High-resolution spectroscopy of [H,C,N]⁺: I. Rotationally resolved vibrational bands of HCN⁺ and HNC

Rovibrational spectra of the open-shell linear cations HCN⁺ (X̃²Π) and HNC⁺ (X̃²Σ⁺) are measured with leak-out spectroscopy in cryogenic 22-pole ion traps. The fundamental ν₁ C-H stretching vibration of HCN⁺ is found at 3056.3412(1) cm^-1 and the lower energy Renner-Teller (RT) component (Σ) of the ν₁ + ν₂ combination band is found at 3340.8480(2) cm^-1. The resulting effective RT vibrational frequency of ≈300 cm^-1 inferred from the comparison of these two bands indicates a large Renner-Teller splitting for HCN⁺. For HNC⁺, the ν₁ N-H stretching vibration is found at 3407.9136(4) cm^-1, much higher than expected from previous matrix work. Thanks to the rotational resolution of these infrared measurements, spectroscopic constants for the electronic fine-structure, molecular rotation, centrifugal distortion, Λ-doubling and spin-rotation interaction have been determined for the vibrational ground and excited states with high confidence. The infrared spectrum of HCN⁺ is rather rich and contains more bands including, e.g., the electronic à ← X̃ transition. The analysis of this band and the pure rotational spectrum of HCN⁺ will be the subject of further publications.