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 with n = 0−7 and the combination bands   with n = 1−6 and   with n = 0−3 in the HCF Ã1A‘ ‘ ← X̃1A‘ 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 27 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 1126 places a lower limit on the à state barrier to dissociation of ∼ 8555 cm-1 above the vibrationless level.

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We report on the vibrational mode dependence of the 19F and 1H hyperfine interaction constants in the A1A" state of HCF, determined using polarization quantum beat spectroscopy. The nuclear spin/overall rotation coupling constants display a pronounced energy dependence and mode selectivity which can be traced to variations in both the A rotational constant and nuclear spin/electron orbital coupling constant a. In particular, modes containing C-F stretching excitation display significantly larger 19F spin-rotation constants, which is explained in terms of a decrease in back donation of electron density into the C 2p(pi) orbitals.

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