The millimeter-wave spectrum of the SiP radical (X2Πi): Rotational perturbations and hyperfine structure

The millimeter/submillimeter-wave spectrum of the SiP radical (X2Πi) 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 ΔEΠ-Σ ∼ 430 cm−1. The data were analyzed using a Hund’s case aβ 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, bF, the spin-dipolar constant, c, and the nuclear spin-orbital parameter, a, indicates that the orbital of the unpaired electron is chiefly pπ 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.