Theoretical study of isomeric structures and low-lying electronic states of the vinyl radical C₂H₃

The molecular structure, intramolecular rearrangement and dissociation energy of C₂H₃ have been studied with high-level ab initio calculations using ACES II and MOLCAS-2 programs. In the structural calculations of C₂H₃, the optimized geometry and vibrational frequencies of X̃ ²A′, the vertical electronic transition energies (à ²A″ ← X̃ ²A′ and B̃ ²A′ ← X̃ ²A′), the vertical ionization potential and the permanent dipole moment of X̃ ²A′ have been computed. The harmonic vibrational frequencies and infrared intensities of C₂H₃ X̃ ²A′ obtained from this calculation will help the spectroscopic observation for the vibrational modes, most of which are unobserved. The calculated vertical transition energy, 25529 cm^-1 for à ²A″ ← X̃ ²A′, and the vertical ionization potential, 8.33 eV from an MRCI method with atomic natural orbitals, are in excellent agreement with the experimental values of 24815 cm^-1 and 8.25 eV, respectively. The vertical transition of B̃ ²A′ ← X̃ ²A′, predicted to be 43910 cm^-1 from this work, will facilitate the experimental search for the undiscovered B̃ state of C₂H₃ through spectroscopic observation. In calculating the intramolecular rearrangement in C₂H₃ X̃ ²A′, using CCSD(T)/Dunning's triple zeta polarizations, the non-classical structure with a hydrogen atom bridged between the C=C bond has been found to lie at least 47 kcal/mol above the classical equilibrium structure. The calculation also indicates that the non-classical C₂H₃ X̃ ²A′ is an unstable isomer, corresponding to a transition state. The computed barrier for the tunnelling of α-H in C₂H₃ X̃ ²A′ is also in excellent agreement with the upper bound limit of < 1500 cm^-1 determined from high-resolution infrared spectroscopy. The dissociation energy of C₂H₃ → C₂H₂ + H and the energy difference between the isomers of acetylene and vinylidene, calculated in the present study, are also consistent with experimental measurements.