Fine structure and hyperfine perturbations in the pure rotational spectrum of the VCl radical in its X⁵ Δ r state

The pure rotational spectrum of the VCl radical in its ⁵Δ r ground state has been recorded in the range 236-417 GHz using millimeter/submillimeter direct absorption techniques. This species was created in an ac discharge of VCl₄ and argon. Ten rotational transitions of V³⁵ Cl were measured in all five Ω ladders; an additional nine transitions of the Ω=1 spin state were recorded in order to evaluate the ⁵¹V hyperfine structure. Hyperfine interactions associated with the C ³⁵ l nucleus were not resolved, consistent with the ionic structure of the molecule. Because of extensive perturbations caused by the low-lying A Π5 r excited state, the rotational spectrum of the ground state has been found to be quite irregular. The four lowest Ω ladders exhibit unusually large lambda-doubling interactions, with the Ω=1 component showing the largest splitting, over 2 GHz in magnitude. The Ω=1 transitions are also shifted to higher frequency relative to the other spin components. In addition, the hyperfine structure varies widely between the Ω ladders, and an avoided crossing is observed in two transitions of both the Ω=1e and 2e components. The data have been analyzed with a case (c) Hamiltonian, and effective rotational, lambda-doubling, and hyperfine constants have been determined for V³⁵ Cl. Higher-order parity-dependent magnetic hyperfine terms dΔ2 and dΔ3 were required in the analysis, derived from perturbation theory, in addition to the usual dΔ parameter. The local perturbations evident in these spectra indicate that the A Π5 r excited state lies within the spin-orbit manifold of the ground state, well below the predicted value of 517 cm-1. Mixing of the A Π5 r and X ⁵Δ r states apparently causes both local and global perturbations in the ground state spectrum.