FOURIER-TRANSFORM INFRARED-SPECTROSCOPY OF THE FIRST CO-STRETCH OVERTONE BAND OF (CH3OH)-C-13

This paper presents a high-resolution Fourier transform infrared study of the first CO-stretch overtone band of (CH3OH)-C-13. The spectrum h as been recorded at the Justus-Liebig University, Giessen, Germany on their Bruker IFS 120 HR Fourier transform spectrometer. We have assign ed 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 e xpansion 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 v(CO) = 2 state w as found to be 0.768 cm(-1), forming a smooth series with that of 0.77 7 cm(-1) obtained in the v(CO) = 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 v(CO) = 2 state are proposed. The value o btained for the barrier height to internal rotation is 377.06 +/- 0.52 cm(-1), nearly indistinguishable from the value 378.65 cm(-1) reporte d for the CO-stretch v(CO) = 1 state. The vibrational energy is found to be 2020.9 +/- 1.4 cm(-1). The harmonic wavenumber for the CO-stretc h vibration in (CH3OH)-C-13 was calculated to be omega = 1029.9 cm(-1) The anharmonicity constant of this vibration is omega x = 6.5 cm(-1) giving x = 6.3 x 10(-3). We have also observed asymmetry-induced K dou bling for the subbands of A symmetry for K values from 1 to 3 at suffi ciently high J values. The size of the splitting coefficients is simil ar to those observed for the CO-stretch fundamental (14), with the exc eption of those for the K = 3A doublet, where the observed splitting i s about 18% larger than that for the ground and CO-stretch v(CO) = 1 s tates. (C) 1998 Academic Press.