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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 IRAS16293$-$2422, one of the next alkenes, isobutene, is a promising candidate tobe searched for in space. We aim to extend the limited line lists of the mainisotopologue of isobutene from the microwave to the millimetre region in orderto obtain a highly precise set of rest frequencies and to facilitate itsdetection in the interstellar medium. We investigated the rotational spectrumof isobutene in the 35$-$370 GHz range using absorption spectroscopy at roomtemperature. Quantum-chemical calculations were carried out to evaluatevibrational frequencies. We determined new or improved spectroscopic parametersfor isobutene up to a sixth-order distortion constant. These new resultsenabled its detection in the G+0.693 molecular cloud for the first time, wherepropene was also recently found. The propene to isobutene column density ratiowas determined to be about 3:1. The observed spectroscopic parameters forisobutene are sufficiently accurate that calculated transition frequenciesshould be reliable up to 700 GHz. This will further help in observing thisalkene in other, warmer regions of the ISM.

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