A rotation-torsion-vibration treatment with three-dimensional internal coordinate approach and additional FTIR spectral assignments for the CH3-bending fundamentals of methanol |
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Authors: | Mohammed Abbouti Temsamani R.M. Lees |
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Affiliation: | a Department of Physical Sciences, University of New Brunswick, Saint John, NB, Canada E2L 4L5 b Department of Physics, University of New Brunswick, Fredericton, NB, Canada E3B 5A3 |
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Abstract: | A theoretical model has been developed to account for certain features of both newly observed and previously reported CH3-bending subbands between 1450 and 1570 cm−1 in the high-resolution Fourier transform infrared spectrum of CH3OH [Can. J. Phys. 79 (2001) 435]. The features include (i) an apparent inversion of the rotationless E-A torsional splitting with respect to the ground state, i.e., the A state located above the E state, (ii) a pronounced upward slope in the K-reduced torsion-vibration energy pattern for the subband origins, and (iii) unexpected A1/A2 inversion of the K=2A and K=3AJ-rotational levels that led to ambiguity in identifying the vibrational mode as or . The model is an effective internal coordinate Hamiltonian constructed in G6 molecular symmetry with the CH3-bends coupled to each other and to torsion and including a- and γ-type Coriolis coupling. With this model, 33 out of 36 experimental upper-state K-term values for newly assigned , and ν10 subbands plus previous ν4 subbands have together been fitted successfully, employing 9 adjustable parameters and 17 fixed parameters to give a standard deviation of 0.14 cm−1. The Pγ Coriolis term appears to be the leading cause of the upward shift in the K-reduced energies. When J-dependence is introduced via a rotational Hamiltonian including b- and c-type Coriolis terms in addition to molecular asymmetry, the observed A1/A2 inversion of the K=2A and 3A rotational levels can also be reproduced. Predictions using the fitted K-rotation-torsion-vibration Hamiltonian show an interesting Coriolis-induced crossover and mixing of the ν5 and ν10 torsion-vibration energy patterns. These predictions played a role in identifying two of the new ν5 subbands in the crossing region, thereby helping to validate the model. |
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Keywords: | Methanol Infrared spectra CH3-bending bands Internal rotation Torsion-vibration interaction Torsional modeling Internal coordinate approach Coriolis coupling |
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