The Third Excited Torsional State of Acetamide

We present the results of our new study of the torsion-rotation spectrum of the acetamide molecule, CH3CONH2. Despite the fact that acetamide was found to be quite abundant in the interstellar medium, its rotation spectrum was studied only up to $J_{\max}=20$ so far. The aim of the present study was to extend the quantum number range involved in the analysis (thus improving the quality of predictions for higher excited rotational states) as well as the frequency range coverage. The new measurements were carried out from the millimeter wave range (starting at 34.5 GHz) to the sub-millimeter wave range (up to 645 GHz) using spectrometers in the Kharkiv and Köln laboratories. Our preliminary fits of the $v_{\mathrm{t}}=0,1,2$ torsional states of acetamide with $J_{\max}=62$ revealed some perturbations, which we attribute to distortions caused by the NH2 wagging state at ∼259 cm−1 (that propagate down to low-lying torsional states via intertorsional interactions). The extension of our assignments to the third excited torsional state of acetamide is a preparatory step toward explicitly accounting for interactions with the NH2 wagging state of acetamide. So far, 421 A-type and 263 E-type rotational transitions up to $J=32$ belonging to the third excited torsional state of acetamide were assigned and added to the analysis. A joint $v_{\mathrm{t}}=0,1,2,3$ dataset involving 8240 rotational transitions was fit using the rho axis method giving a weighted root mean square deviation of 2.8 for a Hamiltonian model that includes 114 parameters.