A ROTATION-TORSION-VIBRATION 3-D INTERNAL COORDINATE TREATMENT FOR THE $CH_{3}$-BENDING FUNDAMENTALS OF METHANOL
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Date
2002
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Ohio State University
Abstract
A theoretical model has been developed to account for certain features of $CH_{3}$-bending subbands of methanol observed between 1450 and $1570 cm^{-1}$. The observed features in $v_{4}$ include (i) an apparent inversion of the rotationless E-A torsional splitting with respect to the ground state, (ii) a pronounced upward slope in the K-reduced torsion-vibration energy pattern for the subband origins, and (iii) $A_{1}-A_{2}$ inversion of the K = 2A and 3A J-rotational levels leading initially to ambiguity in identifying the vibrational mode as $\nu_{4} (A_{1})$ or $\nu_{10} (A_{2})$. The model is an effective internal coordinate Hamiltonian constructed in $G_{6}$ molecular symmetry with the $CH_{3}$-bends coupled to each other and to torsion and including a- and $\gamma$-type Coriolis coupling terms. Experimental upper state energies for $\nu_{4},\nu_{10}$, and $\nu_{5}$ have together been fitted successfully employing 12 adjustable parameters to give a standard deviation of $\pm0.13 cm^{-1}$. J-dependence is introduced via a rotational Hamiltonian including molecular asymmetry, plus b- and c-type Coriolis terms which account for the observed $A_{1}-A_{2}$ inversion of the rotational levels at low K. The computer program for our model was set up in a CH-stretch/$CH_{3}$-bend local mode polyad scheme, with polyad number $p=2(v1+v2+v3)+(v4+v5+v6)$, for ready extension to the $3 \mu$m CH-stretching region in future.
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Author Institution: Department of Physical Sciences, University of New Brunswick