The 130–360 GHz rotational spectrum of syn-2-cyano-1,3-butadiene (C₅H₅N) – a molecule of astrochemical relevance

Abstract

The analysis of syn-2-cyano-1,3-butadiene (C₅H₅N, μ_a = 3.2 D, μ_b = 2.3 D) in its ground vibrational state and two lowest-energy excited vibrational states, ν₂₇ (A^″, 144 cm⁻¹) and ν₁₉ (A′, 163 cm⁻¹), 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 a-type and five b-type Coriolis coupling terms ($G_a$, $G_a^J$, $G_a^K$, $F_{bc}$, $G_b$, $G_b^K$, $F_{ac}$, and $F_{ac}^K$, with or without $F_{ac}^J$). The energy separation between the two states, ΔE_27,19 = 12.307065 (2) cm⁻¹, 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 syn-2-cyano-1,3-butadiene across the radio band.

GRAPHICAL ABSTRACT

KEYWORDS:

• Astrochemistry
• Coriolis coupling
• rotational spectroscopy
• pyridine isomer
• Titan atmosphere

Supplemental material

See supplemental material for least-squares fitting files of 2-cyano-1,3-butadiene, output files from computations, computed vibration-rotation interaction constants, computed vibrational frequencies and infrared intensities, and nominal interstate transitions for the ν₂₇ and ν₁₉ dyad with their corresponding within-state transitions. These files may be found at https://doi.org/10.1080/00268976.2021.1964629.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Science Foundation under Grants CHE-1954270 and CHE-1664912. We thank Michael McCarthy for the loan of an Amplification-Multiplication Chain and the Harvey Spangler Award (to B.J.E.) for the funding that supported the purchase of the corresponding Virginia Diodes Zero-Bias Detector (WR5.1ZBD 140 − 220 GHz). Current and former members of our research team thank Prof. John Stanton for his friendship and for sharing scientific insights through collaborative research over many, many years.