Microwave spectroscopic detection of HCCP in the X 3Σ− electronic state: Phospho-carbene, phospho-allene, or phosphorene?

Microwave spectrum of the HCCP radical was detected for the first time in the X 3Σ− ground electronic state using a source-modulated microwave spectrometer. In total, 24 rotational transitions of HCCP in the 90–360 GHz region, 9 rotational transitions of DCCP in the 260–360 GHz range, and 24 rotational transitions of H13C13CP between 130–360 GHz were measured. Hyperfine structure pertaining to the phosphorus and hydrogen nuclei was observed for HCCP, and in the case of H13C13CP, only for phosphorus. The corresponding hyperfine coupling constants were ascertained in addition to the rotational, centrifugal distortion, and fine structure constants by a least-squares analysis of the measured frequencies. From the hyperfine coupling constants determined, the spin density of unpaired electrons was estimated to be 76% for the phosphorus atom and 42% for the carbon adjacent to the hydrogen. The r0 structure of HCCP was established from the rotational constants of HCCP and its isotopically substituted species: r0(CP)=1.685 Å, r0(CC)=1.241 Å, and r0(CH)=1.057 Å. These structural features are consistent with a linear phospho-allenic form that has been somewhat modified by a phosphorene.