Rotational spectrum of anti- and gauche-4-cyano-1-butyne (C5H5N) - An open-chain isomer of pyridine

The analysis of the rotational spectrum of 4-cyano-1-butyne, an open-chain isomer of pyridine, is presented herein for the first time. Transitions for the ground vibrational states of both conformational isomers, anti and gauche, were observed in the 8-38 GHz and 130-360 GHz frequency ranges and fit to effective sextic distorted-rotor Hamiltonians with low uncertainty (σ_fit < 50 kHz), allowing for their potential detection by radioastronomy or in harsh reaction environments. The microwave region includes hyperfine-split transitions for both conformers, allowing for the experimental determination of the χ_aa, χ_bb, and χ_cc quadrupole coupling constants. In the millimeter-wave region, not every observable ground-state transition could be satisfactorily addressed by a single-state Hamiltonian due to Coriolis coupling with the low-energy vibrationally excited states. Thus, effective single-state fits of the ground states were obtained by including only K_a-series transitions without obviously perturbed energy levels. Importantly, the effective fit does include the most intense transitions throughout the observed frequency ranges. The predicted values (MP2/cc-pVTZ) are in close agreement with the experimental spectroscopic constants. These data provide a sufficient foundation for the identification of anti- and gauche-4-cyano-1-butyne in the interstellar medium by radioastronomy. The gauche conformer is estimated to lie 1.00 (5) kcal/mol higher in energy than the anti conformer, as determined using experimental transition intensities.