Knowledge Bank

Due to the asymmetry of the CH$2$D group, the internal rotation problem in the partially deuterated species of methanol CH$2$DOH is a complicated one as, unlike in the normal species CH$3$OH, the inertia tensor depends on the angle of internal rotation. The CH$2$DOH species also displays a dense far infrared torsional spectrum difficult to assign. Recently 38 torsional subbands of CH$2$DOH have been identified,~{\bf 256} (2009) 204.} but for most of them there is neither an assignment nor an analysis of their rotational structure. In this paper an analysis of the rotation-torsion spectrum of CH$2$DOH will be presented. The rotational structure of 23 torsional subbands have been assigned. These subbands are $\Delta vt\ge 1$ perpendicular subbands with a value of $vt\prime$ up to 10 and values of $K\prime$ and $K″$ ranging from 0 to 9. For all subbands, the $Q$-branch was assigned, for 3 subbands, the $R$- and $P$-branches could also be found. The results of the rotational analysis with an expansion in $J(J+1)$ of the new subbands and of already observed ones~{\bf 192} (1998) 378; and Mukhopadhyay, {\em J.~Mol.~Struct.}~{\bf 695-696} (2004) 357.} will be presented. When available, microwave lines within the lower torsional level, recorded in this work or already measured,~{\bf 146} (1991) 252; and Mukhopadhyay {\em et al.,} {\em J.~Chem.~Phys.}~{\bf 116} (2002) 3710.} were added to the data set. A theoretical approach aimed at calculating the rotation-torsion energy levels has also been developed. It is based on an expansion in terms of rotation-torsion operators with $Cs$ symmetry and accounts for the dependence of the inertia tensor on the angle of internal rotation. This approach will be used to carry out a preliminary global analyses of the wavenumbers and of the frequencies.