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 (νt= 0 below the barrier and νt= 1 straddling the barrier) of the ground vibrational state. The13CH3OH data set contains 725 microwave and millimeter-wave lines and 6283 Fourier-transform far-infrared lines, representing the most recent available information in the quantum number rangesJ≤ 20,K≤ 14. The transitions have been successfully fitted to within the assigned measurement uncertainties of ±50 kHz for most of the microwave and millimeter lines and ±6 MHz for the FIR lines with the use of a program (I. Kleiner and M. Godefroid, private communication) based on the formalism of Herbstet al.(J. Mol. Spectrosc.108,42–57 (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 one-dimensional 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 normal12CH3OH 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.