The challenge of the equilibrium structure of cinnamonitrile: Rotational spectroscopy and semi-experimental approach

The detection of nitrile-containing species in the interstellar medium has highlighted the role of cyano derivatives as proxies for the corresponding non-polar hydrocarbons. Among them, cinnamonitrile can be regarded as a styrene analogue with enhanced polarity, and, thus, as a promising astrochemical candidate. Here, we report the first characterization of its rotational spectrum, combining quantum-chemical calculations and the so-called “Lego-brick” approach with broadband measurements in the 2-18 GHz range. Both E- and Z-cinnamonitrile have been identified in the spectrum, and accurate rotational, centrifugal distortion, and nitrogen quadrupole coupling constants have been obtained. Moreover, 15N- and 13C-containing isotopologues of the most stable E form have been observed in natural abundance and enabled the determination of a (partial) semi-experimental equilibrium structure. Special care was devoted to the treatment of the lowest-frequency torsional mode, whose anharmonicity required a tailored strategy in order to achieve a structure consistent with theoretical predictions. Our results provide both a reliable spectroscopic foundation for astronomical searches of cinnamonitrile and new insights into the structural flexibility of styrene derivatives which will be further explored in future works.