### Abstract

The microwave spectra of the HCCS and DCCS radicals are studied in the frequency range of 160-400 GHz and the rotational transition series are assigned to several low-lying vibronic states in the CCS or H(D)CC bending vibration. Analysis is carried out to obtain effective constants for respective vibronic states. The γ_{eff} constants for the vibronic μ/κ^{2}Σ states are found to be anomalous, in that the variation of the γ_{eff} constants in the same bending mode is large up to 3 GHz and the γ_{eff} value can reach to nearly twice the rotational constants B_{v}. This behavior cannot be understood by the current Renner-Teller theory. We have developed a theory to include cross vibronic interaction between two vibronic ^{2}Σ(v_{t} = 1) states in different bending modes. Since the difference of the vibrational quantum numbers for these states is Δ(v_{4} + v_{5}) = 0, the interaction has a much larger effect than the one considered by Petelin and Kiselev [Int. J. Quantum Chem. 6, 701 (1972)] for the vibronic states with Δ(v_{4} + u_{5}) = ± 2. Calculation with the newly derived expressions for γ_{eff} reproduces the anomaly in HCCS when the Renner parameters are fixed at ε_{4}=-0.37 and ε_{5}=+0.10 from the ab initio calculation, and the parameter |ε_{45}| for the cross vibronic interaction is varied to be 0.4, a value which is obtained for the first time. The relative sign of the above ε_{4} and ε_{5} values is explicitly judged to be correct. In addition, the B_{eff} and the P-doubling constants in the 2Π_{i} and ^{2}Δ_{i} states are found to be effected by a higher-order perturbation of the cross vibronic interaction.

Original language | English |
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Pages (from-to) | 8020-8033 |

Number of pages | 14 |

Journal | Journal of Chemical Physics |

Volume | 105 |

Issue number | 18 |

DOIs | |

Publication status | Published - Jan 1 1996 |

Externally published | Yes |

### ASJC Scopus subject areas

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