Rotation and Rotation–Vibration Spectroscopy of the 0⁺–0^– Inversion Doublet in Deuterated Cyanamide

The pure rotation spectrum of deuterated cyanamide was recorded at frequencies from 118 to 649 GHz, which was complemented by measurement of its high-resolution rotation-vibration spectrum at 8–350 cm^–1. For D₂NCN the analysis revealed considerable perturbations between the lowest <i>K</i>_<i>a</i> rotational energy levels in the 0⁺ and 0^– substates of the lowest inversion doublet. The final data set for D₂NCN exceeded 3000 measured transitions and was successfully fitted with a Hamiltonian accounting for the 0⁺ ↔ 0^– coupling. A smaller data set, consisting only of pure rotation and rotation-vibration lines observed with microwave techniques was obtained for HDNCN, and additional transitions of this type were also measured for H₂NCN. The spectroscopic data for all three isotopic species were fitted with a unified, robust Hamiltonian allowing confident prediction of spectra well into the terahertz frequency region, which is of interest to contemporary radioastronomy. The isotopic dependence of the determined inversion splitting, Δ<i>E</i> = 16.4964789(8), 32.089173(3), and 49.567770(6) cm^–1, for D₂NCN, HDNCN, and H₂NCN, respectively, is found to be in good agreement with estimates from a simple reduced quartic-quadratic double minimum potential