The 130 to 375 GHz Rotational Spectroscopy of s-trans-(Z)-1-Cyano-1,3-butadiene (C5H5N): Analysis of the Lowest-Energy Vibrationally Excited Dyad (ν19 and ν27)

The rotational spectra and analysis of the two lowest-energy vibrationally excited states, ν₁₉ (A′, 126 cm^–1, MP2) and ν₂₇ (A″, 133 cm^–1, MP2), of s-trans-(Z)-1-cyano-1,3-butadiene from 130 to 375 GHz is presented. The state symmetries allow a- and b-type Coriolis coupling, the effects of which are observed due to the close energy spacing of these states. A combined total of 6744 transitions were modeled (σ_fit < 60 kHz) with a partial-octic two-state A-reduced Hamiltonian including eight coupling parameters (G_a, G_a^J, F_bc, F_bc^J, G_b, G_b^J, G_b^JJ, and G_b^K). The vibration–rotation interaction constants, Coriolis ζ terms, and energy difference between the states were determined experimentally and are compared to predicted values (B3LYP/6-311+G(2d,p) and MP2/6-311+G(2d,p)). The coupled-state analysis affords a precise vibrational energy difference of 5.753 373 1 (25) cm^–1 between the two fundamental states. Several matching pairs of a- and b-type resonant transitions, as well as many formally forbidden, nominal interstate transitions, are included in the final least-squares fit. (E)- and (Z)-1-Cyano-1,3-butadiene are possible precursors to or degradation products of pyridine, a fundamental aromatic molecule of interest in the interstellar medium. There is currently some uncertainty concerning the reported identifications of other cyanobutadiene isomers in the interstellar medium. Combined with our previously reported ground-state spectroscopic constants, the vibrationally excited-state constants provided herein enable possible extraterrestrial identification of multiple vibrational states of s-trans-(Z)-1-cyano-1,3-butadiene.