The 130–360 GHz rotational spectrum of <i>syn</i>-2-cyano-1,3-butadiene (C<sub>5</sub>H<sub>5</sub>N) – a molecule of astrochemical relevance
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The analysis of <i>syn</i>-2-cyano-1,3-butadiene (C<sub>5</sub>H<sub>5</sub>N, <i>μ<sub>a</sub></i> = 3.2 D, <i>μ<sub>b</sub></i> = 2.3 D) in its ground vibrational state and two lowest-energy excited vibrational states, ν<sub>27</sub> (A<sup>″</sup>, 144 cm<sup>−1</sup>) and ν<sub>19</sub> (A′, 163 cm<sup>−1</sup>), in the 130–360 GHz frequency region has been completed. Nearly 4200 rotational transitions have been measured in the ground vibrational state for the first time, resulting in the determination of the spectroscopic constants for a complete octic Hamiltonian with low error. Analysis of the two lowest-energy, Coriolis-coupled fundamentals reported herein, each containing circa 3000 transitions, yielded two possible least-squares fitting solutions. Both solutions address perturbation between the two vibrational states, including resonances and several nominal interstate transitions, using four <i>a</i>-type and five <i>b</i>-type Coriolis coupling terms (Ga, GaJ, GaK, Fbc, Gb, GbK, Fac, and FacK, with or without FacJ). The energy separation between the two states, Δ<i>E</i><sub>27,19</sub> = 12.307065 (2) cm<sup>−1</sup>, agrees between the two solutions within their statistical uncertainties, giving confidence that this value is accurate despite the differing Coriolis-coupling terms. The precise rotational and distortion constants determined in this work provide the foundation for an astronomical search for <i>syn</i>-2-cyano-1,3-butadiene across the radio band.
