Ng mixture (Ng = a noble gas).…”

Astrochemically Relevant Radicals and Radical–Molecule Complexes: A New Insight from Matrix Isolation

“…According to the comparison with the results of ab initio calculations, the radicalmolecule complex adopts the most stable parallel configuration. It is worth noting that the combination of experimental and theoretical methods [145] made it possible to assign the whole set of vibronic bands of the C 2 H • • • • CO 2 complex observed in the middle and near IR region (up to ~6200 cm −1 ), which appears to be the first example of a direct manifestation of the effect of weak non-covalent interactions on vibronic transitions in radical-molecule complexes.…”

“…A number of astrochemically relevant radical-molecule complexes with carbon dioxide were characterized by FTIR spectroscopy in recent years. One interesting example is a complex of ethynyl radical (C 2 H • • • • CO 2 ), which was prepared by UV photolysis of propiolic acid in solid argon and krypton matrices [145]:…”