The Internal Rotation Spectroscopy Of DCOOCH₃

Methyl formate, HCOOCH3, is abundant in numerous interstellar clouds. It’s a simple internal rotor with a very dense and intense rotational spectrum, which has led to the identification of a large number of interstellar spectral lines. This is remarkable because the rotational spectrum of methyl formate is complicated by the large amplitude motion of the methyl group (CH3), which is well known as internal rotation or torsion. This motion leads to torsional energy levels, characterized by the quantum number vt=O,1,2,... Each torsional level is in turn split into two substates, characterized by the symmetry labels A(σ=0) and E(σ=±1).In addition, the torsional and overall rotational motions are coupled such that the rotational energy levels and selection rules within torsional substates differ from standard asymmetric top patterns, especially in E substates. Because of the difficulty in analyzing the spectra of internal rotor, several methods have been developed to construct different effective Hamiltonians, such as the principal axis method (PAM), the rho axis method(RAM) and the internal axis method(IAM).The rotational-torsional spectrum of DCOOCH3, singly deuterated isotopomer of methyl formate, has been extended into the millimeter-wave and sub-millimeter-wave regions of the electromagnetic spectrum. The internal rotaion of this compound was treated using the so-called IAM (Internal Axis Method). A total of1712transitions belongs to the A and E torsional substates of the ground torsional state (vt=0), which through rotational quantum number J=64,Kα=36up to a frequency of660GHz, were fited using this global method.31parameters were employed in the final fit, which has an rms deviation of174.6kHz.