Millimeter/Submillimeter-Wave Spectrum of the VCl⁺ Radical in its X ⁴Σ⁻ Ground State

The pure rotational spectrum of the molecular ion VCl⁺ (X ⁴Σ⁻) has been recorded from 274 to 419 GHz using a combination of millimeter direct absorption and velocity modulation spectroscopy. This study is the first spectroscopic measurement of this species in the laboratory, which has also enabled the determination of the ground state term, ⁴Σ⁻. VCl⁺ was produced in an AC discharge of VCl₄ and argon. Fifteen and eleven rotational transitions were recorded for the V³⁵Cl⁺ and V³⁷Cl⁺ isotopologues, respectively. The fine structure splittings of the ⁴Σ⁻ state were found to deviate significantly from a case (b) pattern. Specifically, spin components from adjacent rotational transitions were found to overlap in frequency. Unusual vanadium hyperfine splittings were also observed in the VCl⁺ spectra; the expected vanadium octet was clearly present for the F₂ and F₃ spin components but was partially collapsed and reversed in frequency ordering in the F₁ and F₄ cases. The data were analyzed in a global fit, and rotational, fine structure, and hyperfine constants were determined. For VCl⁺, the values of both the spin−spin and spin-rotation parameters are extremely large, as were the third-order hyperfine and spin-rotation terms, b_s and γ_s. For example, the spin−spin constant is λ = 417 900(22 700) MHz, while γ = 6567(51) MHz. The values of these constants indicate a high density of excited electronic states close to the ground state, which contribute to second and third-order spin−orbit coupling. The hyperfine constants suggest a σ¹π² electron configuration, as opposed to σ¹δ², as found in VS and VO. The bond length of VCl⁺, r₀ = 2.119(2) Å, is shorter than that of VCl by 0.1 Å, which has r₀ = 2.219 Å. The decrease in bond length is attributed to an increase in the electrostatic attraction between V²⁺ and Cl⁻, as opposed to V⁺ and Cl⁻ for VCl.