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

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 X2A′, the vertical electronic transition energies (A2A″ ← X2A′ and B2A′ ← X2A′), the vertical ionization potential and the permanent dipole moment of X2A′ have been computed. The harmonic vibrational frequencies and infrared intensities of C2H3X2A′ 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 A2A″ ← A2A′, 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 B2A′ ← X2A′, predicted to be 43910 cm−1 from this work, will facilitate the experimental search for the undiscovered B̄ state of C2H3 through spectroscopic observation. In calculating the intramolecular rearrangement in C2H3X2A′, using CCSD(T)/Dunningʹs 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 C2H3X2A′ is an unstable isomer, corresponding to a transition state. The computed barrier for the tunnelling of α-H in C2H3X2A′ 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.