Rotational Spectroscopy of 1-Cyano-2-methylenecyclopropane (C₅H₅N)─A Newly Synthesized Pyridine Isomer

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The gas-phase rotational spectrum of 1-cyano-2-methylenecyclopropane (C₁, C₅H₅N), an isomer of pyridine, is presented for the first time, covering the range from 235 to 500 GHz. Over 3600 a-, b-, and c-type transitions for the ground vibrational state have been assigned, measured, and least-squares fit to partial-octic A- and S-reduced distorted-rotor Hamiltonians with low statistical uncertainty (σ_fit = 42 kHz). Transitions for the two lowest-energy fundamental states (ν₂₇ and ν₂₆) and the lowest-energy overtone (2ν₂₇) have been similarly measured, assigned, and least-squares fit to single-state Hamiltonians. Computed vibration–rotation interaction constants (B₀–B_v) using the B3LYP and MP2 levels of theory are compared with the corresponding experimental values. Based upon our preliminary analysis, the next few vibrationally excited states form one or more complex polyads of interacting states via Coriolis and anharmonic coupling. The spectroscopic constants and transition frequencies presented here form the foundation for both future laboratory spectroscopy and astronomical searches for 1-cyano-2-methylenecyclopropane.