Global Fit of Torsion–Rotation Transitions in the Ground and First Excited Torsional States of13C-Methanol

Abstract

A global treatment of reported microwave, millimeter-wave, and Fourier-transform far-infrared (FIR) spectra of the C-13 isotopic species of methanol has been carried out for the first two torsional states ( n t Å 0 below the barrier and n t Å 1 straddling the barrier) of the ground vibrational state. 42-57 (1984)). A convergent global fit was achieved using 55 adjustable and 2 fixed parameters to reach an overall weighted standard deviation of 0.962, giving further confirmation that a traditional onedimensional internal rotation model can accurately describe methanol energy levels up through the first excited torsional state. Our new parameters for C-13 methanol are compared to previous global fitting results for the normal 12 CH 3 OH isotopomer. Calculations employing these parameters reveal possible C-13 assignments for 28 lines appearing in natural abundance in a newly measured methanol microwave atlas from 7 to 200 GHz compiled by the group at Toyama University in Japan.

Torsion–rotation global analysis of the first three torsional states (νt=0, 1, 2) and terahertz database for methanol

“…The present global fit of the first three torsional states (m t = 0, 1, 2) of methanol up to J = 30 is essentially a continuation of our earlier global fits of the torsional ground state (m t = 0, J 6 20) by itself [1] and of the first two torsional states (m t = 0, 1, J 6 20) together [2]. A global fit similar to the present one has been completed for CH 3 18 OH [8], but the available microwave (MW) and millimeter-wave (MMW) data set for that isotopolog is much smaller than the data set treated here. The present fit was stimulated by recent Jet Propulsion Laboratory (JPL) THz measurements, carried out with a new spectrometer [23] based on a number of THz technologies developed for the heterodyne instrument for the far infrared (HIFI), which is scheduled to be flown on the Herschel Space Observatory (HSO) in 2008, and for the Atacama Large Millimeter Array (ALMA).…”

“…Refs. [1][2][3][4][5][6][7][8] are associated in one way or another with the laboratory at the University of New Brunswick in Canada, while Refs. [9][10][11][12][13][14][15][16][17][18][19][20][21][22] are representative torsion-rotation fits from other laboratories.…”

“…These two levels are well above the top of the barrier at V 3 = 374 cm À1 , but they are still well below the lowest small-amplitude vibration m 8 near 1034 cm À1 (with A and E components near 1162 and 1171 cm À1 , respectively, when measured from the bottom of the torsional well [25]). Thus, it should in principle be possible to fit torsion-rotation states of the first three torsional levels using the same torsion-rotation Hamiltonian as was used previously [1][2][3][4][5][6][7][8]. Nevertheless, new difficulties arise, since an increase in torsional excitation leads to more level crossings and perturbations, as well as to a denser spectrum with more line overlap and more anomalous intensity variation within branches (compared to that expected for a rigid molecule).…”

“…1 and 2, however, indicated that a more realistic measurement precision was closer to 150 or 200 kHz [39]. The cause for this is not yet established, but one possibility might be the presence of unknown weak transitions (grass) lying under some of the strong lines in molecules like CH 3 OH that act to perturb the line shape and thereby prevent a correct determination of the line center to the theoretical limit of (linewidth)/ (S/N), when (S/N) > 1000.…”

Raman and infrared spectra of dimethyl ether 13C-isotopologue (CH3O13CH3) from a CCSD(T) potential energy surface

“…Although many isotopic varieties containing 13 C have been spectroscopically characterized to allow their astronomical detection (i.e. for C 4 [2], C 5 [3], methanol [4], methyl formate [5,6], ethyl methyl ether [7]), the 13 C isotopologues of many other species cannot be identified in the ISM due to the lack of their spectral recordings. This is the case of dimethyl ether (DME).…”