I present my own methodology for the spectroscopic study at very low temperatures of non-rigid organic molecules detected in star formation regions of the interstellar medium. As examples of these studies, recent relevant results are shown. The last works on ethylen glycol (EGLY)[1] and dimethyl-ether (DME) [2] are detailed. The firts species, ethylen glycol, was astrophysically identified in 2002 [3]. DME) was found in star-forming regions, where it plays an important role in astrochemical processes [4]. Latter on, DME was observed its firts torsional state [5]. Many medium-sized organic molecules present non-rigidity. Torsional motions and, in general, large amplitude motions, intertransform the different conformers that can stabilize by the formation of intramolecular hydrogen bonds. In the low energy regions of the potential energy surface, these hydrogen bonds can determine the symmetry and they can play important roles on the structure and on the low vibrational energy levels. A variational procedure of reduced dimensionality based on CCSD(T)-F12 calculations is applied to understand the far infrared spectrum of cis-gauche-EGLY [1] where three interacting internal rotations intertransform all the minima. The anisotropy of the surface in the gauche region converts the assignment and classification of the torsional levels into a tricky puzzler. 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. The far infrared spectrum was first observed and assigned by Groner and Durig in 1977 [6]. In 1995 [7-8], some of us performed ab initio calculations concerning the torsional structure. Given the astrophysical interest, the research was reconsidered in 2015 [9] using highly correlated ab initio calculations. New Raman measurements at different temperatures [2] allowed us to reassign relevant torsional bands such as the overtones.

XXXVII Reunión Bienal de la RSEF, Zaragoza, del 15 al 21 de julio de 2019