The pure rotational spectrum of VS (X⁴Σ^-): A combined Fourier transform microwave and millimeter-wave study

The pure rotational spectrum of the vanadium sulfide radical, VS (X ⁴Σ^-), has been measured in the frequency range 5-310 GHz using a combination of millimeter-wave direct absorption and Fourier transform microwave (FTMW) techniques. In the millimeter-wave region, the radical was produced in an AC discharge from the reaction of VCl₄, the vanadium donor, and CS₂. In the FTMW instrument, the molecule was created in a supersonic jet, coupled with a laser ablation/DC discharge source (DALAS), from a mixture of metal vapor and H₂S, heavily diluted in argon. A total of 8 rotational transitions were measured for VS, in which both the quartet fine structure and vanadium hyperfine splittings were resolved. The spectra were analyzed with a Hund's case (b) Hamiltonian, and rotational, spin-rotation, spin-spin, and hyperfine parameters were determined. The precision of the constants from previous optical studies was refined and, for the first time, the vanadium quadrupole constant, eQq = -7.6 (4.0) MHz, and the third order Fermi contact correction, b_S = -0.293 (94) MHz, were established. From the fine structure parameters, the nearby ⁴Π and ²Σ⁺ states were estimated to lie ∼6560 cm ^-1 and ∼7170 cm^-1 above the ground state. The hyperfine constants suggest that the bonding in VS is partly ionic, with a significant degree of covalent character.