A comprehensive high-resolution spectral study of the ground vibrational state of triply deuterated methanol (CD3OH) using synchrotron radiation and backward wave oscillator-based spectrometer: Torsional energies and forbidden transitions.

In this work, systematic and comprehensive studies of the energy states of triply deuterated methanol (CD3OH) in its ground vibrational state have been carried out using spectral studies in the far-infrared (FIR), sub-millimeter-wave (SMMW), and microwave (MW) regions. In the FIR region, a Synchrotron radiation-based Fourier transform spectrometer at the Canadian Light Sources (CLS) has been used to obtain a high-resolution (∼0.0017 cm-1) spectrum over 50-8000 cm-1 with a high signal-to-noise ratio (S/N) under various physical conditions. In the SMMW region, a backward wave-based very fast spectrometer was developed at the Ohio State University by F.C. DeLucia and coworkers. More than 3000 transition wavenumbers in the range of 117-960 GHz have been measured with an estimated uncertainty of 50-100 kHz. In the microwave region, approximately 2000 rotational transition frequencies have been measured in various laboratories over several decades, but have remained unpublished. In the ground torsional state, two K states (viz., K = 0 and 2) are so close to each other that they strongly interact through a ΔK = 2 interaction. In the first excited torsional state, the K = 7 and 9 A states also interact, mixing the states vigorously. Due to the above interactions, a large number of forbidden transitions have been observed, which borrow intensity from the mixed states. The assignments have been thoroughly confirmed using combination loops. The observed transition frequencies have been fitted directly with the term values, which can reproduce the frequencies well within the experimental uncertainty of ±0.0002 cm-1. The Hamiltonian constants have also been obtained by fitting the transition frequencies. However, due to perturbations in the energy levels, they reproduced the transitions up to the second excited torsional state with reasonable accuracy of ∼0.01-0.05 cm-1, and with ∼0.1-0.5 cm-1 for higher-lying states. The pure torsional energies for the first five torsional states have been determined and plotted as a function of the axial component (K) of the rotational angular momentum. A detailed discussion has been included later in this report. Transition frequencies essential to represent the discussions have been included in the text, and more than 12,000 accurately measured transition wave numbers are included in Appendix I. The present work has also been used to calculate the frequencies of many FIR laser lines optically pumped by CO2 laser, with an accuracy an order of magnitude better than previously known.