Molecular structure of the linear C3H radical: Microwave spectrum of the 13C substituted species

Abstract

The rotational spectra of three 13C isotopic species of the linear C3H radical in the 2Πr ground vibronic state and the 2Σμ vibronic state of the ν4 (HCC bending) vibrational mode were observed by using a source-modulated spectrometer. The radical was produced in a free-space cell by a glow discharge in a mixture of C2H2, He, and 13CO. The rotational, spin-rotation interaction, Λ-type doubling, and hyperfine interaction constants were determined from the analysis of the observed spectra, where vibronic interaction between the 2Πr and 2Σμ states was taken into account. The rs structure of C3H, denoted by CαCβCγH, was derived from the rotational constants for the normal, deuterated, and three 13C species: rs(Cα–Cβ)=1.3263(1) Å, rs(Cβ–Cγ)=1.2539(2) Å, and rs(Cγ–H)=1.0171(1) Å. The short C–H distance is interpreted in terms of a large amplitude motion of the ν4 mode, whose vibrational energy is very low due to the Renner–Teller effect. An ab initio molecular orbital calculation was carried out, and the structure in which the HCC angle is bent was found to be slightly more stable than the linear structure. The spin densities of three carbon atoms are derived from the hyperfine interaction constants, and they are discussed in connection to the quasilinear nature of the molecule.