On the Renner-Teller effect and barriers to linearity and dissociation in HCF(Ã ¹A″)

To further investigate the Renner-Teller effect and barriers to linearity and dissociation in the simplest singlet carbene (HCF), we recorded fluorescence excitation spectra of the pure bending transitions 2 ₀ ⁿ with n = 0-7 and the combination bands 1 ₀ ¹ 2 ₀ ⁿ with n = 1-6 and 2 ₀ ⁿ 3 ₀ ¹ with n = 0-3 in the HCF à ¹A← -X̃ ¹A′ system. The spectra were measured under jet-cooled conditions, using a pulsed-discharge source, and rotationally analyzed to yield precise values for the band origins and rotational constants. The derived Ã-state parameters are in excellent agreement with the predictions of ab initio electronic structure theory. The approach to linearity in the à state is evidenced in a sharp increase in the rotational constant A, as first reported by Kable and co-workers, and a minimum in the vibrational intervals near the 2 ⁷ level. A fit of the vibrational intervals for the pure bending levels yields a barrier to linearity of 6300 ± 270 cm ^-1 above the vibrationless level. Our observation of the K _a′ = 1 level of 1 ¹2 ⁶ places a lower limit on the A state barrier to dissociation of ∼ 8555 cm ^-1 above the vibrationless level.