The rotational spectrum of the CCP (X² Π_r) radical and its ¹³C isotopologues at microwave, millimeter, and submillimeter wavelengths

The pure rotational spectrum of CCP (X Πr2) has been measured at microwave, millimeter, and submillimeter wavelengths (17-545 GHz), along with its C13 isotopologues (C13 C13 P, C C13 P, and C13 CP). The spectra of these species were recorded using a combination of millimeter/submillimeter direct absorption methods and Fourier transform microwave (FTMW) techniques. The phosphorus dicarbides were created in the gas phase from the reaction of red phosphorus and acetylene or methane in argon in an ac discharge for the direct absorption experiments, and using P Cl3 as the phosphorus source in a pulsed dc nozzle discharge for the FTMW measurements. A total of 35 rotational transitions were recorded for the main isotopologue, and between 2 and 8 for the C13 - substituted species. Both spin-orbit components were identified for CCP, while only the =12 ladder was observed for C13 C13 P, C C13 P, and C13 CP. Hyperfine splittings due to phosphorus were observed for each species, as well as carbon-13 hyperfine structure for each of the C13 - substituted isotopologues. The data were fitted with a Hund's case (a) Hamiltonian, and rotational, fine structure, and hyperfine parameters were determined for each species. The rm (1) bond lengths established for CCP, r (C-C) =1.289 (1) Å and r (C-P) =1.621 (1) Å, imply that there are double bonds between both the two carbon atoms and the carbon and phosphorus atoms. The hyperfine constants suggest that the unpaired electron in this radical is primarily located on the phosphorus nucleus, but with some electron density also on the terminal carbon atom. There appears to be a minor resonance structure where the unpaired electron is on the nucleus of the end carbon. The multiple double bond structure forces the molecule to be linear, as opposed to other main group dicarbides, such as Si C2, which have cyclic geometries.