Weak intramolecular interaction effects on the torsional spectra of ethylene glycol, an astrophysical species

11 pags., 6 figs., 10 tabs.An elaborate variational procedure of reduced dimensionality based on explicitly correlated coupled clusters calculations is applied to understand the far infrared spectrum of ethylene-glycol, an astrophysical species. This molecule can be classified in the double molecular symmetry group G and displays nine stable conformers, gauche and trans. In the gauche region, the effect of the potential energy surface anisotropy due to the formation of intramolecular hydrogen bonds is relevant. For the primary conformer, stabilized by a hydrogen bond, the ground vibrational state rotational constants are computed to be A = 15 369.57 MHz, B = 5579.87 MHz, and C = 4610.02 MHz corresponding to differences of 6.3 MHz, 7.2 MHz, and 3.5 MHz from the experimental parameters. Ethylene glycol displays very low torsional energy levels whose classification is not straightforward and requires a detailed analysis of the torsional wavefunctions. Tunneling splittings are significant and unpredictable due to the anisotropy of the potential energy surface PES. The ground vibrational state splits into 16 sublevels separated ∼142 cm. The splitting of the >G1 sublevels> was calculated to be ∼0.26 cm in very good agreement with the experimental data (0.2 cm = 6.95 MHz). Transitions corresponding to the three internal rotation modes allow assignment of previously observed Q branches. Band patterns, calculated between 362.3 cm and 375.2 cm, 504 cm and 517 cm, and 223.3 cm and 224.1 cm, that correspond to the tunnelling components of the v fundamental (v = OH-torsional mode), are assigned to the prominent experimental Q branches. by AIP Publishing.This research was supported by the MINECO of Spain grant FIS2013-40626-P. The authors acknowledge the COST Actions CM1405 “MOLIM” and CM1401 “Our Astrochemical History”. R.B. acknowledges the grant No. ECOST-STSMCM1401-020316-071825 and Tunisian Grants. The authors acknowledge the CTI (CSIC) and CESGA for computing facilities.Peer Reviewe