Millimeter-Wave Spectra and Global Torsion–Rotation Analysis for the CH 3 OD Isotopomer of Methanol M. S. Walsh,* Li-Hong Xu,² R. M. Lees,* I. Mukhopadhyay,‡ G. Moruzzi,§ B. P. Winnewisser, ¶ S. Albert, Rebecca A. H. Butler, and F. C. DeLucia *Department of Physics, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3; ² Department of Physical Sciences, University of New Brunswick, Saint John, New Brunswick, Canada E2L 4L5; ‡Laser Programme, Centre for Advanced Technology, Indore 452 013, India; §Dipartimento di Fisica dell’Universita ` di Pisa and INFM, Via Filippo Buonarroti 2, I-56127 Pisa, Italy; ¶ Physikalisch Chemisches Institut, Justus Liebig Universita ¨t, Heinrich Buff Ring 58, D-35392 Giessen, Germany; and Department of Physics, The Ohio State University, 174 West 18th Avenue, Columbus, Ohio 43210 Received May 31, 2000; in revised form July 6, 2000 New millimeter-wave and microwave measurements for CH 3 OD have been combined with previous literature data and with an extended body of Fourier transform far-infrared observations in a full global analysis of the first two torsional states ( v t = 0 and 1) of the ground vibrational state. The fitted CH 3 OD data set contained 564 microwave and millimeter-wave lines and 4664 far-infrared lines, representing the most recent available information in the quantum number ranges J  20 and K  15. A 53-parameter converged global fit was achieved with an overall weighted standard deviation of 1.060, essentially to within the assigned measurement uncertainties of 100 kHz for almost all of the microwave and millimeter-wave lines and 6 MHz for the far-infrared lines. The new parameters for CH 3 OD are compared to previous results obtained for the 12 CH 3 OH, 13 CH 3 OH, and CD 3 OH isotopomers over the same quantum number ranges using the identical fitting program. Strong asymmetry-induced coupling between the accidentally near-degenerate 0E and -1E v t = 0 substates is successfully modeled by the fit. © 2000 Academic Press Key Words: methanol; CH 3 OD; torsion; millimeter-wave spectra; far-infrared spectrum; global fitting. I. INTRODUCTION This analysis of the ground state spectrum of the CH 3 OD species of methanol is the fourth in a series of global fitting studies of the microwave (MW), millimeter-wave (MMW), and far-infrared (FIR) spectra of methanol and its isotopomers (1–4). The data sets have all been chosen to cover closely similar ranges of quantum number to permit consistent inter- comparison of the results. Each of the global fits reproduces the data to within experimental accuracy, using a computer pro- gram (5) based on the formalism of Herbst et al. (6). Here, we will give brief introductory remarks relating to CH 3 OD and refer the reader to the previous literature (1–7) for details of the computer program, the Hamiltonian model used, and the back- ground to the notation for the torsion–rotation quantum num- bers and transition labeling. There have been a number of studies of the MW and MMW spectra of CH 3 OD (8 –13), originally aimed at the structural determination of methanol via isotopic substitution, and later with astrophysical applications in view. CH 3 OD was also in- cluded in Woods’ far-infrared (FIR) study of four methanol isotopomers (14), whose ground state spectra were subse- quently analyzed by Kwan and Dennison (15) in order to better define the torsional potential and Hamiltonian parameters. Re- cently, the high-resolution Fourier transform far-infrared (FT- FIR) spectrum has been investigated for the ground vibrational state, and several fits to model Hamiltonians of differing orders have been reported (16 –19) for a variety of subsets of the FTFIR data. Further ground state information is also available in the form of IR combination differences from the spectrum of the CO–stretching fundamental band (20) and from exploration by laser Stark spectroscopy of torsion–rotation transitions in the vicinity of the HCN laser lines (21). The present paper reports new MMW measurements for CH 3 OD in the ranges from 126 –147 and 482– 499 GHz, plus a number of further measurements in the MW region, which have yielded about 160 new line assignments. These have been combined with previous data in a global fit of a total of 5228 assigned MW, MMW, and FTFIR ground state tran- sitions of CH 3 OD involving levels of the first two torsional states ( v t = 0 and 1) up to a maximum rotational quantum number J of 20. The one-dimensional torsion–rotation model is implemented in a well-tested computer program (5) previously applied successfully for the 12 CH 3 OH (1, 2), 13 CH 3 OH (3), and CD 3 OH (4) isotopomers of methanol and also for acetaldehyde, CH 3 CHO (7). As described previously (4), the principal goals in our pro- gram of global analyses of ground state spectra for methanol isotopomers are as follows. (i) To achieve fits of all observed transitions to within experimental accuracy over quantum number ranges sufficient for rigorous testing of the model. So far, our CH 3 OH, Journal of Molecular Spectroscopy 204, 60 –71 (2000) doi:10.1006/jmsp.2000.8201, available online at http://www.idealibrary.com on 60 0022-2852/00 $35.00 Copyright © 2000 by Academic Press All rights of reproduction in any form reserved.