Fourier Transform Infrared Spectroscopy of the First CO-Stretch Overtone Band of13CH3OH

Abstract

This paper presents a high-resolution Fourier transform infrared study of the first CO-stretch overtone band of13CH3OH. 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 staten= 0 forKvalues up to 6. Each individual subband has been fitted to aJ(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 constantBin the CO-stretchvCO= 2 state was found to be 0.768 cm−1, forming a smooth series with that of 0.777 cm−1obtained in thevCO= 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 thevCO= 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−1reported for the CO-stretchvCO= 1 state. The vibrational energy is found to be 2020.9 ± 1.4 cm−1. The harmonic wavenumber for the CO-stretch vibration in13CH3OH was calculated to be ω = 1029.9 cm−1. The anharmonicity constant of this vibration is ωx= 6.5 cm−1, givingx= 6.3 × 10−3. We have also observed asymmetry-inducedKdoubling for the subbands ofAsymmetry forKvalues from 1 to 3 at sufficiently highJvalues. The size of the splitting coefficients is similar to those observed for the CO-stretch fundamental, with the exception of those for theK= 3Adoublet, where the observed splitting is about 18% larger than that for the ground and CO-stretchvCO= 1 states.