ANALYSIS OF DIODE LASER SPECTRA OF $\nu_{4}$ OF $CD_{3}OH$
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Date
1982
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Ohio State University
Abstract
Diode laser spectra at Doppler limited rosolution of the low J, P and R multiplets of the C-O stretch band $(\upsilon_{4})$ of $CD_{3}OH$, reported upon initially at the last Molecular Spectroscopy Symposium, have been extended, calibrated and analyzed, Calibration was derived from fortuitousaly placed $NH_{3}$ lines and from FTIR spectra calibrated against the $10\mu CO_{2}$ laser lines measured in absorption. Line assignments were made on the basis of survey spectra, relative intensity distributions, combination differences, and the earlier Stark difference $spectra^{1}$. The lines were then LMS fitted to a hindered rotor hamiltonian. Both hindered rotation and molecular asymmatry were handled by successive matrix diogonalization. A Fermi resonance was identified between $\upsilon_{4}$ and the 3rd and 4th harmonics of the torsional mode. This resonance strongly perturbs the low K $\upsilon_{4} = 1, \tau = 1$ levels. It is maximum at K = 1, resulting in $a \pm 2 cm^{-1}$ shift of the interacting levels; it pushes the K=0 levels up by $\sim 0.3 cm^{-1}$; and it pushes the K = 2, 3, 4 levels down by successively decreasing amounts. By including empirical Fermi resonance shifts for the $\upsilon_{4} = 1, \tau = 1, \kappa = 2, 3, 4$ states together with 8 excited state constants, some 160 lines were fit with an rms error of $\pm 0.00088 cm^{-1}$. Intersity perturbations of allowed lines and strengths and positions of forbidden lines induced by the Fermi resonance were adequetly accounted for. Because of the relatively lpw hindered-rotor barrier height, similar Fermi resonances should occur in all vibrational states of all the methanols. In particular, in $\upsilon_{4}$ of $CH_{3}OH$ the interaction with the 3rd and 4th harmonics of the torsional mode is predicted to shift the $\upsilon_{4} = 1, \tau = 1, \kappa = 0$ levels to lower frequency and to perturb all the levels around K = 5. These predictions are consistent with the anomalies noted by Henningson in the analysis of this $band^{2}$.
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$^{1}$W. H. Weber, D. H. Leslie, and C. W. Peters, J. Mol. Spectrosc. 89, 214-222 (1981). $^{2}$J. O. Henningson, J. Mol. Spectrosc. 85, 282-300 (1981).