THE $3 \mu m$ VIBRATION-TORSION-ROTATION ENERGY MANIFOLD OF METHANOL

Author Institution: Department of Physical Sciences, University of New Brunswick; Department of Chemistry, University of AkronThe $3\mu m$ spectrum of methanol is an important gateway to the understanding of molecular dynamics and to the modeling of cometary spectra. The region is extremely complicated due to a dense vibrational structure and network of interactions among the three CH-stretch fundamentals, $\nu_{2}, \nu_{g}, \nu_{3}$, six overtones and combinations of the three $CH_{3}$-bending modes, $\nu_{4}, \nu_{10}, \nu_{5}$, and a variety of overtone combinations of the torsion $\nu_{12}$, with the remaining lower-lying vibrations. We have obtained FT spectra for the $3 \mu m$ region under various conditions. The structure is dense with few easily recognized features above the $v_{3}$ symmetric CH-stretch. However, in an extension of the color-center-laser slit-jet beam spectrum from 2945 to $2975 cm^{-1}$, low K states could be identified, then allowing further assignment and confirmations of the medium K states from FTS. Altogether, about 25 vibration-torsion-K-rotational states have now been firmly assigned up to K = 4. Plots of K-reduced energies place these states into three distinguishable groups assigned as $\nu_{9}, 2\nu_{4}$, and $\nu_{4}+\nu_{10}$, although there are a number of extra subbands in the spectrum arising possibly from interactions with other states. Spectroscopic findings at the present time are: (i) the torsional A/E ordering is inverted for $\nu_{9}$, normal for $2\nu_{4}$, and apparently normal for the presently observed K = 2 states of $\nu_{4}+\nu_{10}$; (ii) the K = 0 torsional A/E splittings are -5.48 and $8.28 cm^{-1}$ for $\nu_{9}$ and $2\nu_{4}$, respectively, and an estimated much lower than ground state value for the $\nu_{4}+\nu_{10}$ combination; (iii) the $\nu_{9}$ and $2\nu_{4}$ states have virtually identical upper state term values around $3092 cm^{-1}$, but show almost equal and opposite linear shifts with K with slopes of $2-3 cm^{-1}$/K-value; (iv) the $\nu_{4}+\nu_{10}$ combination is about $20 cm^{-1}$ lower in energy than $\nu_{9}$ and $2\nu_{4}$, $10 cm^{-1}$ lower than the previous estimates for the band center