Microwave study and global analysis of the lowest three torsional states of [formula omitted

Abstract

The pure rotational spectrum of CH 3 OD in the ground vibrational state has been investigated between 7 and 200 GHz. For transitions with torsional quantum number v t ⩽ 2 and with rotational quantum numbers J ⩽ 21 and | K | ⩽ 9 in the lower state, 248 previously unreported lines have been measured, and 10 new assignments have been made to earlier determinations. In addition, 235 previously identified lines have been re-measured. A global analysis has been carried out for v t ⩽ 2 , J ⩽ 21 , and | K | ⩽ 14 . The final data set includes 994 microwave and millimeter-wave transitions, and over 5800 far-infrared lines. This set was formed by adding the current determinations to an initial data set obtained by merging the data set of Walsh et al. in [J. Mol. Spectrosc. 204 (2000) 60] with that of Mukhopadhyay and Duan [Chem. Phys. 257 (2000) 91]. A good fit was obtained using a torsion–rotation Hamiltonian which has been reduced by the contact transformation formalism so that it can be characterized by 74 adjustable parameters. The root-mean-square values of the residuals were 0.15 and 8.1 MHz for the microwave/millimeter-wave and far-infrared data blocks, respectively. These root-mean-square residuals are the order of the corresponding experimental uncertainties. With the present advances in the understanding of the CH 3 OD energy level pattern, improved predictions can be made for torsion–rotation transitions for states with v t ⩽ 2 states and rotational quantum numbers within the range studied, and moderately beyond.