Ab initio calculation of electronic absorption spectra and ionization potentials of C3H3 radicals

Abstract

C₃H₃ isomers play a major role in combustion chemistry and are of fundamental interest in understanding photodissociation processes of hydrocarbons. Nevertheless, their electronic and ionization spectra are largely unknown. This study presents the first extended investigation of the electronically excited states and ionization potentials of the four most stable isomers, namely propargyl, 1-propynyl, cycloprop-2-enyl, and cycloprop-1-enyl. Electronic spectra are studied by ab initio calculations of the vertical excitation energies at the multi-reference configuration interaction (MRCI) level of theory. Vertical and adiabatic ionization energies are determined by coupled-cluster (RCCSD(T)) and multi-reference perturbation theory (CASPT2). The obtained results are suitable to aid the experimental detection of the different C₃H₃ isomers. Most of all, the calculations show that the assignment of an experimentally observed absorption at 242 nm to the propargyl radical was erroneous. Therefore, some recent experiments, based on this assignment, also need to be re-interpreted because this absorption is due to a yet unknown species.