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Description
An astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range of organic molecules to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict the transitions frequencies very precisely that were not measured in the laboratory. We present the experimental study and its theoretical analysis for two ^13^C-methyl formate isotopologues to detect these two isotopologues for the first time in their excited torsional states, which lie at 130/cm (200K) in Orion-KL. New spectra of HCO-O-^13^CH_3_ (^13^C_2_) methyl formate were recorded with the mm- and submm-wave spectrometer in Lille from 50 to 940GHz. A global fit for v_t_=0 and 1 was accomplished with the BELGI program to reproduce the experimental spectra with greater accuracy. We analysed 5728 and 2881 new lines for v_t_=0 and 1 for HCOO^13^CH_3_. These new lines were globally fitted with 846 previously published lines for v_t_=0. In consequence, 52 parameters of the RAM Hamiltonian were accurately determined and the value of the barrier height (V_3_=369.93168(395)/cm) was improved. We report the detection of the first excited torsional states (v_t_=1) in Orion-KL for the ^13^C_2_ and ^13^C_1_ methyl formate based on the present analysis and previously published data. We provide column densities, isotopic abundances, and vibrational temperatures for these species. Following this work, accurate prediction can be provided. This permits detecting 135 features of the first excited torsional states of ^13^C-methyl formate isotopologues in Orion-KL in the 80-280GHz frequency range, without missing lines.
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