The millimeter-wave spectrum of the SiP radical (X²Π_i): Rotational perturbations and hyperfine structure

The millimeter/submillimeter-wave spectrum of the SiP radical (X²Π_i) has been recorded using direct absorption spectroscopy in the frequency range of 151-532 GHz. SiP was synthesized in an AC discharge from the reaction of SiH₄ and gas-phase phosphorus, in argon carrier gas. Both spin-orbit ladders were observed. Fifteen rotational transitions were measured originating in the Ω = 3/2 ladder, and twelve in the Ω = 1/2 substate, each exhibiting lambda doubling and, at lower frequencies, hyperfine interactions from the phosphorus nuclear spin of I = 1/2. The lambda-doublets in the Ω = 1/2 levels appeared to be perturbed at higher J, with the f component deviating from the predicted pattern, likely due to interactions with the nearby excited A²Σ⁺ electronic state, where ΔE_Π-Σ ∼ 430 cm^-1. The data were analyzed using a Hund's case a_β Hamiltonian and rotational, spin-orbit, lambda-doubling, and hyperfine parameters were determined. A ²Π/²Σ deperturbation analysis was also performed, considering spin-orbit, spin-electronic, and L-uncoupling interactions. Although SiP is clearly not a hydride, the deperturbed parameters derived suggest that the pure precession hypothesis may be useful in assessing the ²Π/²Σ interaction. Interpretation of the Fermi contact term, b_F, the spin-dipolar constant, c, and the nuclear spin-orbital parameter, a, indicates that the orbital of the unpaired electron is chiefly p_π in character. The bond length in the v = 0 level was found to be r₀ = 2.076 Å, suggestive of a double bond between the silicon and phosphorus atoms.