Millimeter wave spectroscopy of flexible molecules relevant to astrochemistry

The investigation of phenomena related to the chemistry of the Cosmos is strongly based on the identification of molecules by spectroscopic methods and a knowledge of their spectroscopic parameters or their transition frequencies is needed. Rotational spectroscopy, with its high resolution, is the most powerful tool for the unambigous identification of molecules and laboratory work is needed to provide the community with the spectral features to analyze the cosmological surveys. Many of the molecules detected in space are complex organic molecules containing chains of carbon atoms and which therefore show a high degree of molecular flexibility. The high number of low energy conformations and the presence of large amplitude motions on shallow potential energy surfaces are peculiar to this kind of systems. The presence of a high number of stable conformers - often interconverting through small energy barriers - usually gives rise to very complex rotational spectra, which represent a challenge for spectroscopic and computational methods. Spectroscopic strategies for the rotational study of flexible molecules include the use of the cold and isolated conditions of a free jet expansion and laser ablation sources for the non volatile systems while the computational methods must deal with complex conformational surfaces and large amplitude motions which can be coupled to the overall rotation causing a hyperfine splitting of the rotational transitions. We will give examples of rotational spectroscopy of highly flexible organic molecules of astrophysical interest studied by the Free Jet Spectroscopy Group in Bologna with instruments going from 6 to 78 GHz. They include methylacetate [1], methylaminoethanol and biological building blocks like C4 sugars [2] The interpretation of their rotational spectra with appropriate models is essential to correctly predict their spectral features at higher frequencies. [1] M. Tudorie, I. Kleiner, J. T. Hougen, S. Melandri, L. W. Sutikdja, W. Stahl, J. Mol. Spectr., 269, 211 (2011) [2] B. M. Giuliano, S. Blanco, S. Melandri, and W. Caminati, Astrophy. J. Suppl,.179, 355 (2008); B. M. Giuliano, S. Blanco, S. Melandri, W. Caminati, Chem. Phys. Lett., 467, 74 (2008