The torsional spectrum of dimethyl-ether. New experiments and assignments

Abstract

Dimethyl-ether (DME) was found in star-forming regions, where it plays an important role in astrochemical processes [1]. Mono-deuterated DME [2] and singly-13C substituted DME [3] were detected as well. Despite this astrophysical interest, there is a lack of accurate spectral data of its isotopic species, which difficults further identification in the interstellar medium. The absorption spectrum of DME is quite dense and its analysis challenging because DME is an asymmetric top molecule with two internal rotors undergoing large amplitude motions. Thus, understanding such large amplitude internal motions on a quantitative level rely on quantum chemical models validated by laboratory data. However, one of the torsional bands is forbidden in the infrared spectra, that preventing the experimental determination of some of the interaction parameters.

We have recorded Raman spectra of DME and of 13C-DME at room temperature, and of cooled DME in supersonic jet. We will introduce first the experimental approach [4-5], and then we will present some Raman spectra of DME at different spectral regions. The spectral region of the torsional overtones of DME is rather complex due to their coupling with the C-O-C bending mode, and the presence of many hot bands. The spectrum of cooled DME allowed us to assign unequivocally the torsional overtones and their first hot bands. In turn, the observed frequencies have been used to refine 3-D quantum calculations employing a torsion-torsion-bending Hamiltonian [6], along with an ab initio potential surface [7].