Fourier Transform Infrared Spectroscopy of the First CO-Stretch Overtone Band of 13CH3OH.

This paper presents a high-resolution Fourier transform infrared study of the first CO-stretch overtone band of 13CH3OH. The spectrum has been recorded at the Justus-Liebig University, Gießsen, Germany on their Bruker IFS 120 HR Fourier transform spectrometer. We have assigned parallel subbands in the torsional state n = 0 for K values up to 6. Each individual subband has been fitted to a J(J + 1) power series expansion in order to obtain the subband origin and the state-specific energy expansion coefficients for the first CO-stretch overtone state. The average rotational constant B in the CO-stretch vCO = 2 state was found to be 0.768 cm-1, forming a smooth series with that of 0.777 cm-1 obtained in the vCO = 1 state and the ground state value of 0.787 cm-1. Modeling of the excited state torsion-vibration energy level structure derived from the subband origins is then discussed and molecular parameters in the vCO = 2 state are proposed. The value obtained for the barrier height to internal rotation is 377.06 +/- 0.52 cm-1, nearly indistinguishable from the value 378.65 cm-1 reported for the CO-stretch vCO = 1 state. The vibrational energy is found to be 2020.9 +/- 1.4 cm-1. The harmonic wavenumber for the CO-stretch vibration in 13CH3OH was calculated to be omega = 1029.9 cm-1. The anharmonicity constant of this vibration is omegax = 6.5 cm-1, giving x = 6.3 x 10(-3). We have also observed asymmetry-induced K doubling for the subbands of A symmetry for K values from 1 to 3 at sufficiently high J values. The size of the splitting coefficients is similar to those observed for the CO-stretch fundamental, with the exception of those for the K = 3A doublet, where the observed splitting is about 18% larger than that for the ground and CO-stretch vCO = 1 states. Copyright 1998 Academic Press.