A rotation–torsion–vibration treatment with three-dimensional internal coordinate approach and additional FTIR spectral assignments for the CH3-bending fundamentals of methanol
✍ Scribed by Mohammed Abbouti Temsamani; Li-Hong Xu; R.M. Lees
- Publisher
- Elsevier Science
- Year
- 2003
- Tongue
- English
- Weight
- 258 KB
- Volume
- 218
- Category
- Article
- ISSN
- 0022-2852
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✦ Synopsis
A theoretical model has been developed to account for certain features of both newly observed and previously reported CH 3bending subbands between 1450 and 1570 cm À1 in the high-resolution Fourier transform infrared spectrum of CH 3 OH [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 A 1 =A 2 inversion of the K ¼ 2A and K ¼ 3A J-rotational levels that led to ambiguity in identifying the vibrational mode as m 4 ðA 1 Þ or m 10 ðA 2 Þ. 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 c-type Coriolis coupling. With this model, 33 out of 36 experimental upper-state K-term values for newly assigned m 4 ; m 5 , and m 10 subbands plus previous m 4 subbands have together been fitted successfully, employing 9 adjustable parameters and 17 fixed parameters to give a standard deviation of 0.14 cm À1 . The P c 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 A 1 =A 2 inversion of the K ¼ 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 m 5 and m 10 torsionvibration energy patterns. These predictions played a role in identifying two of the new m 5 subbands in the crossing region, thereby helping to validate the model.