Laboratory Characterization and Interstellar Search for a New Interstellar Candidate: 2,3-Butadienal.

Three aldehydes with unsaturated substituents: propenal, 3-hydroxypropenal, and propynal, have been detected in the interstellar medium (ISM). The structurally related molecule 2,3-butadienal, an aldehyde with an allenyl substituent, therefore represents a compelling new target for interstellar searches. To date, however, no laboratory measurements of its rotational spectrum have been reported. Here, we present the first measurements and analysis of the rotational spectrum of the most stable trans-2,3-butadienal in the 150-485 GHz frequency range, with the aim of providing the accurate spectroscopic predictions required for astronomical observations. Guided by quantum chemical calculations, we assign the ground state and the two lowest excited vibrational states and analyze the latter within a Coriolis-coupled framework. The resulting high-precision spectroscopic parameters enable reliable frequency predictions over a broad range of quantum numbers. Using this new spectroscopic model, we searched for 2,3-butadienal in public astronomical data toward TMC-1, Sgr B2(N), and G +0.693-0.027. No transitions of this molecule were detected in any of these sources. Upper limits to its column density were therefore derived, and the implications for its possible formation pathways in the ISM are discussed. The comprehensive spectroscopic characterization reported here provides a robust foundation for future astronomical searches and will facilitate efforts to detect 2,3-butadienal in diverse interstellar environments.