Rotational spectroscopy of the two conformers of 3-methylbutyronitrile (C₄H₉CN) between 2 and 400 GHz

Abstract

We present high-resolution rotational spectroscopy of the two conformers of 3-methylbutyronitrile (C4H9CN). Spectra were taken between 2 and 24 GHz by means of Fourier transform microwave spectroscopy. Spectra between 36 and 403 GHz were recorded by means of frequency modulated (FM) absorption spectroscopy. The analysis yields precise rotational constants and higher order distortion constants, as well as a set of 14N nuclear electric quadrupole coupling parameters for each of the two conformers. In addition, quantum chemical calculations were performed in order to assist the assignments. Frequency calculations yield insight into the vibrational energy structure of the two conformers, from which partition functions and vibrational correction factors are determined. These factors are used to determine experimentally and computationally the energy difference between the two conformers, which is revealed to be negligible. Overall, this study provides precise spectroscopic constants for the search of 3-methylbutyronitrile in the interstellar medium. In particular, this molecule is a perfect test case for our knowledge of branched molecule formation in space.

High-Resolution Rotational Spectroscopy and Interstellar Search for Isopropyl Isothiocyanate

“…Based on the accurate laboratory rotational spectroscopic data determined for (CH 3 ) 2 CHNCS, a spectroscopic search for (CH 3 ) 2 CHNCS toward Sgr B2(N) was carried out with a Shanghai Tianma 65 m radio telescope (TMRT) at a Ku band (12)(13)(14)(15)(16)(17)(18). As there are radio frequency interferences at 12.6 and 13.2 GHz, only transitions at 15 GHz were observed.…”

“…The rotational spectra of isopropyl alcohol, − isopropyl mercaptan, 2-nitropropane, and isopropyl amine − have been analyzed in detail. For larger sized branched alkyl molecules, laboratory investigations of butyl cyanides, e.g., tertiary-butyl cyanide, , 2-cyanobutane, and 3-methylbutyronitrile, have been carried out. Although the rotational spectroscopic parameters of branched molecules determined in the laboratory are available, isopropyl cyanide still ranks as the only one that has been detected in the ISM to date.…”

Astrochemistry During the Formation of Stars

“…With the new era of sensitive spectral broadband datasets at submillimeter wavelengths, laboratory measurements at high frequencies are becoming even more crucial. The spectroscopy community has already begun to extend the spectral characterization of a number of COMs into the submm domain (e.g., Wehres et al 2018b;Kolesniková et al 2018;Motiyenko et al 2019). But even the extension of the spectroscopy of COMs from the cm range, investigated several decades ago, to the mm range already represents significant progress (e.g., Cernicharo et al 2016;Alonso et al 2016;Martin-Drumel et al 2019).…”

Molecular complexity in the interstellar medium

“…The chemical models presented in Belloche et al (2014), using the astrochemical code MAGICKAL (Garrod 2013), reproduce the abundance ratio i /n of 0.4 measured for propyl cyanide, and further simulations by Garrod et al (2017) predict that for butyl cyanide, C 4 H 9 CN, one of the branched isomers should even dominate over the straight chain form. These predictions and the rotational spectroscopy work performed on these isomers in parallel (Müller et al 2017;Wehres et al 2018) motivated us to search for the isomers of butyl cyanide and perform the ReMoCA survey (Re-exploring Molecular Complexity with ALMA) toward Sgr B2(N) during ALMA's Cycle 4, improving by a factor of three both the angular resolution and the sensitivity compared to EMoCA. The high angular resolution of 0.5" was key to resolve the emission around the main hot core Sgr B2(N1).…”