High resolution laser spectroscopy of asymmetrically deuterated cyclopentadienyl radicals : a study of vibronic degeneracy resolution and Jahn-Teller distortion

The rotationally resolved, laser induced fluorescence, excitation spectra of the partially deuterated cyclopentadienyl radicals, C5H4D and C5HD4, have been observed at low temperature in a supersonic free jet expansion. The observed electronic transition in the uv region corresponds to the à 2A″2■X̃ 2E″1 transition in the symmetric cyclopentadienyl isotopomers with D5h symmetry. In the reduced C2v symmetry of the C5HD4 and C5H4D isotopomers, this electronic transition splits into two distinct vibronic bands, separated by about 9 cm−1, which arise from the two vibronic components X̃1 and X̃2 into which the X̃ state is resolved when the symmetry is lowered. In C5H4D the ground X̃1 state has 2A2 symmetry and a permanently distorted, elongated allyl-like structure while the low-lying X̃2 state has 2B2 symmetry and a compressed dienelike structure. The symmetries of the energy levels and the distortions are reversed for the C5HD4 species. A detailed theoretical model is developed to describe the splitting and the rotational structure of the X̃1 and X̃2 states. Application of this model yields a precise value for the alternation of the C–C bond lengths in the distorted structures. This value is shown to be the same as that describing the dynamically pseudorotating, Jahn–Teller distorted structures in the D5h isotopomers. Additionally, the theory explains anomalous values of the observed inertial defects and relates them to physically meaningful quantities for all the isotopomers in both their X̃1 and X̃2 states. Our results are compared to previous experimental work and ab initio calculations on the cyclopentadienyl radical.