Rotational spectrum of 4-methylcyanoallene (CH₃CH=C=CH-CN), a chiral molecule of potential astrochemical interest^⋆

¹ Institut de Physique de Rennes (IPR), UMR 6251 UR1-CNRS, Département de Physique Moléculaire, Université de Rennes 1, Campus de Beaulieu, Bâtiment 11 C, 35042 Rennes Cedex, France
e-mail: sophie.carles@univ-rennes1.fr
² Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, University of Oslo, PO Box 1033 Blindern, NO-0315 Oslo, Norway
³ Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, CNRS, UMR 6226, 11 Allée de Beaulieu, CS 50837, 35708 Rennes Cedex 7, France

Received: 4 October 2013
Accepted: 29 December 2013

Abstract

Context. A successful identification of an interstellar compound requires that its spectrum has first been assigned in the laboratory. New and sensitive radiotelescopes, such as ALMA, will make it possible to detect interstellar molecules in much smaller concentrations than before. Cyanoallene (CH₂=C=CH-CN) has recently been observed in the dense molecular cloud TMC-1 by means of its rotational spectrum. Its methyl congener, 4-methylcyanoallene (CH₃CH=C=CH-CN), may also be present in the interstellar medium (ISM). This chiral compound exists in two forms, which are mirror images. Chirality is an essential feature of life. So far, no chiral compounds have been detected in the ISM.

Aims. The synthesis and assignment of the rotational spectrum of CH₃CH=C=CH-CN, will facilitate the potential detection of this compound in the ISM.

Methods. The spectrum of 4-methylcyanoallene has been recorded between 13 and 116 GHz using the microwave spectrometer at the University of Oslo. The spectroscopic study has been augmented with high-level quantum chemical calculations at the B3LYP/cc-pVTZ and CCSD/cc-pVTZ levels of theory.

Results. The rotational spectra of CH₃CH=C=CH-CN in the ground vibrational state and in the first vibrationally excited state are reported for the first time and accurate spectroscopic constants have been obtained from a large number of transitions.