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 (Ã 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 Ã 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.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry