First high-resolution spectral study of the fundamental CD2-twisting band of asymmetrically deuterated methanol CHD2OH in the trans- (e0) configuration, using Synchrotron radiation

Abstract

This paper reports the analysis of the high-resolution infrared (IR) spectrum for asymmetrically deuterated methanol, CHD₂OH (Methanol-D₂), corresponding to the CD₂ twisting band at the trans- species (e₀), recorded at low temperature with an excellent signal-to-noise (S/N) ratio of about 500:1 for strong isolated lines. In spite of the complexity and perturbation in the spectra, assignments were possible for the band for K values of 0–10 and with maximum J values of about 30 in most cases. In total, about 1000 spectral lines have been assigned. Localized perturbations have been observed for most of the sub-bands, which have been treated partially with 2 × 2 matrices with interaction parameters. Spectral lines have been analyzed in terms of state-dependent molecular parameters (7 parameters for each sub-state, Kσ). Assignments have been rigorously confirmed using various combination relations, and the combination loop defects suggest the uncertainty of the measured wavenumbers better than ±0.0002 cm^-1. In addition, about 100 usually forbidden Q-branch transitions were detected for the parallel transitions from gauche-states in the ground state terminating at the trans- states in the twisting mode. The state-dependent molecular parameters obtained can reproduce the observed wavenumbers within experimental uncertainty. An extended atlas of >10,000 assigned transitions with accurate wavenumbers has been prepared and is being submitted in the Appendix as a depository, which should provide laboratory backup for the detection of this critical isotopomer of methanol species in interstellar space and produce optically pumped coherent radiation. It was also possible to assign some forbidden transitions which connect the gauche state (o₁) in the ground state with the trans state (e₀) in the twist mode. This study represents the first time the high-resolution analysis of the CD₂-twisting mode in CHD₂OH is being performed.