Excited electronic states of protonated acetylene

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Abstract
The classical structure of protonated C2H2, and the nonclassical, bridge arrangement for the same species, have been studied along the rearrangement pathway with the intention of throwing new light on the role played by the electronically excited state of the full system in the collisions between a proton and the C2H2 molecule. Furthermore, the potential energy on one possible dissociation path into acetylene and proton has been obtained. Extensive, multiple reference double‐excitation configuration‐interaction (MRD‐CI) calculations have been carried out for the first time with the specific aim of locating energies and geometries of the ground and of the lower‐lying excited states of the C2H+3 species as it goes from one structure to the other and from the C2V geometry to separated C2H2+H+ or C2H+2+H fragments. The results show an interesting behavior of the excited states and provide a preliminary understanding of the full potential energy surfaces thought to be involved in charge‐transfer (CT) inelastic collisions at low and intermediate energies (E≤100 eV).