Microwave spectroscopy of the HCCS and DCCS radicals (X̃ 2Πi) in excited vibronic states: A study of the Renner–Teller effect

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 γeff constants for the vibronic μ/κ2Σ states are found to be anomalous, in that the variation of the γeff constants in the same bending mode is large up to 3 GHz and the γeff value can reach to nearly twice the rotational constants Bv. This behavior cannot be understood by the current Renner–Teller theory. We have developed a theory to include cross vibronic interaction between two vibronic 2Σ(vt=1) states in different bending modes. Since the difference of the vibrational quantum numbers for these states is Δ(v4+v5)=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(v4+v5)=±2. Calculation with the newly derived expressions for γeff reproduces the anomaly in HCCS when the Renner parameters are fixed at ε4=−0.37 and ε5=+0.10 from the ab initio calculation, and the parameter |ε45| 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 ε4 and ε5 values is explicitly judged to be correct. In addition, the Beff and the P-doubling constants in the 2Πi and 2Δi states are found to be effected by a higher-order perturbation of the cross vibronic interaction.