Ethynylarsine (2a) is synthesized by reaction of arsenic trichloride with ethynyltributylstannane followed by a chemoselective reduction of the formed ethynyldichloroarsine (la). The base-induced rearrangement of compound 2a on a solid base in VGSR conditions leads to ethylidynearsine (3a) characterized by 1 H and 13C NMR and by microwave spectroscopy. The rotational spectrum of 3a is recorded in the frequency range 90-470 GHz. Rotational and centrifugal distortion constants are determined and the quadrupole hyperfine structure is analyzed. The structure of CH~CIAS has been estimated from the rotational constants of eight different isotopic species and from ab initio calculations. Compound 3a exhibits a low stability at 0 OC in deuteriochloroform (~1 1 2 ca. 1 h) or in the gaseous phase at a pressure of 10 Pa (~1 / 2 ca. 30 min).

Low pressure pyrolysis at 600 K of bis(fluoroformyl) peroxide, FC(O)OOC(O)F, yields the fluorocarboxyl radical, FCO2, in a concentration high enough to allow its detection by millimeter wave and infrared spectroscopy. The radical was first identified from its high resolution infrared spectrum obtained using a Fourier transform infrared interferometer. Observation and identification of its millimeter wave (MMW) spectrum were made possible by reliable ab initio calculations at the level of open-shell coupled cluster theory using large basis sets. The excellent agreement between the experimental and theoretical results confirms the structure of the FCO2 radical and the efficiency of the synthesis. The analysis of the MMW spectrum has given a set of ground state parameters including rotational, quartic centrifugal distortion, fine and hyperfine constants.

The transient FPO molecule was produced in a flow by pyrolysis of 5% F₂POPF₂ in Ar at 1300–1400°C. High resolution (0.008 cm⁻¹) Fourier transform infrared spectra of the a-type ν₁ and ν₂ bands centered at 1297.54 and 819.57 cm⁻¹, respectively, were measured and fitted to excited state parameters up to quartic centrifugal distortion constants. Millimeter-wave spectra between 300 and 370 GHz of FPO in the ground and ʋ₃ = 1 excited states were recorded, and 124 and 86 lines, respectively, including for both states a- and b-type transitions, were measured and fitted to a Watson A-type Hamiltonian up to sextic centrifugal distortion terms. High-level ab initio calculations with large basis sets were performed for FPO to provide reliable structural parameters as well as harmonic [CCSD(T)/AVQZ + 1] and anharmonic [MP2/VQZ + 1] force fields up to quartic terms. The spectroscopic constants derived from these force fields are generally in excellent agreement with experiment. The calculations moreover suggest anharmonic interactions between ν₂ and 2ν₃, and between ν₁ and ν₂ + ν₃. Deperturbation of the ʋ₁ = 1 and ʋ₂ = 1 levels was done, and the results are in support of a band center of ν₃ close to 412 cm⁻¹. Consistent experimental and theoretical equilibrium structures were determined for FPO, with r_e(PO) 145.3 pm, r_e(PF) 157.3 pm and θ_e(FPO) 110.1°. The collision-controlled 1/e life-time of FPO generated by an electric discharge in an F₂POPF₂/Ar mixture at 8–10 Pa and at room temperature is 8 ms

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