Millimetre and submillimetre spectroscopy of isobutene and its detection in the molecular cloud G+0.693

TL;DR

This study extends the spectroscopic data of isobutene to facilitate its detection in space, successfully identifying it in a molecular cloud and comparing its abundance to propene.

Contribution

The paper provides highly accurate spectroscopic parameters for isobutene, enabling reliable detection in the interstellar medium and expanding the line list from microwave to millimetre wavelengths.

Findings

First detection of isobutene in the G+0.693 molecular cloud.

Propene to isobutene ratio of about 3:1.

Spectroscopic parameters accurate up to 700 GHz.

Abstract

Isobutene ((CH$_3$)$_2$C=CH$_2$) is one of the four isomers of butene (C$_4$H$_8$). Given the detection of propene (CH$_3$CH=CH$_2$) toward TMC-1, and also in the warmer environment of the solar-type protostellar system IRAS 16293$-$2422, one of the next alkenes, isobutene, is a promising candidate to be searched for in space. We aim to extend the limited line lists of the main isotopologue of isobutene from the microwave to the millimetre region in order to obtain a highly precise set of rest frequencies and to facilitate its detection in the interstellar medium. We investigated the rotational spectrum of isobutene in the 35$-$370 GHz range using absorption spectroscopy at room temperature. Quantum-chemical calculations were carried out to evaluate vibrational frequencies. We determined new or improved spectroscopic parameters for isobutene up to a sixth-order distortion constant. These new results enabled its detection in the G+0.693 molecular cloud for the first time, where propene was also recently found. The propene to isobutene column density ratio was determined to be about 3:1. The observed spectroscopic parameters for isobutene are sufficiently accurate that calculated transition frequencies should be reliable up to 700 GHz. This will further help in observing this alkene in other, warmer regions of the ISM.