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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