Knowledge Bank

Diode laser spectra at Doppler limited rosolution of the low J, P and R multiplets of the C-O stretch band $(\upsilon 4)$ of $CD3OH$, reported upon initially at the last Molecular Spectroscopy Symposium, have been extended, calibrated and analyzed, Calibration was derived from fortuitousaly placed $NH3$ lines and from FTIR spectra calibrated against the $10\mu CO2$ 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 $spectra1$. 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 $a\pm 2cm-1$ shift of the interacting levels; it pushes the K=0 levels up by $\sim 0.3cm-1$; 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 $\pm 0.00088cm-1$. 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 $CH3OH$ 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 $band2$.