The microwave and millimeter rotational spectra of the PCN radical (X̃ ³∑ ^-)

The pure rotational spectrum of the PCN radical (X̃ ³∑ ^-) has been measured for the first time using a combination of millimeter/submillimeter direct absorption and Fourier transform microwave (FTMW) spectroscopy. In the millimeter instrument, PCN was created by the reaction of phosphorus vapor and cyanogen in the presence of an ac discharge. A pulsed dc discharge of a dilute mixture of PCl ₃ vapor and cyanogen in argon was the synthetic method employed in the FTMW machine. Twenty-seven rotational transitions of PCN and six of P ¹³CN in the ground vibrational state were recorded from 19 to 415 GHz, all which exhibited fine structure arising from the two unpaired electrons in this radical. Phosphorus and nitrogen hyperfine splittings were also resolved in the FTMW data. Rotational satellite lines from excited vibrational states with v ₂ = 1-3 and v ₁ = 1 were additionally measured in the submillimeter range. The data were analyzed with a Hund's case (b) effective Hamiltonian and rotational, fine structure, and hyperfine constants were determined. From the rotational parameters of both carbon isotopologues, the geometry of PCN was established to be linear, with a P-C single bond and a C-N triple bond, structurally comparable to other non-metal main group heteroatom cyanides. Analysis of the hyperfine constants suggests that the two unpaired electrons reside almost exclusively on the phosphorus atom in a π ² configuration, with little interaction with the nitrogen nucleus. The fine structure splittings in the vibrational satellite lines differ significantly from the pattern of the ground state, with the effect most noticeable with increasing v ₂ quantum number. These deviations likely result from spin-orbit vibronic perturbations from a nearby 1∑ ⁺ state, suggested by the data to lie ∼12 000 cm ^-1 above the ground state.