### Abstract

The molecular structure, intramolecular rearrangement and dissociation energy of C_{2}H_{3} have been studied with high-level ab initio calculations using ACES II and MOLCAS-2 programs. In the structural calculations of C_{2}H_{3}, the optimized geometry and vibrational frequencies of X̃ ^{2}A′, the vertical electronic transition energies (Ã ^{2}A″ ← X̃ ^{2}A′ and B̃ ^{2}A′ ← X̃ ^{2}A′), the vertical ionization potential and the permanent dipole moment of X̃ ^{2}A′ have been computed. The harmonic vibrational frequencies and infrared intensities of C_{2}H_{3} X̃ ^{2}A′ 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 ^{2}A″ ← X̃ ^{2}A′, 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 ^{2}A′ ← X̃ ^{2}A′, predicted to be 43910 cm^{-1} from this work, will facilitate the experimental search for the undiscovered B̃ state of C_{2}H_{3} through spectroscopic observation. In calculating the intramolecular rearrangement in C_{2}H_{3} X̃ ^{2}A′, 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 C_{2}H_{3} X̃ ^{2}A′ is an unstable isomer, corresponding to a transition state. The computed barrier for the tunnelling of α-H in C_{2}H_{3} X̃ ^{2}A′ is also in excellent agreement with the upper bound limit of < 1500 cm^{-1} determined from high-resolution infrared spectroscopy. The dissociation energy of C_{2}H_{3} → C_{2}H_{2} + H and the energy difference between the isomers of acetylene and vinylidene, calculated in the present study, are also consistent with experimental measurements.

Original language | English |
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Pages (from-to) | 43-56 |

Number of pages | 14 |

Journal | Chemical Physics |

Volume | 206 |

Issue number | 1-2 |

DOIs | |

Publication status | Published - 1996 May 15 |

### ASJC Scopus subject areas

- Physics and Astronomy(all)
- Physical and Theoretical Chemistry

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## Cite this

_{2}H

_{3}.

*Chemical Physics*,

*206*(1-2), 43-56. https://doi.org/10.1016/0301-0104(95)00441-6