Laboratory Measurement of the Pure Rotational Transitions of HCNH+ and Its Isotopic Species

The pure rotational transitions of the protonated hydrogen cyanide ion, HCNH⁺, and its isotopic species, HCND⁺ and DCND⁺, were measured in the 107-482 GHz region with a source-modulated microwave spectrometer. The ions were generated in a cell with a magnetically confined DC-glow discharge of HCN and/or DCN. The rotational constant B₀ and the centrifugal distortion constant D₀ for each ion were precisely determined by a least-squares fitting to the observed spectral lines. The observed rotational transition frequencies by laboratory spectroscopy and the predicted ones are accurate to about 30-40 kHz and are useful as rest frequencies for astronomical searches of HCNH⁺ and HCND⁺.